Aerographer's Mate

Module 5—Basic Meteorology

NAVEDTRA 14312

NONRESIDENTTRAININGCOURSE

June 2001

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

Although the words “he,” “him,” and“his” are used sparingly in this course toenhance communication, they are notintended to be gender driven or to affront ordiscriminate against anyone.

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

PREFACE

By enrolling in this self-study course, you have demonstrated a desire to improve yourself and theNavy. Remember, however, this self-study course is only one part of the total Navy trainingprogram. Practical experience, schools, selected reading, and your desire to succeed are alsonecessary to successfully round out a fully meaningful training program.

COURSE OVERVIEW: In completing this nonresident training course, you will demonstrate aknowledge of the subject matter by correctly answering questions on the following subjects:Fundamentals of Meteorology, Atmospheric Physics, Atmospheric Circulation, Air Masses,Fronts, Atmospheric Phenomena, Climate and Climatology.

THE COURSE: This self-study course is organized into subject matter areas, each containinglearning objectives to help you determine what you should learn along with text and illustrationsto help you understand the information. The subject matter reflects day-to-day requirements andexperiences of personnel in the rating or skill area. It also reflects guidance provided by EnlistedCommunity Managers (ECMs) and other senior personnel, technical references, instructions,etc., and either the occupational or naval standards, which are listed in the Manual of NavyEnlisted Manpower Personnel Classifications and Occupational Standards, NAVPERS 18068.

THE QUESTIONS: The questions that appear in this course are designed to help youunderstand the material in the text.

VALUE: In completing this course, you will improve your military and professional knowledge.Importantly, it can also help you study for the Navy-wide advancement in rate examination. Ifyou are studying and discover a reference in the text to another publication for furtherinformation, look it up.

2001 Edition Prepared byAGC(AW/SW) RICK KROLAK

Published byNAVAL EDUCATION AND TRAINING

PROFESSIONAL DEVELOPMENTAND TECHNOLOGY CENTER

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NAVSUP Logistics Tracking Number

0504-LP-026-4050

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Sailor’s Creed

“I am a United States Sailor.

I will support and defend theConstitution of the United States ofAmerica and I will obey the ordersof those appointed over me.

I represent the fighting spirit of theNavy and those who have gonebefore me to defend freedom anddemocracy around the world.

I proudly serve my country’s Navycombat team with honor, courageand commitment.

I am committed to excellence andthe fair treatment of all.”

TABLE OF CONTENTS

APPENDIX

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CHAPTER PAGE

INDEX .................................................................................................................................. INDEX-1

1. Fundamentals of Meteorology ............................................................................... 1-1

2. Atmospheric Physics ............................................................................................. 2-1

3. Atmospheric Circulation........................................................................................ 3-1

4. Air Masses and Fronts........................................................................................... 4-1

5. Atmospheric Phenomena....................................................................................... 5-1

6. Climate and Climatology....................................................................................... 6-1

I. Glossary................................................................................................................ AI-1

SUMMARY OF THE AEROGRAPHER'S MATETRAINING SERIES

The following training manuals of the AG training series are available:

AG MODULE 1, NAVEDTRA 12881, Surface Weather Observations

This module covers the basic procedures that are involved with conducting surface weatherobservations. It begins with a discussion of surface observation elements, followed by adescription of primary and backup observation equipment that is used aboard ships and at shorestations. Module 1 also includes a complete explanation of how to record and encode surfaceMETAR observations using WMO and NAVMETOCCOM guidelines. The module concludeswith a description of WMO plotting models and procedures.

AG MODULE 2, NAVEDTRA 12882, Miscellaneous Observations and Codes

This module concentrates on the observation procedures, equipment, and codes associatedwith upper-air observations and bathythermograph observations. Module 2 also discussesaviation weather codes, such as TAFs and PIREPs, and includes a chapter on surf observationprocedures. Radiological fallout and chemical contamination plotting procedures are alsoexplained.

AG MODULE 3, NAVEDTRA 12883, Environmental Satellites and Weather Radar

This module describes the various types of environmental satellites, satellite imagery, andassociated terminology. It also discusses satellite receiving equipment. In addition, Module 3contains information on the Weather Surveillance Radar-1988 Doppler (WSR-88D). It includesa discussion of electromagnetic energy and radar propagation theory, and explains the basicprinciples of Doppler radar. The module also describes the configuration and operation of theWSR-88D, as well as WSR-88D products.

AG MODULE 4, NAVEDTRA 12884, Environmental Communications and administration

This module covers several of the most widely used environmental communications systemswithin the METOC community. It also describes the software programs and products associatedwith these systems. The module concludes with a discussion of basic administration procedures.

AG MODULE 5, NAVEDTRA 14312, Basic Meteorology

This training manual introduces the Aerographer's Mate to the basic fundamentals ofmeteorology, atmospheric physics, atmospheric circulation, air masses, fronts, atmosphericphenomena, climate and climatology.

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NOTE

Additional modules of the AG training series are in development. Check the NETPDTC websitefor details at http://www.cnet.navy.mil/netpdtc/nac/neas.htm. For ordering information, checkNAVEDTRA 12061, Catalog of Nonresident Training Courses, which is also available from theNETPDTC website.

INSTRUCTIONS FOR TAKING THE COURSE

ASSIGNMENTS

The text pages that you are to study are listed at thebeginning of each assignment. Study these pagescarefully before attempting to answer the questions.Pay close attention to tables and illustrations and readthe learning objectives. The learning objectives statewhat you should be able to do after studying thematerial. Answering the questions correctly helps youaccomplish the objectives.

SELECTING YOUR ANSWERS

Read each question carefully, then select the BESTanswer. You may refer freely to the text. The answersmust be the result of your own work and decisions. Youare prohibited from referring to or copying the answersof others and from giving answers to anyone else takingthe course.

SUBMITTING YOUR ASSIGNMENTS

To have your assignments graded, you must be enrolledin the course with the Nonresident Training CourseAdministration Branch at the Naval Education andTraining Professional Development and TechnologyCenter (NETPDTC). Following enrollment, there aretwo ways of having your assignments graded: (1) usethe Internet to submit your assignments as youcomplete them, or (2) send all the assignments at onetime by mail to NETPDTC.

Grading on the Internet: Advantages to Internetgrading are:

� you may submit your answers as soon as youcomplete an assignment, and

� you get your results faster; usually by the nextworking day (approximately 24 hours).

In addition to receiving grade results for eachassignment, you will receive course completionconfirmation once you have completed all the

assignments. To submit your assignment answers viathe Internet, go to:

http://courses.cnet.navy.mil

Grading by Mail: When you submit answer sheets bymail, send all of your assignments at one time. Do NOTsubmit individual answer sheets for grading. Mail all ofyour assignments in an envelope, which you eitherprovide yourself or obtain from your nearestEducational Services Officer (ESO). Submit answersheets to:

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Answer Sheets: All courses include one "scannable"answer sheet for each assignment. These answer sheetsare preprinted with your SSN, name, assignmentnumber, and course number. Explanations forcompleting the answer sheets are on the answer sheet.

Do not use answer sheet reproductions: Use only theoriginal answer sheets that we provide—reproductionswill not work with our scanning equipment and cannotbe processed.

Follow the instructions for marking your answers onthe answer sheet. Be sure that blocks 1, 2, and 3 arefilled in correctly. This information is necessary foryour course to be properly processed and for you toreceive credit for your work.

COMPLETION TIME

Courses must be completed within 12 months from thedate of enrollment. This includes time required toresubmit failed assignments.

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PASS/FAIL ASSIGNMENT PROCEDURES

If your overall course score is 3.2 or higher, you willpass the course and will not be required to resubmitassignments. Once your assignments have been gradedyou will receive course completion confirmation.

If you receive less than a 3.2 on any assignment andyour overall course score is below 3.2, you will begiven the opportunity to resubmit failed assignments.You may resubmit failed assignments only once.Internet students will receive notification when theyhave failed an assignment—they may then resubmitfailed assignments on the web site. Internet studentsmay view and print results for failed assignments fromthe web site. Students who submit by mail will receivea failing result letter and a new answer sheet forresubmission of each failed assignment.

COMPLETION CONFIRMATION

After successfully completing this course, you willreceive a letter of completion.

ERRATA

Errata are used to correct minor errors or deleteobsolete information in a course. Errata may also beused to provide instructions to the student. If a coursehas an errata, it will be included as the first page(s) afterthe front cover. Errata for all courses can be accessedand viewed/downloaded at:

http://www.cnet.navy.mil/netpdtc/nac/neas.htm

STUDENT FEEDBACK QUESTIONS

We value your suggestions, questions, and criticismson our courses. If you would like to communicate withus regarding this course, we encourage you, if possible,to use e-mail. If you write or fax, please use a copy ofthe Student Comment form that follows this page.

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NAVAL RESERVE RETIREMENT CREDIT

If you are a member of the Naval Reserve, you willreceive retirement points if you are authorized toreceive them under current directives governingretirement of Naval Reserve personnel. For NavalReserve retirement, this course is evaluated at 9points. (Refer to Administrative Procedures for NavalReservists on Inactive Duty, BUPERSINST 1001.39,for more information about retirement points.)

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Student Comments

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Course Title: Aerographer's Mate, Module 5—Basic Meteorology

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CHAPTER 1

FUNDAMENTALS OF METEOROLOGY

Meteorology is the study of atmosphericphenomena. This study consists of physics, chemistry,and dynamics of the atmosphere. It also includes manyof the direct effects the atmosphere has upon Earth’ssurface, the oceans, and life in general. In this manualwe will study the overall fundamentals of meteorology,a thorough description of atmospheric physics andcirculation, air masses, fronts, and meteorologicalelements. This information supplies the necessarybackground for you to understand chart analysis,tropical analysis, satellite analysis, and chartinterpretation.

SYSTEM OF MEASUREMENT

LEARNING OBJECTIVE: Recognize theunits of measure used in the Metric System andthe English System and how these systems ofmeasurement are used in Meteorology.

To work in the field of meteorology, you must havea basic understanding of the science of measurement(metrology). When you can measure what you aretalking about and express it in numerical values, youthen have knowledge of your subject. To measure howfar something is moved, or how heavy it is, or how fastit travels; you may use a specific measurement system.There are many such systems throughout the worldtoday. The Metric System (CGS,centimeter-gram-second) has been recognized for usein science and research. Therefore, that system isdiscussed in the paragraphs that follow, with briefpoints of comparison to the English System (FPS,foot-pound-second). The metric units measure length,weight, and time, respectively. The derivation of thoseunits is described briefly.

LENGTH

To familiarize you with the conventional units ofmetric length, start with the meter. The meter is slightlylarger than the English yard (39.36 inches vs. 36inches). Prefixes are used in conjunction with the meterto denote smaller or larger units of the meter. Eachlarger unit is ten times larger than the next smaller unit.(See table 1-1.).

Table 1-1.—Common Prefixes in the Metric System

Prefix1 SymbolDecimal

ValueScientific Notation

Kilo K 1000 103

Hecto H 100 102

Deka D 10 101

Deci d .1 10-1

Centi c .01 10-2

Milli m .001 10-3

1These prefixes are used with all metric units such asmeters, grams, liters, and seconds (eg., kilometers,hectometers, centiliters, milliseconds).

Since the C in CGS represents centimeters (cm)you should see from table 1-1 that the centimeter isone-hundredth of a meter, .O1M, or 10-2 M.Conversely, 1 M equals 100 cm. To describe a gram,the G in the CGS system, you must first have afamiliarization with area and volume.

AREA AND VOLUME

A square has four equal sides and it is a one-planefigure—like a sheet of paper. To determine how muchsurface area is enclosed within the square you multiplythe length of one side by the length of the other equalside. If the sides were 1 centimeter (cm) in length thearea of the square would be 1 cm × 1 cm = 1 square cm,or 1 cm2. If squares having an area of 1 cm2 werestacked on top of each other until the stack was 1 cmtall, you would end up with a cube whose sides wereeach 1-cm in length. To determine the volume of thecube you simply multiply the length by the width andheight. Because each side is 1 cm you end up with avolume of 1 cubic centimeter (cm3) (1 cm × 1 cm × 1cm = 1 cm3). More simply stated, multiply the area ofone side of the cube by the height of the cube. Once youunderstand how the volume of a cube is determined,you are now ready to review the G in the CGS system.

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WEIGHT

The conventional unit of weight in the metricsystem is the gram (gm). You could use table 1-1 andsubstitute the word gram for meter and the symbol (gm)for the symbol (M). You would then have a table formetric weight. The gram is the weight of 1 cm3 of purewater at 4°C. At this point it may be useful to comparethe weight of an object to its mass. The weight of the 1cm3 of water is 1 gin. Weight and mass are proportionalto each other. However, the weight of the 1 cm3 of waterchanges as you move away from the gravitationalcenter of Earth. In space the 1 cm3 of water isweightless, but it is still a mass. Mass is expressed as afunction of inertia/acceleration, while weight is afunction of gravitational force. When we express themovement of an object we use the terms mass andacceleration.

TIME

Time is measured in hours, minutes, and seconds inboth systems. Hence, the second need not be explainedin the CGS system. With knowledge of how the CGSsystem can be used to express physical entities, younow have all the background to express such things asdensity and force.

DENSITY

With the previous explanation of grams andcentimeters, you should be able to understand howphysical factors can be measured and described. Forexample, density is the weight something has per unitof volume. The density of water is given as 1 gram percubic centimeter or 1 gm/cm. By comparison, thedensity of water in the English system is 62.4 poundsper cubic foot or 62.4 lb/ft3.

FORCE

Force is measured in dynes. A dyne is the force thatmoves a mass of 1 gram, 1 centimeter per squaresecond. This is commonly written as gin cm per sec2,gin cm/sec/sec or gm/cm/sec2. The force necessary fora gram to be accelerated at 980.665 cm/sec2 at 45°latitude is 980.665 dynes. For more detailed conversionfactors commonly used in meteorology andoceanography, refer to Smithsonian MeteorologyTables.

REVIEW QUESTIONS

Q1-1. What units does the metric (CGS) systemmeasure?

Q1-2. What is the difference between weight andmass?

Q1-3. What does a dyne measure?

EARTH-SUN RELATIONSHIP

LEARNING OBJECTIVE: Describe howradiation and insolation are affected by theEarth-Sun relationship.

The Sun is a great thermonuclear reactor about 93million miles from Earth. It is the original source ofenergy for the atmosphere and life itself. The Sun’senergy is efficiently stored on Earth in such things asoil, coal, and wood. Each of these was produced bysome biological means when the Sun acted upon livingorganisms. Our existence depends on the Sun becausewithout the Sun there would be no warmth on Earth, noplants to feed animal life, and no animal life to feedman.

The Sun is important in meteorology because allnatural phenomena can be traced, directly or indirectly,to the energy received from the Sun. Although the Sunradiates its energy in all directions, only a small portionreaches our atmosphere. This relatively small portion ofthe Sun’s total energy represents a large portion of theheat energy for our Earth. It is of such importance inmeteorology that every Aerographer’s Mate shouldhave at least a basic knowledge about the Sun and theeffects it has on Earth’s weather.

SUN

The Sun may be regarded as the only source of heatenergy that is supplied to earth’s surface and theatmosphere. All weather and motions in the atmosphereare due to the energy radiated from the Sun.

The Sun’s core has a temperature of 15,000,000°Kand a surface temperature of about 6,000°K (10,300°F).The Sun radiates electromagnetic energy in alldirections. However, Earth intercepts only a smallfraction of this energy. Most of the electromagneticenergy radiated by the Sun is in the form of light waves.Only a tiny fraction is in the form of heat waves. Evenso, better than 99.9 percent of Earth’s heat is derivedfrom the Sun in the form of radiant energy.

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Solar Composition

The Sun may be described as a globe of gas heatedto incandescence by thermonuclear reactions fromwithin the central core.

The main body of the Sun, although composed ofgases, is opaque and has several distinct layers. (See fig.1-1.) The first of these layers beyond the radiative zoneis the convective zone. This zone extends very nearly tothe Sun’s surface. Here, heated gases are raisedbuoyantly upwards with some cooling occurring andsubsequent convective action similar to that, whichoccurs within Earth’s atmosphere. The next layer is awell-defined visible surface layer referred to as thephotosphere. The bottom of the photosphere is the solarsurface. In this layer the temperature has cooled to asurface temperature of 6,000°K at the bottom to4,300°K at the top of the layer. All the light and heat of

the Sun is radiated from the photosphere. Above thephotosphere is a more transparent gaseous layerreferred to as the chromosphere with a thickness ofabout 1,800 miles (3,000 km). It is hotter than thephotosphere. Above the chromosphere is the corona, alow-density high temperature region. It is extended farout into interplanetary space by the solar wind—asteady outward streaming of the coronal material.Much of the electromagnetic radiation emissionsconsisting of gamma rays through x-rays, ultraviolet,visible and radio waves, originate in the corona.

Within the solar atmosphere we see the occurrenceof transient phenomena (referred to as solar activity),just as cyclones, frontal systems, and thunderstormsoccur within the atmosphere of Earth. This solaractivity may consist of the phenomena discussed in thefollowing paragraphs that collectively describe thefeatures of the solar disk (the visual image of the outer

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CORONA

CHRO

MO

SPHER

E

PHOTOSPHERE

RADIATIVE ZONE

SOLARATMOSPHERE

SEVERALSOLARDIAMETERSIN DEPTH 3,000Km

SOLAR SURFACETEMPERATUREAPPROX. 6,000 K

O

CENTRAL CORE(THERMONUCLEARREACTIONS) APPROX.15,000,000 KO

CO

NVEC

TIVE

ZONE

AGf0101

Figure 1-1.—One-quarter cross-section depicting the solar structure.

surface of the sun as observed from outside regions).(See fig. 1-2).

Solar Prominences/Filaments

Solar prominences/filaments are injections of gasesfrom the chromosphere into the corona. They appear asgreat clouds of gas, sometimes resting on the Sun’ssurface and at other times floating free with no visibleconnection. When viewed against the solar disk, theyappear as long dark ribbons and are called filaments.When viewed against the solar limb (the dark outeredge of the solar disk), they appear bright and are calledprominences. (See fig. 1-2.) They display a variety ofshapes, sizes, and activity that defy general description.

They have a fibrous structure and appear to resist solargravity. They may extend 18,500 to 125,000 miles(30,000 to 200,000 km) above the chromosphere. Themore active types have temperatures of 10,000°K ormore and appear hotter than the surroundingatmosphere.

Sunspots

Sunspots are regions of strong localized magneticfields and indicate relatively cool areas in thephotosphere. They appear darker than theirsurroundings and may appear singly or in morecomplicated groups dominated by larger spots near thecenter. (See fig. 1-2).

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AGf0102

SOLAR PROMINENCES

PLAGE

FLARE

SUNSPOTS

(FILAMENTS)

Figure 1-2.—Features of the solar disk.

Sunspots begin as small dark areas known as pores.These pores develop into full-fledged spots in a fewdays, with maximum development occurring in about 1to 2 weeks. When sunspots decay the spot shrinks insize and its magnetic field also decreases in size. Thislife cycle may consist of a few days for small spots tonear 100 days for larger groups. The larger spotsnormally measure about 94,500 miles (120,000 kin)across. Sunspots appear to have cyclic variations inintensity, varying through a period of about 8 to 17years. Variation in number and size occurs throughoutthe sunspot cycle. As a cycle commences, a few spotsare observed at high latitudes of both solarhemispheres, and the spots increase in size and number.They gradually drift equatorward as the cycleprogresses, and the intensity of the spots reach amaximum in about 4 years. After this period, decay setsin and near the end of the cycle only a few spots are leftin the lower latitudes (5° to 10°).

Plages

Plages are large irregular bright patches thatsurround sunspot groups. (See fig. 1-2). They normallyappear in conjunction with solar prominences orfilaments and may be systematically arranged in radialor spiral patterns. Plages are features of the lowerchromosphere and often completely or partiallyobscure an underlying sunspot.

Flares

Solar flares are perhaps the most spectacular of theeruptive features associated with solar activity. (See fig.1-2). They look like flecks of light that suddenly appearnear activity centers and come on instantaneously asthough a switch were thrown. They rise sharply to peakbrightness in a few minutes, then decline moregradually. The number of flares may increase rapidlyover an area of activity. Small flare-like brighteningsare always in progress during the more active phase ofactivity centers. In some instances flares may take theform of prominences, violently ejecting material intothe solar atmosphere and breaking into smallerhigh-speed blobs or clots. Flare activity appears to varywidely between solar activity centers. The greatest flareproductivity seems to be during the week or 10 dayswhen sunspot activity is at its maximum.

Flares are classified according to size andbrightness. In general, the higher the importanceclassification, the stronger the geophysical effects.Some phenomena associated with solar flares haveimmediate effects; others have delayed effects (15minutes to 72 hours after flare).

Solar flare activity produces significant disruptionsand phenomena within Earth’s atmosphere. Duringsolar flare activity, solar particle streams (solar winds)are emitted and often intercept Earth. These solarparticles are composed of electromagnetic radiation,which interacts with Earth’s ionosphere. This results inseveral reactions such as: increased ionization(electrically charging neutral particles), photo chemicalchanges (absorption of radiation), atmospheric heating,electrically charged particle motions, and an influx ofradiation in a variety of wavelengths and frequencieswhich include radio and radar frequencies.

Some of the resulting phenomena include thedisruption of radio communications and radardetection. This is due to ionization, incoming radiowaves, and the motion of charged particles. Satelliteorbits can be affected by the atmospheric heating andsatellite transmissions may be affected by all of thereactions previously mentioned. Geomagneticdisturbances like the aurora borealis and auroraAustralia result primarily from the motion ofelectrically charged particles within the ionosphere.

EARTH

Of the nine planets in our solar system, Earth is thethird nearest to (or from) the Sun. Earth varies indistance from the Sun during the year. The Sun is 94million miles (150,400,000 km) in summer and 91million miles (145,600,000 km) in winter.

Motions

Earth is subject to four motions in its movementthrough space: rotation about its axis, revolution aroundthe Sun, processional motion (a slow conical movementor wobble) of the axis, and the solar motion (themovement of the whole solar system with space). Ofthe four motions affecting Earth, only two are of anyimportance to meteorology.

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The first motion is rotation. Earth rotates on its axisonce every 24 hours. One-half of the Earth’s surface istherefore facing the Sun at all times. Rotation aboutEarth’s axis takes place in an eastward direction. Thus,the Sun appears to rise in the east and set in the west.(See fig. 1-3.)

The second motion of Earth is its revolution aroundthe Sun. The revolution around the Sun and the tilt ofEarth on its axis are responsible for our seasons. Earthmakes one complete revolution around the Sun inapproximately 365 1/4 days. Earth’s axis is at an angleof 23 1/2° to its plane of rotation and points in a nearlyfixed direction in space toward the North Star (Polaris).

Solstices and Equinoxes

When Earth is in its summer solstice, as shown forJune in figure 1-4, the Northern Hemisphere is inclined23 1/2° toward the Sun. This inclination results in moreof the Sun’s rays reaching the Northern Hemispherethan the Southern Hemisphere. On or about June 21,

direct sunlight covers the area from the North Poledown to latitude 66 1/2°N (the Arctic Circle). The areabetween the Arctic Circle and the North Pole isreceiving the Sun’s rays for 24 hours each day. Duringthis time the most perpendicular rays of the Sun arereceived at 23 l/2°N latitude (the Tropic Of Cancer).Because the Southern Hemisphere is tilted away fromthe Sun at this time, the indirect rays of the Sun reachonly to 66 1/2°S latitude (the Antarctic Circle).Therefore, the area between the Antarctic Circle andthe South Pole is in complete darkness. Note carefullythe shaded and the not shaded area of Earth in figure 1-4for all four positions.

At the time of the equinox in March and again inSeptember, the tilt of Earth’s axis is neither toward noraway from the Sun. For these reasons Earth receives anequal amount of the Sun’s energy in both the NorthernHemisphere and the Southern Hemisphere. During thistime the Sun’s rays shine most perpendicularly at theequator.

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Agf0103

MIDNIGHTSUNSET

SUNRISENOON

Figure 1-3.—Rotation of the Earth about its axis (during equinoxes).

In December, the situation is exactly reversed fromthat in June. The Southern Hemisphere now receivesmore of the Sun’s direct rays. The most perpendicularrays of the Sun are received at 23 1/2°S latitude (theTropic Of Capricorn). The southern polar region is nowcompletely in sunshine and the northern polar region iscompletely in darkness.

Since the revolution of Earth around the Sun is agradual process, the changes in the area receiving theSun’s rays and the changes in seasons are gradual.However, it is customary and convenient to mark thesechanges by specific dates and to identify them byspecific names. These dates are as follows:

1. March 21. The vernal equinox, when Earth’saxis is perpendicular to the Sun’s rays. Spring begins inthe Northern Hemisphere and fall begins in theSouthern Hemisphere.

2. June 21. The summer solstice, when Earth’saxis is inclined 23 1/2° toward the Sun and the Sun hasreached its northernmost zenith at the Tropic of Cancer.Summer officially commences in the NorthernHemisphere; winter begins in the SouthernHemisphere.

3. September 22. The autumnal equinox, whenEarth’s axis is again perpendicular to the Sun’s rays.This date marks the beginning of fall in the NorthernHemisphere and spring in the Southern Hemisphere. Itis also the date, along with March 21, when the Sunreaches its highest position (zenith) directly over theequator.

4. December 22. The winter solstice, when theSun has reached its southernmost zenith position at theTropic of Capricorn. It marks the beginning of winter inthe Northern Hemisphere and the beginning of summerin the Southern Hemisphere.

In some years, the actual dates of the solstices andthe equinoxes vary by a day from the dates given here.This is because the period of revolution is 365 1/4 daysand the calendar year is 365 days except for leap yearwhen it is 366 days.

Because of its 23 1/2° tilt and its revolution aroundthe Sun, five natural light (or heat) zones according tothe zone's relative position to the Sun's rays mark Earth.Since the Sun is ALWAYS at its zenith between theTropic of Cancer and the Tropic of Capricorn, this is thehottest zone. It is called the Equatorial Zone, the TorridZone, the Tropical Zone, or simply the Tropics.

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AGF0104

MARCH 21

SEPTEMBER22

JUNE 21

SUN

N/P

S/P

N/P

S/P

N/P

S/P

DECEMBER22

N/P

S/P

Figure 1-4.—Revolution of Earth around the sun.

The zones between the Tropic of Cancer and theArctic Circle and between the Tropic of Capricorn andthe Antarctic Circle are the Temperate Zones. Thesezones receive sunshine all year, but less of it in theirrespective winters and more of it in their respectivesummers.

The zones between the Arctic Circle and the NorthPole and between the Antarctic Circle and the SouthPole receive the Sun’s rays only for parts of the year.(Directly at the poles there are 6 months of darknessand 6 months of sunshine.) This, naturally, makes themthe coldest zones. They are therefore known as theFrigid or Polar Zones.

RADIATION

The term "radiation" refers to the process by whichelectromagnetic energy is propagated through space.Radiation moves at the speed of light, which is 186,000miles per second (297,600 km per second) and travelsin straight lines in a vacuum. All of the heat received byEarth is through this process. It is the most importantmeans of heat transfer.

Solar radiation is defined as the totalelectromagnetic energy emitted by the Sun. The Sun’s

surface emits gamma rays, x-rays, ultraviolet, visiblelight, infrared, heat, and electromagnetic waves.Although the Sun radiates in all wavelengths, abouthalf of the radiation is visible light with most of theremainder being infrared. (See figure 1-5.)

Energy radiates from a body by wavelengths,which vary inversely with the temperature of that body.Therefore, the Sun, with an extremely hot surfacetemperature, emits short wave radiation. Earth has amuch cooler temperature (15°C average) and thereforereradiates the Sun’s energy or heat with long waveradiation.

INSOLATION

Insolation (an acronym for INcoming SOLarradiATION) is the rate at which solar radiation isreceived by a unit horizontal surface at any point on orabove the surface of Earth. In this manual, insolation isused when speaking about incoming solar radiation.

There are a wide variety of differences in theamounts of radiation received over the various portionsof Earth’s surface. These differences in heating areimportant and must be measured or otherwisecalculated to determine their effect on the weather.

1-8

SCHEMATIC DIAGRAM 0F THE DISTRIBUTION OF ENERGY IN THE SOLAR SPECTRUM.(NOT TO SCALE). THE NUMBERS ARE PERCENTAGES OF THE SOLAR CONSTANT . THEFIGURE FOR THE RADIO ENERGY IS FOR THE OBSERVED BAND FROM 15 TO 30,000 MHZ.

EXTREME U.V.AND RAYS

NEARU.V.

VISIBLE INFRARED RADIO

41

527

10 -3

10 10 10 10 10 10 10 10 10 10 10-6 -4 -2 2 4 6 8 10 12 14 16

10 -10

WAVELENGTHS IN MILLIMICRONS

VISIBLE SPECTRUM

AGf0105

COSMICRAYS

GAMMARAYS

X-RAYS ULTRA-VIOLETRAYS

INFRA-REDRAYS

HERTZIANWAVES

RADIOWAVES

LONGELECTRICAL

OSCILLATIONS

400mu 700mu

1

Figure 1-5.—Electromagnetic spectrum.

The insolation received at the surface of Earthdepends upon the solar constant (the rate at which solarradiation is received outside Earth’s atmosphere), thedistance from the Sun, inclination of the Sun’s rays, andthe amount of insolation depleted while passingthrough the atmosphere. The last two are the importantvariable factors.

Depletion of Solar Radiation

If the Sun’s radiation was not filtered or depleted insome manner, our planet would soon be too hot for lifeto exist. We must now consider how the Sun’s heatenergy is both dispersed and depleted. This isaccomplished through dispersion, scattering,reflection, and absorption.

DISPERSION.—Earlier it was learned thatEarth’s axis is inclined at an angle of 23 1/2°. Thisinclination causes the Sun’s rays to be received on thesurface of Earth at varying angles of incidence,depending on the position of Earth. When the Sun’srays are not perpendicular to the surface of Earth, theenergy becomes dispersed or spread out over a greaterarea (figure.1-6). If the available energy reaching theatmosphere is constant and is dispersed over a greaterarea, the amount of energy at any given point within thearea decreases, and therefore the temperature is lower.Dispersion of insolation in the atmosphere is caused bythe rotation of Earth. Dispersion of insolation also takesplace with the seasons in all latitudes, but especially inthe latitudes of the polar areas.

SCATTERING.—About 25 percent of theincoming solar radiation is scattered or diffused by theatmosphere. Scattering is a phenomenon that occurswhen solar radiation passes through the air and some ofthe wavelengths are deflected in all directions bymolecules of gases, suspended particles, and water

vapor. These suspended particles then act like a prismand produce a variety of colors. Various wavelengthsand particle sizes result in complex scattering affectsthat produce the blue sky. Scattering is also responsiblefor the red Sun at sunset, varying cloud colors at sunriseand sunset, and a variety of optical phenomena.

Scattering always occurs in the atmosphere, butdoes not always produce dramatic settings. Undercertain radiation wavelength and particle sizeconditions all that can be seen are white clouds and awhitish haze. This occurs when there is a high moisturecontent (large particle size) in the air and is calleddiffuse reflection. About two-thirds of the normallyscattered radiation reaches earth as diffuse skyradiation. Diffuse sky radiation may account for almost100 percent of the radiation received by polar stationsduring winter.

REFLECTION.—Reflection is the processwhereby a surface turns a portion of the incident backinto the medium through which the radiation came.

A substance reflects some insolation. This meansthat the electromagnetic waves simply bounce backinto space. Earth reflects an average of 36 percent of theinsolation. The percent of reflectivity of allwavelengths on a surface is known as its albedo. Earth’saverage albedo is from 36 to 43 percent. That is, Earthreflects 36 to 43 percent of insolation back into space.In calculating the albedo of Earth, the assumption ismade that the average cloudiness over Earth is 52percent. All surfaces do not have the same degree ofreflectivity; consequently, they do not have the samealbedo. Some examples are as follows:

1. Upper surfaces of clouds reflect from 40 to 80percent, with an average of about 55 percent.

2. Snow surfaces reflect over 80 percent ofincoming sunlight for cold, fresh snow and as lowas 50 percent for old, dirty snow.

3. Land surfaces reflect from 5 percent ofincoming sunlight for dark forests to 30 percent fordry land.

4. Water surfaces (smooth) reflect from 2 percent,when the Sun is directly overhead, to 100 percentwhen, the Sun is very low on the horizon. Thisincrease is not linear. When the Sun is more than25°above the horizon, the albedo is less than 10percent. In general, the albedo of water is quite low.

When Earth as a whole is considered, clouds aremost important in determining albedo.

1-9

SUN

OBLIQUE RAYSDISPERSED OVERA LARGER AREATHANPERPENDICULARRAYS

S

N

EQUATOR

AGf0106

Figure 1-6.—Dispersion of insolation.

ABSORPTION.—Earth and its atmosphereabsorb about 64 percent of the insolation. Land andwater surfaces of Earth absorb 51 percent of thisinsolation. Ozone, carbon dioxide, and water vapordirectly absorb the remaining 13 percent. These gasesabsorb the insolation at certain wavelengths. Forexample, ozone absorbs only a small percentage of theinsolation. The portion or type the ozone does absorb iscritical since it reduces ultraviolet radiation to a levelwhere animal life can safely exist. The most importantabsorption occurs with carbon dioxide and water vapor,which absorb strongly over a broader wavelength band.Clouds are by far the most important absorbers ofradiation at essentially all wavelengths. In sunlightclouds reflect a high percentage of the incident solarradiation and account for most of the brightness ofEarth as seen from space.

There are regions, such as areas of clear skies,where carbon dioxide and water vapor are at aminimum and so is absorption. These areas are calledatmospheric windows and allow insolation to passthrough the atmosphere relatively unimpeded.

Greenhouse Effect

The atmosphere conserves the heat energy of Earthbecause it absorbs radiation selectively. Most of thesolar radiation in clear skies is transmitted to Earth’ssurface, but a large part of the outgoing terrestrialradiation is absorbed and reradiated back to the surface.This is called the greenhouse effect. A greenhousepermits most of the short-wave solar radiation to passthrough the glass roof and sides, and to be absorbed bythe floor, ground or plants inside. These objectsreradiate energy at their temperatures of about 300°K,which is a higher temperature than the energy that wasinitially received. The glass absorbs the energy at thesewavelengths and sends part of it back into thegreenhouse, causing the inside of the structure tobecome warmer than the outside. The atmosphere actssimilarly, transmitting and absorbing in somewhat thesame way as the glass. If the greenhouse effect did notexist, Earth’s temperature would be 35°C cooler thanthe 15°C average temperature we now enjoy, becausethe insolation would be reradiated back to space.

Of course, the atmosphere is not a contained spacelike a greenhouse because there are heat transportmechanisms such as winds, vertical currents, andmixing with surrounding and adjacent cooler air.

RADIATION (HEAT) BALANCE IN THEATMOSPHERE

The Sun radiates energy to Earth, Earth radiatesenergy back to space, and the atmosphere radiatesenergy also. As is shown in figure 1-7, a balance ismaintained between incoming and outgoing radiation.This section of the lesson explains the various radiationprocesses involved in maintaining this critical balanceand the effects produced in the atmosphere.

We have learned that an object reradiates energy ata higher temperature. Therefore, the more the Sun heatsEarth, the greater the amount of heat energy Earthreradiates. If this rate of heat loss/gain did not balance,Earth would become continuously colder or warmer.

Terrestrial (Earth) Radiation

Radiation emitted by Earth is almost entirelylong-wave radiation. Most of the terrestrial radiation isabsorbed by the water vapor in the atmosphere andsome by other gases (about 8 percent is radiateddirectly to outer space). This radiant energy isreradiated in the atmosphere horizontally andvertically. Horizontal flux (flow or transport) of energyneed not be considered due to a lack of horizontaltemperature differences. The vertical, upward ordownward, flux is of extreme significance.

Convection and turbulence carry aloft some of thisradiation. Water vapor, undergoing thecondensation-precipitation-evaporation cycle(hydrological cycle), carries the remainder into theatmosphere.

Atmospheric Radiation

The atmosphere reradiates to outer space most ofthe terrestrial radiation (about 43 percent) andinsolation (about 13 percent) that it has absorbed. Someof this reradiation is emitted earthward and is known ascounterradiation. This radiation is of great importancein the greenhouse effect.

Heat Balance and Transfer in the Atmosphere

Earth does not receive equal radiation at all pointsas was shown in figure 1-4. The east-west rotation ofEarth provides equal exposure to sunlight but latitudeand dispersion do affect the amount of incidentradiation received. The poles receive far less incidentradiation than the equator. This uneven heating is calleddifferential insolation.

1-10

Due to this differential insolation the tropicalatmosphere is constantly being supplied heat and thetemperature of the air is thus higher than in areaspoleward. Because of the expansion of warm air, thiscolumn of air is much thicker and lighter than over thepoles. At the poles Earth receives little insolation andthe column or air is less thick and heavier. Thisdifferential in insolation sets up a circulation thattransports warm air from the Tropics poleward aloft andcold air from the poles equatorward on the surface. (Seefig. 1-8.) Modifications to this general circulation arediscussed in detail later in this training manual.

This is the account of the total radiation. Some ofthe radiation makes several trips, being absorbed,reflected, or reradiated by Earth or the atmosphere.Insolation comes into the atmosphere and all of it isreradiated. How many trips it makes while in ouratmosphere does not matter. The direct absorption ofradiation by Earth and the atmosphere and thereradiation into space balance. If the balance did notexist, Earth and its atmosphere, over a period of time,would steadily gain or lose heat.

Although radiation is considered the mostimportant means of heat transfer, it is not the onlymethod. There are others such as conduction,convection, and advection that also play an importantpart in meteorological processes.

1-11

OUTER SPACE

UPPER ATMOSPHERE

AGf0107

NOTE:36% OF INCOMING INSULATIONINITIALLY REFLECTED

RADIATED DIRECTLYTO OUTER SPACE8%

RERADIATEDBY ATMOSPHERETO OUTER SPACE56%

INCOMING

SHORT WAVE

RADIATION

REFLECTED BY

EARTH, CLOUDS

AND ATMOSPHERE

TO OUTER SPACE

36%

51%ABSORBEDBY EARTH

13%ABSORBED BYATMOSPHERE

OUTGOINGLONG WAVERADIATION

HYDROLOGICCYCLE

Figure 1-7.—Radiation balance in the atmosphere.

NORTHPOLE

SOUTHPOLE

EQUATOR AREA OFGREATEST

INSOLATION

AREA OF LEASTINSOLATION

AREA OF LEASTINSOLATION

AGf0108

Figure 1-8.—Beginning of a circulation.

REVIEW QUESTIONS

Q1-4. What are sunspots?

Q1-5. In the Southern Hemisphere, approximatelywhat date will the greatest amount ofincoming solar radiation be received?

Q1-6. What percent of the earth's insolation do landand water absorb?

Q1-7.What is the effect on a polar air column inrelation to a column of air over the equator?

PRESSURE

LEARNING OBJECTIVE: Describe howpressure is measured and determine how theatmosphere is affected by pressure.

DEFINITION AND FORMULA

Pressure is the force per unit area. Atmosphericpressure is the force per unit area exerted by theatmosphere in any part of the atmospheric envelope.Therefore, the greater the force exerted by the air forany given area, the greater the pressure. Although thepressure varies on a horizontal plane from day to day,the greatest pressure variations are with changes inaltitude. Nevertheless, horizontal variations of pressureare ultimately important in meteorology because thevariations affect weather conditions.

Pressure is one of the most important parameters inmeteorology. Knowledge of the distribution of air andthe resultant variations in air pressure over the earth isvital in understanding Earth’s fascinating weatherpatterns.

Pressure is force, and force is related toacceleration and mass by Newton’s second law. Thislaw states that acceleration of a body is directlyproportional to the force exerted on the body andinversely proportional to the mass of that body. It maybe expressed as

aF

mor F ma= =

“A” is the acceleration, “F” is the force exerted, and"in" is the mass of the body. This is probably the mostimportant equation in the mechanics of physics dealingwith force and motion.

NOTE: Be sure to use units of mass and not units ofweight when applying this equation.

STANDARDS OF MEASUREMENT

Atmospheric pressure is normally measured inmeteorology by the use of a mercurial or aneroidbarometer. Pressure is measured in many differentunits. One atmosphere of pressure is 29.92 inches ofmercury or 1,013.25 millibars. These measurementsare made under established standard conditions.

STANDARD ATMOSPHERE

The establishment of a standard atmosphere wasnecessary to give scientists a yardstick to measure orcompare actual pressure with a known standard. In theInternational Civil Aeronautical Organization (ICAO),the standard atmosphere assumes a mean sea leveltemperature of 59°F or 15°C and a standard sea levelpressure of 1,013.25 millibars or 29.92 inches ofmercury. It also has a temperature lapse rate (decrease)of 3.6°F per 1000 feet or 0.65°C per 100 meters up to 11kilometers and a tropopause and stratospheretemperature of -56.5°C or -69.7°F.

VERTICAL DISTRIBUTION

Pressure at any point in a column of water, mercury,or any fluid, depends upon the weight of the columnabove that point. Air pressure at any given altitudewithin the atmosphere is determined by the weight ofthe atmosphere pressing down from above. Therefore,the pressure decreases with altitude because the weightof the atmosphere decreases.

It has been found that the pressure decreases by halffor each 18,000-foot (5,400-meter) increase in altitude.Thus, at 5,400 meters one can expect an averagepressure of about 500 millibars and at 36,000 feet(10,800 meters) a pressure of only 250 millibars, etc.Therefore, it may be concluded that atmosphericpressures are greatest at lower elevations because thetotal weight of the atmosphere is greatest at thesepoints.

There is a change of pressure whenever either themass of the atmosphere or the accelerations of themolecules within the atmosphere are changed.Although altitude exerts the dominant control,temperature and moisture alter pressure at any givenaltitude—especially near Earth’s surface where heatand humidity, are most abundant. The pressurevariations produced by heat and humidity with heatbeing the dominant force are responsible for Earth’swinds through the flow of atmospheric mass from anarea of higher pressure to an area of lower pressure.

1-12

PASCAL’S LAW

Pascal's Law is an important law in atmosphericphysics. The law states that fluids (including gases suchas Earth’s atmosphere) transmit pressure in alldirections. Therefore, the pressure of the atmosphere isexerted not only downward on the surface of an object,but also in all directions against a surface that isexposed to the atmosphere.

REVIEW QUESTIONS

Q1-8. What is the definition of pressure?

Q1-9. With a sea level pressure reading of 1000 mb,what would be the approximate pressure at18,000 feet?

Q1-10. What environmental changes have the biggesteffect on pressure changes?

TEMPERATURE

LEARNING OBJECTIVE: Describe howtemperature is measured and determine howthe atmosphere is affected by temperature.

DEFINITION

Temperature is the measure of molecular motion.Its intensity is determined from absolute zero (Kelvinscale), the point which all molecular motion stops.Temperature is the degree of hotness or coldness, or itmay be considered as a measure of heat intensity.

TEMPERATURE SCALES

Long ago it was recognized that uniformity in themeasurement of temperature was essential. It would beunwise to rely on such subjective judgments oftemperature as cool, cooler, and coolest; therefore,arbitrary scales were devised. Some of them aredescribed in this section. They are Fahrenheit, Celsius,and absolute (Kelvin) scales. These are the scales usedby the meteorological services of all the countries in theworld. Table 1-2 shows a temperature conversion scalefor Celsius, Fahrenheit, and Kelvin.

Fahrenheit Scale

Gabriel Daniel Fahrenheit invented the Fahrenheitscale about 1710. He was the first to use mercury in athermometer. The Fahrenheit scale has 180 divisions or

degrees between the freezing (32°F) and boiling(212°F) points of water.

Celsius Scale

Anders Celsius devised the Celsius scale during the18th century. This scale has reference points withrespect to water of 0°C for freezing and 100°C forboiling. It should be noted that many publications stillrefer to the centigrade temperature scale. Centigradesimply means graduated in 100 increments, and hasrecently and officially adopted the name of itsdiscoverer, Celsius.

Absolute Scale (Kelvin)

Another scale in wide use by scientists in manyfields is the absolute scale or Kelvin scale, developedby Lord Kelvin of England. On this scale the freezingpoint of water is 273°K and the boiling point of water is373°K. The absolute zero value is considered to be apoint at which theoretically no molecular activityexists. This places the absolute zero at a minus 2730 onthe Celsius scale, since the degree divisions are equal insize on both scales. The absolute zero value on theFahrenheit scale falls at minus 459.6°F.

Scale Conversions

Two scales, Fahrenheit and Celsius, are commonlyused. With the Celsius and Fahrenheit scales, it is oftennecessary to change the temperature value of one scaleto that of the other. Generally a temperature conversiontable, like table 1-2, is used or a temperature computer.If these are not available, you must then use one of thefollowing mathematical methods to convert one scale toanother.

Mathematical Methods

It is important to note that there are 100 divisionsbetween the freezing and boiling points of water on theCelsius scale. There are 180 divisions between thesame references on the Fahrenheit scale. Therefore, onedegree on the Celsius scale equals nine-fifths degree onthe Fahrenheit scale. In converting Fahrenheit values toCelsius values the formula is:

C (F 32 )5

9= − °

In converting Celsius values to Fahrenheit values theformula is:

1-13

F9

5C 32= + °

One way to remember when to use 9/5 and when touse 5/9 is to keep in mind that the Fahrenheit scale hasmore divisions than the Celsius scale. In going fromCelsius to Fahrenheit, multiply by the ratio that is

larger; in going from Fahrenheit to Celsius, use thesmaller ratio.

Another method of converting temperatures fromone scale to another is the decimal method. Thismethod uses the ratio 1°C equals 1.8°F. To findFahrenheit from Celsius, multiply the Celsius value by1.8 and add 32. To find Celsius from Fahrenheit,

1-14

1Fahrenheit temperatures are rounded to the nearest 0.5 degree. For a more exact conversion, utilize thepsychrometric computer or the mathematical method.

Table1-2

Table 1-2.—Temperature Conversion Scale

subtract 32 from the Fahrenheit and divide theremainder by 1.8.

Examples:

1. F = l.8C + 32

Given: 24°C. Find: °F24 × 1.8 = 43.243.2 + 32 = 75.2 or 75°F.

2. C =F 32

1.8

−

Given: 96°F. Find: °C.96 – 32 = 6464 + 1.8 = 35.5 or 36°C

To change a Celsius reading to an absolute value,add the Celsius reading to 273° algebraically. For

example, to find the absolute value of -35°C, you wouldadd minus 35° to 273°K algebraically. That is, you take273° and combine 35° so you use the minus (-) functionto arrive at 238°K.

To change a Fahrenheit reading to an absolutevalue, first convert the Fahrenheit reading to itsequivalent Celsius value. Add this value algebraicallyto 273°. Consequently, 50°F is equivalent to 2830absolute, arrived at by converting 50°F to 10°C andthen adding the Celsius value algebraically to 273°.

VERTICAL DISTRIBUTION

Earth’s atmosphere is divided into layers or zonesaccording to various distinguishing features. (See fig.1-9). The temperatures shown here are generally basedon the latest “U.S. Extension to the ICAO Standard

1-15

AURORA

TEMPERATURE(KINETIC)

PRESSURE(MB)

NOCTILUCENTCLOUDS

0 500 1000 1500 2000

600

500

400

300

200

100

700

2500

10

10

10

10

10

STANDARDTEMPERATURE

TROPOPAUSE

NACREOUSCLOUDS

MT EVEREST180 190 200 210 220 230 240 250 260 270 280 290 300 DEGREES K

DEGREES C

DEGREES F

100

200

300

500

700850

1000

50

50

25

10100

150

200

10-1

1

250

10-2

-3

300

GEOMETRICHEIGHT

F REGION

E REGION

D REGION

10

KILOMETERS

100

90

60

80

70

40

50

10

20

30

STRATOSPHERE

STRATOPAUSE

MESOPAUSE

THERMOSHPERE

MESOSPHERE

TROPOSPHERE

GEOMETRIC HEIGHT

KILOMETERS

(400) MILES

(300)

(200)

(100)

-4

-5

-6

-7

-8

(MILES)(60)

PRESSURE(MB)

TEMPERATURE

(10)

(20)

(30)

(40)

(50)

THOUSOF FEET

-90

-140 -120 -100

-80 -70

-80

-60 -50

-60

-40

-40

-30

-20

-20

0

-10

20

0

32 40

10

60

20

80

30

OZONOSPHERE

E REGION

D REGION

DEG. K

IONOSPH

ERE

F REGION2

F REGION1

AGf0109

Figure 1-9.—Earth's Atmosphere.

Atmosphere” and are representative of mid-latitudeconditions. The extension shown in the insert isspeculative. These divisions are for reference ofthermal structure (lapse rates) or other significantfeatures and are not intended to imply that these layersor zones are independent domains. Earth is surroundedby one atmosphere, not by a number ofsub-atmospheres.

The layers and zones are discussed under twoseparate classifications. One is theMETEOROLOGICAL classification that defines zonesaccording to their significance for the weather. Theother is the ELECTRICAL classification that defineszones according to electrical characteristics of gases ofthe atmosphere.

Meteorological Classification

In the meteorological classification (commencingwith Earth’s surface and proceeding upward) we havethe troposphere, tropopause, stratosphere, stratopause,mesosphere, mesopause, thermosphere, and theexosphere. These classifications are based ontemperature characteristics. (See fig. 1-9 for someexamples.)

TROPOSPHERE.—The troposphere is the layerof air enveloping Earth immediately above Earth’ssurface. It is approximately 5 1/2 miles (29,000 ft or 9kin) thick over the poles, about 7 1/2 miles (40,000 ft or12.5 kin) thick in the mid-latitudes, and about 11 1/2miles (61,000 ft or 19 kin) thick over the Equator. Thefigures for thickness are average figures; they changesomewhat from day to day and from season to season.The troposphere is thicker in summer than in winter andis thicker during the day than during the night. Almostall weather occurs in the troposphere. However, somephenomena such as turbulence, cloudiness (caused byice crystals), and the occasional severe thunderstormtop occur within the tropopause or stratosphere.

The troposphere is composed of a mixture ofseveral different gases. By volume, the composition ofdry air in the troposphere is as follows: 78 percentnitrogen, 21 percent oxygen, nearly 1-percent argon,and about 0.03 percent carbon dioxide. In addition, itcontains minute traces of other gases, such as helium,hydrogen, neon, krypton, and others.

The air in the troposphere also contains a variableamount of water vapor. The maximum amount of watervapor that the air can hold depends on the temperatureof the air and the pressure. The higher the temperature,the more water vapor it can hold at a given pressure.

The air also contains variable amounts ofimpurities, such as dust, salt particles, soot, andchemicals. These impurities in the air are importantbecause of their effect on visibility and the part theyplay in the condensation of water vapor. If the air wereabsolutely pure, there would be little condensation.These minute particles act as nuclei for thecondensation of water vapor. Nuclei, which have anaffinity for water vapor, are called HYGROSCOPICNUCLEI.

The temperature in the troposphere usuallydecreases with height, but there may be inversions forrelatively thin layers at any level.

TROPOPAUSE.—The tropopause is a transitionlayer between the troposphere and the stratosphere. It isnot uniformly thick, and it is not continuous from theequator to the poles. In each hemisphere the existenceof three distinct tropopauses is generally agreedupon—one in the subtropical latitudes, one in middlelatitudes, and one in subpolar latitudes. They overlapeach other where they meet.

The tropopause is characterized by little or nochange in temperature with increasing altitude. Thecomposition of gases is about the same as that for thetroposphere. However, water vapor is found only invery minute quantities at the tropopause and above it.

STRATOSPHERE.—The stratosphere directlyoverlies the tropopause and extends to about 30 miles(160,000 ft or 48 kilometers). Temperature varies littlewith height in the stratosphere through the first 30,000feet (9,000 meters); however, in the upper portion thetemperature increases approximately linearly to valuesnearly equal to surface temperatures. This increase intemperature through this zone is attributed to thepresence of ozone that absorbs incoming ultravioletradiation.

STRATOPAUSE.—The stratopause is the top ofthe stratosphere. It is the zone marking another reversalwith increasing altitude (temperature begins todecrease with height).

MESOSPHERE.—The mesosphere is a layerapproximately 20 miles (100,000 ft or 32 kilometers)thick directly overlaying the stratopause. Thetemperature decreases with height.

MESOPAUSE.—The mesopause is the thinboundary zone between the mesosphere and thethermosphere. It is marked by a reversal oftemperatures; i.e., temperature again increases withaltitude.

1-16

THERMOSPHERE.—The thermosphere, asecond region in which the temperature increases withheight, extends from the mesopause to the exosphere.

EXOSPHERE.—The very outer limit of Earth’satmosphere is regarded as the exosphere. It is the zonein which gas atoms are so widely spaced they rarelycollide with one another and have individual orbitsaround Earth.

Electrical Classification

The primary concern with the electricalclassification is the effect on communications andradar. The electrical classification outlines threezones—the troposphere, the ozonosphere, and theionosphere.

TROPOSPHERE.—The troposphere is importantto electrical transmissions because of the immensechanges in the density of the atmosphere that occur inthis layer. These density changes, caused by differencesin heat and moisture, affect the electronic emissionsthat travel through or in the troposphere. Electricalwaves can be bent or refracted when they pass throughthese different layers and the range and area ofcommunications may be seriously affected.

OZONOSPHERE.—This layer is nearlycoincident with the stratosphere. As was discussedearlier in this section, the ozone is found in this zone.Ozone is responsible for the increase in temperaturewith height in the stratosphere.

IONOSPHERE.—The ionosphere extends fromabout 40 miles (200,000 ft or 64 kilometers) to anindefinite height. Ionization of air molecules in thiszone provides conditions that are favorable for radiopropagation. This is because radio waves are sentoutward to the ionosphere and the ionized particlesreflect the radio waves back to Earth.

HEAT TRANSFER

The atmosphere is constantly gaining and losingheat. Wind movements are constantly transporting heatfrom one part of the world to another. It is due to theinequalities in gain and loss of heat that the air is almostconstantly in motion. Wind and weather directlyexpress the motions and heat transformations.

Methods

In meteorology, one is concerned with fourmethods of heat transfer. These methods are

conduction, convection, advection, and radiation. Heatis transferred from Earth directly the atmosphere byradiation, conduction, and advection. Heat istransferred within the atmosphere by radiation,conduction, and convection. Advection, a form ofconvection, is used in a special manner in meteorology.It is discussed as a separate method of heat transfer. Asradiation was discussed earlier in the unit, this sectioncovers conduction, convection, and advection.

CONDUCTION.—Conduction is the transfer ofheat from warmer to colder matter by contact. Althoughof secondary importance in heating the atmosphere, it isa means by which air close to the surface of Earth heatsduring the day and cools during the night.

CONVECTION.—Convection is the method ofheat transfer in a fluid resulting in the transport andmixing of the properties of that fluid. Visualize a pot ofboiling water. The water at the bottom of the pot isheated by conduction. It becomes less dense and rises.Cooler and denser water from the sides and the top ofthe pot rushes in and replaces the rising water. In time,the water is thoroughly mixed. As long as heat isapplied to the pot, the water continues to transfer heatby convection. The transfer of heat by convection inthis case applies only to what is happening to the waterin the pot. In meteorology, the term convection isnormally applied to vertical transport.

Convection occurs regularly in the atmosphere andis responsible for the development of air turbulence.Cumuliform clouds showers and thunderstorms occurwhen sufficient moisture is present and strong verticalconvection occurs. Vertical transfer of heat in theatmosphere (convection) works in a similar manner.Warmer, less dense air rises and is replaced bydescending cooler, denser air, which acquires heat.

Specific Heat

The specific heat of a substance shows how manycalories of heat it takes to raise the temperature of 1gram of that substance 1°C. Since it takes 1 calorie toraise the temperature of 1 gram of water 1°C, thespecific heat of water is 1. The specific heat of asubstance plays a tremendous role in meteorologybecause it is tied directly to temperature changes. Forinstance, the specific heat of earth in general is 0.33.This means it takes only 0.33 calorie to raise thetemperature of 1 gram of earth 1°C. Stated another way,earth heats and cools three times as fast as water.Therefore, assuming the same amount of energy(calories) is available, water heats (and cools) at a

1-17

slower rate than land does. The slower rate of heatingand cooling of water is the reason temperature extremesoccur over land areas while temperatures over waterareas are more consistent.

The specific heat of various land surfaces is alsodifferent, though the difference between one landsurface and another is not as great as between land andwater. Dry sand or bare rock has the lowest specificheat. Forest areas have the highest specific heat. Thisdifference in specific heat is another cause fordifferences in temperature for areas with different typesof surfaces even when they are only a few miles apart;this difference is important in understanding thehorizontal transport of heat (advection) on a smallerscale.

Advection is a form of convection, but inmeteorology it means the transfer of heat or otherproperties HORIZONTALLY. Convection is the termreserved for the VERTICAL transport of heat. In thismanual the words convection and advection are used tomean the vertical and horizontal transfer ofatmospheric properties, respectively.

Horizontal transfer of heat is achieved by motion ofthe air from one latitude and/or longitude to another. Itis of major importance in the exchange of air betweenpolar and equatorial regions. Since large masses of airare constantly on the move somewhere on Earth’ssurface and aloft, advection is responsible fortransporting more heat from place to place than anyother physical motion. Transfer of heat by advection isachieved not only by the transport of warm air, but alsoby the transport of water vapor that releases heat whencondensation occurs.

REVIEW QUESTIONS

Q1-11. What is the definition of Temperature?

Q1-12. What are 20 C converted to Fahrenheit?

Q1-13. Name the zones of the earth's atmosphere inascending order.

Q1-14. What are the four methods of heat transfer?

Q1-15. What is the horizontal transport of heatcalled?

MOISTURE

LEARNING OBJECTIVE: Describe howmoisture affects the atmosphere.

ATMOSPHERIC MOISTURE

More than two-thirds of Earth’s surface is coveredwith water. Water from this extensive source iscontinually evaporating into the atmosphere, coolingby various processes, condensing, and then falling tothe ground again as various forms of precipitation. Theremainder of Earth’s surface is composed of solid landof various and vastly different terrain features.Knowledge of terrain differences is very important inanalyzing and forecasting weather. The world’s terrainvaries from large-scale mountain ranges and deserts tominor rolling hills and valleys. Each type of terrainsignificantly influences local wind flow, moistureavailability, and the resulting weather.

Moisture in the atmosphere is found in threestates—solid, liquid, and gaseous. As a solid, it takesthe form of snow, hail, and ice pellets, frost, ice-crystalclouds, and ice-crystal fog. As a liquid, it is found asrain, drizzle, dew, and as the minute water dropletscomposing clouds of the middle and low stages as wellas fog. In the gaseous state, water forms as invisiblevapor. Vapor is the most important single element in theproduction of clouds and other visible weatherphenomena. The availability of water vapor for theproduction of precipitation largely determines theability of a region to support life.

The oceans are the primary source of moisture forthe atmosphere, but lakes, rivers, swamps, moist soil,snow, ice fields, and vegetation also furnish it. Moistureis introduced into the atmosphere in its gaseous state,and may then be carried great distances by the windbefore it is discharged as liquid or solid precipitation.

WATER VAPOR CHARACTERISTICS

There is a limit to the amount of water vapor thatair, at a given temperature, can hold. When this limit isreached, the air is said to be saturated. The higher the airtemperature, the more water vapor the air can holdbefore saturation is reached and condensation occurs.(See fig. 1-10.) For approximately every 20°F (11°C)increase in temperature between 0°F and 100°F (-18°Cand 38°C), the capacity of a volume of air to hold watervapor is about doubled. Unsaturated air, containing agiven amount of water vapor, becomes saturated if itstemperature decreases sufficiently; further coolingforces some of the water vapor to condense as fog,clouds, or precipitation.

1-18

The quantity of water vapor needed to producesaturation does not depend on the pressure of otheratmospheric gases. At a given temperature, the sameamount of water vapor saturates a given volume of air.This is true whether it be on the ground at a pressure of1000 mb or at an altitude of 17,000 ft (5,100 meters)with only 500 mb pressure, if the temperature is thesame. Since density decreases with altitude, a givenvolume of air contains less mass (grams) at 5,100meters than at the surface. In a saturated volume, therewould be more water vapor per gram of air at thisaltitude than at the surface.

Temperature

Although the quantity of water vapor in a saturatedvolume of atmosphere is independent of the airpressure, it does depend on the temperature. The higherthe temperature, the greater the tendency for liquidwater to turn into vapor. At a higher temperature,therefore, more vapor must be injected into a givenvolume before the saturated state is reached and dew orfog forms. On the other hand, cooling a saturated

volume of air forces some of the vapor to condense andthe quantity of vapor in the volume to diminish.

Condensation

Condensation occurs if moisture is added to the airafter it is saturated, or if cooling of the air reduces thetemperature below the saturation point. As shown infigure 1-11, the most frequent cause of condensation iscooling of the air from the following results: (a) airmoves over a colder surface, (b) air is lifted (cooled byexpansion), or (c) air near the ground is cooled at nightas a result of radiation cooling.

Pressure (Dalton’s Law)

The English physicist, John Dalton, formulated thelaws relative to the pressure of a mixture of gases. Oneof the laws states that the partial pressures of two ormore mixed gases (or vapors) are the same as if eachfilled the space alone. The other law states that the totalpressure is the sum of all the partial pressures of gasesand vapors present in an enclosure.

1-19

SATURATED CONDENSING

HEATCOOL

60 Fo

80 Fo

60 Fo

AGf0110

Figure 1-10.—Saturation of air depends on its temperature.

WARM COLDER COLDER

AIR MOVES IN OVERCOLDER SURFACE.

COOLED BYEXPANSION.

RADIATION COOLING

LIFTING

A B C

AGf0111

Figure 1-11.—Causes of condensation.

For instance, water vapor in the atmosphere isindependent of the presence of other gases. The vaporpressure is independent of the pressure of the dry gasesin the atmosphere and vice versa. However, the totalatmospheric pressure is found by adding all thepressures—those of the dry air and the water vapor.

TERMS

The actual amount of water vapor contained in theair is usually less than the saturation amount. Theamount of water vapor in the air is expressed in severaldifferent methods. Some of these principal methods aredescribed in the following portion of this section.

Relative Humidity

Although the major portion of the atmosphere isnot saturated, for weather analysis it is desirable to beable to say how near it is to being saturated. Thisrelationship is expressed as relative humidity. Therelative humidity of a volume of air is the ratio (inpercent) between the water vapor actually present andthe water vapor necessary for saturation at a giventemperature. When the air contains all of the watervapor possible for it to hold at its temperature, therelative humidity is 100 percent (See fig. 1-12). Arelative humidity of 50 percent indicates that the aircontains half of the water vapor that it is capable ofholding at its temperature.

Relative humidity is also defined as the ratio(expressed in percent) of the observed vapor pressure tothat required for saturation at the same temperature andpressure.

Relative humidity shows the degree of saturation,but it gives no clue to the actual amount of water vapor

in the air. Thus, other expressions of humidity areuseful.

Absolute Humidity

The mass of water vapor present per unit volume ofspace, usually expressed in grams per cubic meter, isknown as absolute humidity. It may be thought of as thedensity of the water vapor.

Specific Humidity

Humidity may be expressed as the mass of watervapor contained in a unit mass of air (dry air plus thewater vapor). It can also be expressed as the ratio of thedensity of the water vapor to the density of the air(mixture of dry air and water vapor). This is called thespecific humidity and is expressed in grams per gram orin grams per kilogram. This value depends upon themeasurement of mass, and mass does not change withtemperature and pressure. The specific humidity of aparcel of air remains constant unless water vapor isadded to or taken from the parcel. For this reason, airthat is unsaturated may move from place to place orfrom level to level, and its specific humidity remainsthe same as long as no water vapor is added or removed.However, if the air is saturated and cooled, some of thewater vapor must condense; consequently, the specifichumidity (which reflects only the water vapor)decreases. If saturated air is heated; its specifichumidity remains unchanged unless water vapor isadded to it. In this case the specific humidity increases.The maximum specific humidity that a parcel can haveoccurs at saturation and depends upon both thetemperature and the pressure. Since warm air can holdmore water vapor than cold air at constant pressure, thesaturation specific humidity at high temperatures is

1-20

THE DIFFERENCE BETWEENACTUAL TEMP. AND DEW POINTTEMP. IS AN INDICATION OFHOW CLOSE THE AIR IS TOSATURATION.

IF COOLED TO DEW POINTTEMP. OR ADDITIONALWATER VAPOR IS ADDEDTO SATURATED AIR,CONDENSATION OCCURS.

DRY AIR

WATER VAPOR

AIR TEMP

DEW POINT

RELATIVE HUMIDITY

40 60 80 80

80

80

8040

100 100100 100

60 60

50

OO O O O O

OO O O O O

% %% %%

F

F

AGf0112

Figure 1-12.—Relative humidity and dew point.

greater than at low temperatures. Also, since moist airis less dense than dry air at constant temperature, aparcel of air has a greater specific humidity atsaturation if the pressure is low than when the pressureis high.

Mixing Ratio

The mixing ratio is defined as the ratio of the massof water vapor to the mass of dry air and is expressed ingrams per gram or in grams per kilogram. It differsfrom specific humidity only in that it is related to themass of dry air instead of to the total dry air plus watervapor. It is very nearly equal numerically to specifichumidity, but it is always slightly greater. The mixingratio has the same characteristic properties as thespecific humidity. It is conservative (values do notchange) for atmospheric processes involving a changein temperature. It is non conservative for changesinvolving a gain or loss of water vapor.

Previously it was learned that air at any giventemperature can hold only a certain amount of watervapor before it is saturated. The total amount of vaporthat air can hold at any given temperature, by weightrelationship, is referred to as the saturation mixingratio. It is useful to note that the following relationshipexists between mixing ratio and relative humidity.Relative humidity is equal to the mixing ratio dividedby the saturation mixing ratio, multiplied by 100. If anytwo of the three components in this relationship areknown, the third may be determined by simplemathematics.

Dew Point

The dew point is the temperature that air must becooled, at constant pressure and constant water vaporcontent, in order for saturation to occur. The dew pointis a conservative and very useful element. Whenatmospheric pressure stays constant, the dew pointreflects increases and decreases in moisture in the air. Italso shows at a glance, under the same conditions, howmuch cooling of the air is required to condensemoisture from the air.

REVIEW QUESTIONS

Q1-16. Name the three states in which moisture in theatmosphere may be found.

Q1-17. What is the primary source of atmosphericmoisture?

Q1-18. What is the difference between relativehumidity and absolute humidity?

Q1-19. What is the definition of mixing ratio?

Q1-20. What information does the dew pointtemperature provide to meteorologists?

SUMMARY

In this chapter, we introduced the basicfundamentals of meteorology. It is important to have abasic knowledge of systems of measurement, how theearth and sun relate to each other, and how pressure,temperature and moisture are measured and calculated.An understanding of the basic fundamentals isnecessary before proceeding on to the next chapter.

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CHAPTER 2

ATMOSPHERIC PHYSICS

The science of physics is devoted to finding,defining, and reaching solutions to problems. It is thebasic science that deals with motion, force, and energy.Physics, therefore, not only breeds curiosity of one’senvironment, but it provides a means of acquiringanswers to questions that continue to arise.Atmospheric physics is a branch of physicalmeteorology that deals with a combination of dynamicand thermodynamic processes that account for theexistence of numerous atmospheric conditions.

To understand the weather elements and to analyzemeteorological situations you must know how to applythe fundamental principles of physics. This does notmean that you must be able to understand all of thecomplicated theories of meteorology. It does mean,however, that you should have a working knowledge ofelementary physics. You should learn how to apply therules of physics to understand how the atmosphereworks. This is necessary to perform your duties as anAerographer’s Mate in a creditable manner.

MOTION

LEARNING OBJECTIVE: Describe thelaws of motion and determine how motion isaffected by external forces.

Any general discussion of the principles of physicsmust contain some consideration of the way in whichmass, force, and motion are related. In physics, the lawsof motion state that an object at rest never starts to moveby itself; a push or a pull must be exerted on it by someother object. This also applies to weather. Weather hascomplex motions in the vertical and horizontal planes.To fully understand how and why weather moves, youmust have a basic knowledge of motion and thoseexternal forces that affect motion.

TERMS

In dealing with motion several terms should bedefined before you venture into the study of motion.These terms are inertia, speed, direction, velocity, andacceleration.

Inertia

An object at rest never moves unless something orsomeone moves it. This is a property of all forms ofmatter (solid, liquid, or gas). Inertia, therefore, is theproperty of matter to resist any change in its state of restor motion.

Speed

Speed is the rate at which something moves in agiven amount of time. In meteorology, speed is the termthat is used when only the rate of movement is meant. Ifthe rate of movement of a hurricane is 15 knots, we sayits speed is 15 knots per hour.

Direction

Direction is the line along which something movesor lies. In meteorology, we speak of direction as towardor the direction from which an object is moving. Forexample, northerly winds are winds COMING FROMthe north.

Velocity

Velocity describes both the rate at which a bodymoves and the direction in which it is traveling. If thehurricane, with its speed of 15 knots per hour, isdescribed as moving westward, it now hasvelocity—both a rate and direction of movement.

Acceleration

This term applies to a rate of change of the speedand/or the velocity of matter with time. If a hurricane,which is presently moving at 15 knots, is moving at 18knots 1 hour from now and 21 knots 2 hours from now,it is said to be accelerating at a rate of 3 knots per hour.

LAWS OF MOTION

Everything around us is in motion. Even a bodysupposedly at rest on the surface of Earth is in motionbecause the body is actually moving with the rotation ofEarth; Earth, in turn, is turning in its orbit around theSun. Therefore, the terms rest and motion are relative

2-1

terms. The change in position of any portion of matter ismotion. The atmosphere is a gas and is subject to muchmotion. Temperature, pressure, and density act toproduce the motions of the atmosphere. These motionsare subject to well-defined physical laws. Anexplanation of Newton’s laws of motion can help you tounderstand some of the reasons why the atmospheremoves as it does.

Newton’s First Law

Sir Isaac Newton, a foremost English physicist,formulated three important laws relative to motion. Hisfirst law, the law of inertia, states, every body continuesin its state of rest or uniform motion in a straight lineunless it is compelled to change by applied forces.”Although the atmosphere is a mixture of gases and hasphysical properties peculiar to gases, it still behaves inmany respects as a body when considered in the termsof Newton’s law. There would be no movement of greatquantities of air unless there were forces to cause thatmovement. For instance, air moves from one area toanother because there is a force (or forces) great enoughto change its direction or to overcome its tendency toremain at rest.

Newton’s Second Law

Newton’s second law of motion, force, andacceleration states, “the change of motion of a body isproportional to the applied force and takes place in thedirection of the straight line in which that force isapplied.” In respect to the atmosphere, this means that achange of motion in the atmosphere is determined bythe force acting upon it, and that change takes place inthe direction of that applied force.

From Newton’s second law of motion the followingconclusions can be determined:

1. If different forces are acting upon the samemass, different accelerations are produced that areproportional to the forces.

2. For different masses to acquire equalacceleration by different forces, the forces must beproportional to the masses.

3. Equal forces acting upon different massesproduce different accelerations that are proportional tothe masses.

Newton’s Third Law

Newton’s third law of motion states, “to everyaction there is always opposed an equal reaction; or, themutual actions of two bodies upon each other arealways equal, and directed to contrary parts.” In otherwords forces acting on a body originate in other bodiesthat make up its environment. Any single force is onlyone aspect of a mutual interaction between two bodies.

WORK

Work is done when a force succeeds in overcominga body’s inertia and moving the body in the directionthe force is applied. The formula is

W = F × d

where W is work, F is force and d is the distance moved.The amount of work done is the product of themagnitude of the force and the distance moved.

Work is measured in the English system by thefoot-pound; that is, if 1 pound of force acts through adistance of 1 foot, it performs 1 foot-pound of work. Inthe metric CGS system, force is measured in dynes,distance is measured in centimeters, and work isdenoted in ergs. An erg is the work done by a force ofone dyne exerted for a distance of one centimeter.Another unit used to measure work is the joule. It issimply 10,000,000 ergs, and is equivalent to just underthree-fourths of a foot-pound.

ENERGY

Energy is defined as the ability to do work. Energyis conservative, meaning it may be neither created nordestroyed. It is defined in two forms—potential energyand kinetic energy. As its name implies, potentialenergy is the amount of energy that MAY BEAVAILABLE to a body due to its position. It isprimarily due to the force of gravity. The higher a bodyis raised above the surface, the greater its POTENTIALenergy. Kinetic energy is the energy available to a bodydue to its motion through a field. The total amount ofenergy a body possesses is the sum of its potential andkinetic energies. The total amount of energy availableto a body determines how much work it canaccomplish.

FORCE

There are two types of forces the AG dealswith—contact force and action at a distance force.Contact force is the force that occurs when pressure is

2-2

put on an object directly through physical contact. Anexample of contact force is the force your hand exertswhen you push your coffee cup across a table. Contactforce may act in several different directions at once aswell. For example, the force exerted by water in a can isequally exerted on the sides and the bottom of the can.In addition, an upward force is transmitted to an objecton the surface of the water. Forces that act throughempty space without contact are known as action at adistance force. An example of this force is gravity.

Vectors

Problems often arise that make it necessary to dealwith one or more forces acting on a body. To solveproblems involving forces, a means of representingforces must be found. True wind speed at sea involvestwo different forces and is obtained through the use ofthe true wind computer. Ground speed and course ofaircraft are computed by adding the vector representingaircraft heading and true air speed to the vectorrepresenting the wind direction and speed. Incomputation of the effective fallout wind and otherradiological fallout problems, the addition of forces isused. From these examples, it is evident that theaddition and subtraction of forces has manyapplications in meteorology.

A force is completely described when itsmagnitude, direction, and point of application aregiven. A vector is a line that represents both magnitudeand direction; therefore, it may be used to describe aforce. The length of the line represents the magnitudeof the force. The direction of the line represents thedirection in which the force is being applied. Thestarting point of the line represents the point ofapplication of the force. (See fig. 2-1.) To represent aforce of 10 pounds or 10 knots of wind acting towarddue east on point A, draw a line 10 units long, starting atpoint A and extending in a direction of 090°.

Composition of Forces

If two or more forces are acting simultaneously at apoint, the same effect can be produced by a single forceof the proper size and direction. This single force,which is equivalent to the action of two or more forces,is called the resultant. Putting component forcestogether to find the resultant force is called compositionof forces. (See fig. 2-2.) The vectors representing theforces must be added to find the resultant. Because avector represents both magnitude and direction, themethod for adding vectors differs from the procedureused for scalar quantities (quantities having onlymagnitude and no direction). To find the resultant forcewhen a force of 5 pounds and a force of 10 pounds areapplied at a right angle to point A, refer to figure 2-2.

The resultant force may be found as follows:Represent the given forces by vectors AB and ACdrawn to a suitable scale. At points B and C drawdashed lines perpendicular to AB and AC, respectively.From point A, draw a line to the point of intersection X,of the dashed lines. Vector AX represents the resultantof the two forces. Thus, when two mutuallyperpendicular forces act on a point, the vectorrepresenting the resultant force is the diagonal of arectangle. The length of AX, if measured on the samescale as that for the two original forces, is the resultantforce; in this case approximately 11.2 pounds. Theangle gives the direction of the resultant force withrespect to the horizontal.

Mathematically, the resultant force ofperpendicular forces can be found by using thePythagorean theorem which deals with the solution ofright triangles. The formula is C2 = a2 + b2. This statesthat the hypotenuse, side “C” (our unknown resultantforce) squared is equal to the sum of side “a” (one of ourknown forces) squared and side “b” (another of ourknown forces) squared.

2-3

N

A

10 LB

AG5f0201

Figure 2-1.—Example of a vector.AG5f0202

C X

5 LB

10 LBBA

Figure 2-2.—Composition of two right angle forces.

If we substitute the known information in figure2-2 we have the following:

C2 = Unknown resultant force

a2 = 5 lb or the known force on one side of ourright triangle, side BX (same as side AC)

b2 = 10 lb or the known force on the other sideof our right triangle, side AB

Setting up the equation we have:

C2 = a2 +

C2 = 52 + 102

C2 = 25 + 100

C2 = 125

C = 125

C = 11.18034

To find the resultant of two forces that are not atright angles, the following graphic method may beused. (See fig. 2-3).

Let AB and AC represent the two forces drawnaccurately to scale. From point C draw a line parallel toAB and from point B draw a line parallel to AC. Thelines intersect at point X. The force AX is the resultantof the two forces AC and AB. Note that the two dashedlines and the two given forces make a parallelogramACXB. Arriving at the resultant in this manner is calledthe parallelogram method. The resultant force anddirection of the resultant is found by measuring thelength of line AX and determining the direction of lineAX from the figure drawn to scale. This method appliesto any two forces acting on a point whether they act atright angles or not. Note that the parallelogrambecomes a rectangle for forces acting at right angles.With a slight modification, the parallelogram method ofaddition applies also to the reverse operation ofsubtraction. Consider the problem of subtracting forceAC from AB. (See fig. 2-4.)

First, force AC is reversed in direction giving -AC(dashed line). Then, forces -AC and AB are added bythe parallelogram method, giving the resulting AX,which in this case is the difference between forces ABand AC. A simple check to verify the results consists ofadding AX to AC; the sum or resultant should beidentical with AB.

Application of Vectors and Resultant Forces

The methods presented for computing vectors andresultant forces are the simplest and quickest methodsfor the Aerographer’s Mate. The primary purposes ofusing vectors and resultant forces are for computingradiological fallout patterns and drift calculations forsearch and rescue operations.

REVIEW QUESTIONS

Q2-1. What is the definition of speed?

Q2-2. What is the correct formula for work?

Q2-3. What are the two types forces that AGs dealwith?

MATTER

LEARNING OBJECTIVE: Recognize howpressure, temperature, and density affect theatmosphere. Describe how the gas laws areapplied in meteorology.

Matter is around us in some form everywhere in ourdaily lives—the food we eat, the water we drink, andthe air we breathe. The weather around us, such as hail,rain, invisible water vapor (humidity), etc., are all

2-4

AG5f0203

C

A(R) RESULTANT

B

X

Figure 2-3.—Graphic method of the composition of forces.

C

B

C X

A

AG5f0204

Figure 2-4.—Parallelogram method of subtracting forces.

matter. Matter is present in three forms—solids,liquids, and gases. A good working knowledge of thephysical properties of matter and how matter canchange from one form to another can help youunderstand what is happening in our atmosphere thatproduces the various meteorological occurrences welive with every day.

DEFINITIONS

Matter is anything that occupies space and hasweight. Two basic particles make up the composition ofall matter—the atom and the molecule. The molecule isthe smallest particle into which matter can be dividedwithout destroying its characteristic properties. Inphysics, the molecule is the unit of matter. Moleculesare composed of one or more atoms. The atom is thesmallest particle of an element of matter that can existeither alone or in combination with others of the sameor of another element. The atom and atomic structure isconstantly under study and has revealed a whole newarray of subatomic particles. To date, a new definitionfor atom has not been developed.

A compound is a substance (or matter) formed bycombining two or more elements. Thus, ordinary tablesalt is a compound formed by combining twoelements—sodium and chlorine. Elements andcompounds may exist together without forming newcompounds. Their atoms do not combine. This isknown as a mixture. Air is a familiar mixture. Everysample of air contains several kinds of molecules whichare chiefly molecules of the elements oxygen, nitrogen,and argon, together with the compounds of water vaporand carbon dioxide. Ocean water, too, is anothermixture, made up chiefly of water and salt molecules,with a smaller number of molecules of many othercompounds as well as molecules of several elements.

STATES OF MATTER

Matter is found in all of the following three states:

1. Solid. Solids are substances that have a definitevolume and shape and retain their original shape andvolume after being moved from one container toanother, such as a block of wood or a stone.

2. Liquid. A liquid has a definite volume, becauseit is almost impossible to put it into a smaller space.However, when a liquid is moved from one container toanother, it retains its original volume, but takes on theshape of the container into which it is moved. Forexample, if a glass of water is poured into a largerbucket or pail, the volume remains unchanged. The

liquid occupies a different space and shape in that itconforms to the walls of the container into which it ispoured.

3. Gas. Gases have neither a definite shape nor adefinite volume. Gases not only take on the shape of thecontainer into which they are placed but expand and fillit, no matter what the volume of the container.

Since gases and liquids flow easily, they are bothcalled fluids. Moreover, many of the laws of physicsthat apply to liquids apply equally well to gases.

PHYSICAL PROPERTIES

Since matter is anything that occupies space andhas weight, it can be said that all kinds of matter havecertain properties in common. These properties areinertia, mass, gravitation, weight, volume, and density.These properties are briefly covered in this section andare referred to as the general properties of matter.

Inertia

Inertia of matter is perhaps the most fundamental ofall attributes of matter. It is the tendency of an object tostay at rest if it is in a position of rest, or to continue inmotion if it is moving. Inertia is the property thatrequires energy to start an object moving and to stopthat object once it is moving.

Mass

Mass is the quantity of matter contained in asubstance. Quantity does not vary unless matter isadded to or subtracted from the substance. For example,a sponge can be compressed or allowed to expand backto its original shape and size, but the mass does notchange. The mass remains the same on Earth as on thesun or moon, or at the bottom of a valley or the top of amountain. Only if something is taken away or added toit is the mass changed. Later in the unit its meaning willhave a slightly different connotation.

Gravitation

All bodies attract or pull upon other bodies. In otherwords, all matter has gravitation. One of Newton’s lawsstates that the force of attraction between two bodies isdirectly proportional to the product of their masses andinversely proportional to the square of the distancebetween their two centers. Therefore, a mass has lessgravitational pull on it at the top of a mountain than ithas at sea level because the center is displaced farther

2-5

away from the gravitational pull of the center of Earth.However, the mass remains the same even though thegravitational pull is different. Gravity also varies withlatitude. It is slightly less at the equator than at the polesdue to the equator’s greater distance from the center ofEarth.

Weight

The weight of an object is a measure of itsgravitational attraction. The weight depends upon themass or quantity that it contains and the amount ofgravitational attraction Earth has for it. Weight is aforce, and as such it should be expressed in units offorce. Since gravity varies with latitude and heightabove sea level, so must weight vary with the samefactors. Therefore, a body weighs more at the polesthan at the equator and more at sea level than atop amountain. In a comparison of mass and weight, massremains constant no matter where it is, but weightvaries with latitude and height above sea level.

Volume

Volume is the measure of the amount of space thatmatter occupies. The volume of rectangular objects isfound directly by obtaining the product of their length,width, and depth. For determining the volume of liquidsand gases, special graduated containers are used.

Density

The mass of a unit volume of a substance or massper unit volume is called density. Usually we speak ofsubstances being heavier or lighter than another whencomparing equal volumes of the two substances.

Since density is a derived quantity, the density of anobject can be computed by dividing its mass (or weight)by its volume. The formula for determining the densityof a substance is

DM

V(or D M V)= = = ÷

where D stands for density, M for mass, and V forvolume.

From this formula, it is obvious that with massremaining unchanged, an increase in volume causes adecrease in density. A decrease in volume causes anincrease in density.

The density of gases is derived from the same basicformula as the density of a solid. Pressure and

temperature also affect the density of gases. This effectis discussed later in this unit under Gas Laws.

CHANGES OF STATE

A change of state (or change of phase) of asubstance describes the change of a substance from asolid to a liquid, liquid to a vapor (or gas), vapor to aliquid, liquid to a solid, solid to vapor, or vapor to asolid. In meteorology you are concerned primarily withthe change of state of water in the air. Water is present inthe atmosphere in any or all of the three states (solid,liquid, and vapor) and changes back and forth from onestate to another. The mere presence of water isimportant, but the change of state of that water in the airis significant because it directly affects the weather.The solid state of water is in the form of ice or icecrystals. The liquid state of water is in the form ofraindrops, clouds, and fogs. The vapor state of water isin the form of unseen gases (water vapor) in the air.

Heat Energy

Energy is involved in the various changes of statethat occur in the atmosphere. This energy is primarily inthe form of heat. Each of the changes of state processeseither uses heat from the atmosphere or releases heatinto the atmosphere. The heat used by a substance inchanging its state is referred to as the latent heat and isusually stated in calories.

The calorie is a unit of heat energy. It is the amountof heat required to raise the temperature of 1 gram ofwater 1°C. A closer look at some of the major changesof state of the atmosphere helps to clarify latent heat.Refer to figure 2-5 during the following discussions.

Liquid to Solid and Vice Versa

Fusion is the change of state from a solid to a liquidat the same temperature. The number of gram caloriesof heat necessary to change 1 gram of a substance fromthe solid to the liquid state is known as the latent heat offusion. To change 1 gram of ice to 1 gram of water at aconstant temperature and pressure requires roughly 80calories of heat. This is called the latent heat of fusion.Fusion uses heat. The source of this heat is thesurrounding air.

The opposite of fusion is freezing—a liquidchanges into a solid. Since it requires 80 calories tochange 1 gram of ice to 1 gram of water, this sameamount of heat is released into the air when 1 gram ofwater is changed to ice.

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Liquid to Gas and Vice Versa

Water undergoes the process of evaporation whenchanging from the liquid to a gaseous state. Accordingto the molecular theory of matter, all matter consists ofmolecules in motion. The molecules in a bottled liquidare restricted in their motion by the walls of thecontainer. However, on a free surface exposed to theatmosphere, the motion of the molecules in the liquid isrestricted by the weight of the atmosphere or, moreprecisely, by the atmospheric pressure. If the speed ofthe liquid molecules is sufficiently high, they escapefrom the surface of the liquid into the atmosphere. Asthe temperature of the liquid is increased, the speed ofthe molecules is increased, and the rate at which themolecules escape from the surface also increases.Evaporation takes place only from the free or exposedsurface of a substance.

During the process of evaporation, heat is released.This heat is absorbed by the water that has vaporized.The amount absorbed is approximately 539 calories pergram of water at a temperature of 100°C. On the otherhand, the amount is 597.3 calories, if the evaporationtakes place at a water temperature of 0°C. This energyis required to keep the molecules in the vapor state and

is called the latent heat of vaporization. Since the waterneeds to absorb heat in order to vaporize, heat must besupplied or else evaporation cannot take place. The airprovides this heat. For this reason, evaporation is said tobe a cooling process, because by supplying the heat forvaporization, the temperature of the surrounding air islowered.

Condensation is the opposite of evaporationbecause water vapor undergoes a change in state fromgas back to liquid. However, a condition of saturationmust exist before condensation can occur. That is, theair must contain all the water vapor it can hold (100percent relative humidity) before any of it can condensefrom the atmosphere. In the process of condensation,the heat that was absorbed in evaporation by the watervapor is released from the water vapor into the air and iscalled the latent heat of condensation. As you mightexpect, condensation warms the surrounding air.

Solid to Gas and Vice Versa

Sublimation is the change of state from a soliddirectly to a vapor or vice versa at the sametemperature. In physics and chemistry, sublimation isregarded as the change of state from solid to vapor only,

2-7

FREEZING

FREEZING

CONDENSA IT ONCOOLING

COOLING

MELTING EVAPORATIONHEATING

HEATINGMELTING

COOLING & CONDENSATION

HEATING & EVAPORATION

SUBLIMATION

SUBLIMATION

1 gm

ICE 0 C

1 gm

ICE 0 C

1 gm

ICE -10 C

1 gm

ICE 0 C

1 gmINVISIBLE

WATER VAPOR100 C

1 gmINVISIBLE

WATER VAPOR100 C

1 gmWATER

0 C

1 gmWATER

0 C

1 gmINVISIBLE

WATER VAPOR0 C

TOTAL CALORIESADDED TO

THE AIR

727

723

677

TOTAL CALORIESTAKEN FROM

THE AIR

727

723

677

80 597 50

4 80 100+539

677

NOTE:1. EVAPORATION COOLS AIR.2. CONDENSATION HEATS.3. CALORIES SHOWN TO NEAREST

WHOLE FIGURES.

1 gmINVISIBLE

WATER VAPOR0 C

AG5f0205

Figure 2-5.—Thermal history of 1 gram of ice during changes of state.

but meteorologists do not make this distinction. Theheat of sublimation equals the heat of fusion plus theheat of vaporization for a substance. The caloriesrequired for water to sublime are: 80 + 597.3 = 677.3, ifthe vapor has a temperature of 0°C.

In the sublimation process of vapor passing directlyinto the solid form without going through the liquidphase, the calories released are the same as those for thesublimation of a solid to a gas. Sublimation of watervapor to ice frequently takes place in the atmospherewhen supercooled water vapor crystallizes directly intoice crystals and forms cirriform clouds.

REVIEW QUESTIONS

Q2-4. What are the two basic particles that make upthe composition of matter?

Q2-5. What is the correct formula for density?

Q2-6. What is fusion?

GAS LAWS

LEARNING OBJECTIVE: Recognize howpressure, temperature, and density affect theatmosphere and describe how the gas laws areapplied in meteorology.

Since the atmosphere is a mixture of gases, itsbehavior is governed by well-defined laws.Understanding the gas laws enables you to see that thebehavior of any gas depends upon the variations intemperature, pressure, and density.

To assist in comparing different gases and inmeasuring changes of gases it is necessary to have astandard or constant to measure these changes against.The standard used for gases are: a pressure of 760millimeters of mercury (1,013.25 mb) and atemperature of 0°C. These figures are sometimesreferred to as Standard Temperature and Pressure(STP).

KINETIC THEORY OF GASES

The Kinetic theory of gases refers to the motions ofgases. Gases consist of molecules that have no inherenttendency to stay in one place as do the molecules of asolid. Instead, the molecules of gas, since they aresmaller than the space between them, are free to moveabout. The motion is in straight lines until the linescollide with each other or with other obstructions,making their overall motion random. When a gas isenclosed, its pressure depends on the number of times

the molecules strike the surrounding walls. The numberof blows that the molecules strike per second againstthe walls remains constant as long as the temperatureand the volume remain constant.

If the volume (the space occupied by the gas) isdecreased, the number of blows against the wall isincreased, thereby increasing the pressure if thetemperature remains constant. Temperature is ameasure of the molecular activity of the gas moleculesand a measure of the internal energy of a gas. When thetemperature is increased, there is a correspondingincrease in the speed of the molecules; they strike thewalls at a faster rate, thereby increasing the pressureprovided the volume remains constant. Therefore,there is a close relationship of volume, pressure, anddensity of gases.

BOYLE’S LAW

Boyle’s law states that the volume of a gas isinversely proportional to its pressure, provided thetemperature remains constant. This means that if thevolume is halved, the pressure is doubled. An exampleof Boyle’s law is a tire pump. As the volume of thepump’s cylinder is decreased by pushing the handledown, the pressure at the nozzle is increased. Anotherway of putting it is, as you increase the pressure in thecylinder by pushing down the handle, you also decreasethe volume of the cylinder.

The formula for Boyle’s law is as follows:

VP = V’P’V = initial volumeP = initial pressureV’ = new volumeP’ = new pressure

For example, assume 20 cm3 of gas has a pressureof 1,000 mb. If the pressure is increased to 1,015 mband the temperature remains constant, what will be thenew volume? Applying the formula, we have

V = 20 cm3

P = 1000 mbV’ = Unknown in cm3

P’ = 1015 mbV • P = V’ • P’20 • 1,000 = V’ • 1,01520,000 = V’ • 1,015

V’ =20 000

1015

,

,V’ = 19.71 cm3

2-8

Boyle’s law does not consider changes intemperature. Since our atmosphere is constantlychanging temperature at one point or another,temperature must be considered in any practicalapplication and understanding of gas laws.

CHARLES’ LAW

In the section on the kinetic theory of gases, it wasexplained that the temperature of a gas is a measure ofthe average speed of the molecules of the gas. It wasalso shown that the pressure the gas exerts is a measureof the number of times per second that the moleculesstrike the walls of the container and the speed at whichthey strike it. Therefore, if the temperature of a gas in aclosed container is raised, the speed of the moleculeswithin the gas increases. This causes the molecules tostrike the sides of the container more often per secondand with more force because they are moving faster.Thus, by increasing the temperature, the pressure isincreased.

Charles’ law states if the volume of an enclosed gasremains constant, the pressure is directly proportionalto the absolute temperature. Therefore, if the absolutetemperature is doubled, the pressure is doubled; if theabsolute temperature is halved, the pressure is halved.Experiments show that the volume increases by 1/273for a 1°C rise in temperature. (Remember, 0°C is equalto 273°K.) An example of Charles’ law is a bottle ofsoda or beer. When the soda or beer is cold, very littlepressure is released when the bottle is opened. When awarm soda or beer is opened, it often results in enoughpressure buildup in the bottle to squirt soda or beer outof the top. Sometimes, warm soda or beer explodesspontaneously when exposed to too much direct heatsuch as sunlight.

The formulas for Charles’ law are as follows:

VT’ = V’T, where pressure is assumed to beconstant, and

PT’ = P’T, where volume is constant

V = initial volumeT = initial temperature (absolute)V’ = new volumeT’ = new temperature (absolute)

For example, assume that 10 cm3 of a gas has atemperature of 200° absolute. If the temperature isincreased to 300° absolute, what will be the newvolume? Applying the formula, we have

V = 10 cm3

T = 200°KV’ = Unknown in cm3

T’ = 300°K10 • 300 = V’ • 2003000 = V’ • 200

V’ =3000

200V’ = 15 cm3

The same type relationship can be computed byapplying T’ (new temperature) and P’ (new pressure)using the formula PT’ = P’T where the volume isassumed to remain constant.

UNIVERSAL GAS LAW

The universal gas law is a combination of Boyle’slaw and Charles’ law. It states that the product of theinitial pressure, initial volume, and new temperature(absolute scale) of an enclosed gas is equal to theproduct of the new pressure, new volume, and initialtemperature. The formula is as follows:

PVT’ = P’V’T

P = initial pressureV = initial volumeT = initial temperature (absolute)P’ = new pressureV = new volume (absolute)T = new temperature (absolute)

For example, assume the pressure of a 500 cm3

volume of gas is 600 mb and the temperature is 30°C(303 absolute). If the temperature is increased to 45°C(318° absolute) and the volume is decreased to 250cm3, what will be the new pressure of the volume?Applying the formula, we have

P = 600 mbV = 500 cm3

T = 303°KP’ = Unknown pressure in mbV’ = 250 cm3

T’ = 318°K600 • 500 • 318 = P’ • 250 • 30395,400,000 = P’75,750

P’ =95 400 000

75 750

, ,

,P’ = 1,259.4 mb

2-9

EQUATION OF STATE

The equation of state is a general gas law forfinding pressure, temperature, or density of a dry gas.Rather than using volume, this formula uses what iscalled gas constant. A gas constant is a molecularweight assigned to various gases. Actually, air does nothave a molecular weight because it is a mixture of gasesand there is no such thing as an air molecule. However,it is possible to assign a so-called molecular weight todry air that makes the equation of state work. The gasconstant for air is 2,870 and for water vapor it is 1,800when the pressure is expressed in millibars and thedensity is expressed in metric tons per cubic meter. Thegas constant may be expressed differently dependingon the system of units used.

The following formula is an expression of theequation of state:

P = ρRT

P = pressure in millibarsρ = density (Greek letter rho)R = specific gas constantT = temperature (absolute)

The key to this formula is the equal sign thatseparates the two sides of the formula. This equal signmeans that the same value exists on both sides; bothsides of the equation are equal. If the left side of theequation (pressure) changes, a corresponding changemust occur on the right side (either in the density ortemperature) to make the equation equal again.Therefore, an increase of the total value on one side ofthe Equation of State must be accompanied by anincrease of the total value on the other side. The same istrue of any decrease on either side.

NOTE: Since R is a constant it will always remainunchanged in any computation.

The right side of the equation can balance out anychange in either density or temperature without havinga change on the left side (pressure). If, for example, anincrease in temperature is made on the right side, theequation may be kept in balance by decreasing density.This works for any value in the equation of state.

From this relationship, we can draw the followingconclusions:

1. A change in pressure, density (mass orvolume), or temperature requires a change in one orboth of the others.

2. With the temperature remaining constant, anincrease in density results in an increase in atmosphericpressure. Conversely, a decrease in density results in adecrease in pressure.

NOTE: Such a change could occur as a result of achange in the water vapor content.

3. With an increase in temperature, the pressureand/or density must change. In the free atmosphere, atemperature increase frequently results in expansion ofthe air to such an extent that the decrease in densityoutweighs the temperature increase, and the pressureactually decreases. Likewise, a temperature increaseallows an increase in moisture, which in turn decreasesdensity (mass of moist air is less than that of dry air).Couple this with expansion resulting from thetemperature increase and almost invariably, the finalresult is a decrease in pressure.

At first glance, it may appear that pressureincreases with an increase in temperature. Earlier,however, it was noted that this occurs when volume (thegas constant) remains constant. This condition wouldbe unlikely to occur in the free atmosphere becausetemperature increases are associated with densitydecreases, or vice versa. The entire concept of theequation of state is based upon changes in densityrather than changes in temperature.

HYDROSTATIC EQUATION

The hydrostatic equation incorporates pressure,temperature, density, and altitude. These are the factorsthat meteorologists must also deal with in any practicalapplication of gas laws. The hydrostatic equation,therefore, has many applications in dealing withatmospheric pressure and density in both the horizontaland vertical planes. The hydrostatic equation itself willbe used in future units and lessons to explain pressuregradients and vertical structure of pressure centers.Since the equation deals with pressure, temperature,and density, it is briefly discussed here.

The hypsometric formula is based on thehydrostatic equation and is used for either determiningthe thickness between two pressure levels or reducingthe pressure observed at a given level to that at someother level. The hypsometric formula states that thedifference in pressure between two points in theatmosphere, one above the other, is equal to the weightof the air column between the two points. There are twovariables that must be considered when applying thisformula to the atmosphere. They are temperature anddensity.

2-10

From Charles’ law we learned that when thetemperature increases, the volume increases and thedensity decreases. Therefore, the thickness of a layer ofair is greater when the temperature increases. To findthe height of a pressure surface in the atmosphere (suchas in working up an adiabatic chart), these two variables(temperature and density) must be taken intoconsideration. By working upward through theatmosphere, the height of that pressure surface can becomputed by adding thicknesses together. A good toolfor determining height and thickness of layers is theSkew-T Log P diagram, located in AWS/TR-79/006.

Since there are occasions when Skew-Ts are notavailable, a simplified version of the hypsometricformula is presented here. This formula for computingthe thickness of a layer is accurate within 2 percent;therefore, it is suitable for all calculations that theAerographer’s Mate would make on a daily basis.

The thickness of a layer can be determined by thefollowing formula:

Z = (49,080 + 107t) • Po P

Po P

−+

Z = altitude difference in feet (unknownthickness of layer)

49,080 = A constant (representing gravitation andheight of the D-mb level above thesurface)

107 = A constant (representing density andmean virtual temperature)

t = mean temperature in degrees Fahrenheit

Po = pressure at the bottom point of the layer

P = pressure at the top point of the layer

For example, let us assume that a layer of airbetween 800 and 700 millibars has a mean temperatureof 30°F. Applying the formula, we have

Z = (49,080 + 107 × 30) • 800 700

800 700

−+

Z = (49,080 + 3,210) • 100

1 500,

Z = (52,290) • 1

15

Z = 3,486 feet (1,063 meters)(1 meter = 3.28 feet)

REVIEW QUESTIONS

Q2-7. What three things does the behavior of gasesdepend on?

Q2-8. According to Boyle's Law, how is volume andpressure related?

Q2-9. According to Charles' Law, how istemperature and pressure related?

Q2-10. What is the formula for the Universal GasLaw?

ATMOSPHERIC ENERGY

LEARNING OBJECTIVE: Describe theadiabatic process and determine how stabilityand instability affect the atmosphere.

There are two basic kinds of atmospheric energyimportant to AGs—kinetic and potential. Kineticenergy is energy that performs work due to presentmotion while potential energy is energy that is storedfor later action. Kinetic energy is discussed first inrelation to its effect on the behavior of gases.

According to the kinetic theory of gases, thetemperature of a gas is dependent upon the rate at whichthe molecules are moving about and is proportional tothe kinetic energy of the moving molecules. The kineticenergy of the moving molecules of a gas is the internalenergy of the gas; it follows that an increase intemperature is accompanied by an increase in theinternal energy of the gas. Likewise, an increase in theinternal energy results in an increase in the temperatureof the gas. This relationship, between heat and energy,is called thermodynamics.

An increase in the temperature of a gas or in itsinternal energy can be produced by the addition of heator by performing work on the gas. A combination ofthese can also produce an increase in temperature orinternal energy. This is in accordance with the first lawof thermodynamics.

FIRST LAW OF THERMODYNAMICS

This law states that the quantity of energy suppliedto any system in the form of heat is equal to work doneby the system plus the change in internal energy of thesystem. In the application of the first law ofthermodynamics to a gas, it may be said that the twomain forms of energy are internal energy and workenergy. Internal energy is manifested as sensible heat orsimply temperature. Work energy is manifested aspressure changes in the gas. In other words, work is

2-11

required to increase the pressure of a gas and work isdone by the gas when the pressure diminishes. Itfollows that if internal energy (heat) is added to asimple gas, this energy must show up as an increase ineither temperature or pressure, or both. Also, if work isperformed on the gas, the work energy must show up asan increase in either pressure or temperature, or both.

An example of the thermodynamic process is amanual tire pump. The pump is a cylinder enclosed by apiston. In accordance with the first law ofthermodynamics, any increase in the pressure exertedby the piston as you push down on the handle results inwork being done on the air. As a consequence, eitherthe temperature and pressure must be increased or theheat equivalent of this work must be transmitted to thesurrounding bodies. In the case of a tire pump, the workdone by the force on the piston is changed into anincrease in the temperature and the pressure in the air. Italso results in some increase in the temperature of thesurrounding body by conduction.

If the surrounding body is considered to beinsulated so it is not heated, there is no heat transferred.Therefore, the air must utilize this additional energy asan increase in temperature and pressure. This occurs inthe adiabatic process.

THE ADIABATIC PROCESS

The adiabatic process is the process by which a gas,such as air, is heated or cooled, without heat beingadded to or taken away from the gas, but rather byexpansion and compression. In the atmosphere,adiabatic and nonadiabatic processes are taking placecontinuously. The air near the ground is receiving heatfrom or giving heat to the ground. These arenonadiabatic processes. However, in the freeatmosphere somewhat removed from Earth’s surface,the short-period processes are adiabatic. When a parcelof air is lifted in the free atmosphere, pressuredecreases. To equalize this pressure, the parcel mustexpand. In expanding, it is doing work. In doing work,it uses heat. This results in a lowering of temperature aswell as a decrease in the pressure and density. When aparcel of air descends in the free atmosphere, pressureincreases. To equalize the pressure, the parcel mustcontract. In doing this, work is done on the parcel. Thiswork energy, which is being added to the parcel, showsup as an increase in temperature. The pressure anddensity increase in this case also.

Terms

In discussing the adiabatic process several termsare used that you should understand.

LAPSE RATE.—In general, lapse rate is the rateof decrease in the value of any meteorological elementwith elevation. However, it is usually restricted to therate of decrease of temperature with elevation; thus, thelapse rate of the temperature is synonymous with thevertical temperature gradient. The temperature lapserate is usually positive, which means that thetemperature decreases with elevation.

INVERSION.—Inversions describe theatmospheric conditions when the temperature increaseswith altitude, rather than decreases as it usually does.Inversions result from the selective absorption ofEarth’s radiation by the water vapor in the air, and alsofrom the sinking, or subsidence, of air, which results inits compression and, therefore, heating. Either effectalone may cause an inversion; combined, the inversionis stronger.

When air is subsiding (sinking), the compressed airheats. This frequently produces a subsidence inversion.When subsidence occurs above a surface inversion, thesurface inversion is intensified. Such occurrences arecommon in wintertime high-pressure systems. The airin the inversion layer is very stable, and the cold airabove the inversion acts as a lid trapping fog, smoke,and haze beneath it. Poor visibility in the lower levels ofthe atmosphere results, especially near industrial areas.Such conditions frequently persist for days, notably inthe Great Basin region of the western United States. Aninversion is a frequent occurrence (especially at night)in the Tropics and in the Polar regions. For nightconditions all over the world, polar and tropical regionsincluded, it may be said that low- level inversions arethe rule rather than the exception.

ISOTHERMAL.—In the isothermal lapse rate, nocooling or warming is noted and the rate is neutral withheight—no change in temperature with height.

Adiabatic Heating and Cooling

Air is made up of a mixture of gases that is subjectto adiabatic heating when it is compressed andadiabatic cooling when it is expanded. As a result, airrises seeking a level where the pressure of the body ofair is equal to the pressure of the air that surrounds it.There are other ways air can be lifted, such as throughthe thermodynamic processes of a thunderstorm ormechanically, such as having colder, denser air move

2-12

under it or by lifting as it flows up over a mountainslope.

As the air rises, the pressure decreases whichallows the parcel of air to expand. This continues until itreaches an altitude where the pressure and density areequal to its own. As it expands, it cools through athermodynamic process in which there is no transfer ofheat or mass across the boundaries of the system inwhich it operates (adiabatic process). As air rises, itcools because it expands by moving to an altitudewhere pressure and density is less. This is calledadiabatic cooling. When the process is reversed and airis forced downward, it is compressed, causing it to heat.This is called adiabatic heating, (See fig. 2-6.)

Remember, in an adiabatic process an increase intemperature is due only to COMPRESSION when theair sinks or subsides. A decrease in temperature is due

only to EXPANSION when air rises, as with convectivecurrents or air going over mountains. There is noaddition or subtraction of heat involved. The changes intemperature are due to the conversion of energy fromone form to another.

STABILITY AND INSTABILITY

The atmosphere has a tendency to resist verticalmotion. This is known as stability. The normal flow ofair tends to be horizontal. If this flow is disturbed, astable atmosphere resists any upward or downwarddisplacement and tends to return quickly to normalhorizontal flow. An unstable atmosphere, on the otherhand, allows these upward and downward disturbancesto grow, resulting in rough (turbulent) air. An exampleis the towering thunderstorm that grows as a result of alarge intense vertical air current.

2-13

AG5f0206

MOIST AIR BEING LIFTEDBY COLD FRONT.

AS THE LIFTED AIR EXPANDS,IT COOLS ADIABATICALLY.

COLD FRONT

WHEN AIR RISES IN ALTITUDE,IT EXPANDS AND COOLS AT ITSADIABATIC LAPSE RATE.

ADIABATICCOOLING

WHEN AIR DESCENDS IN ALTITUDE,IT COMPRESSES AND HEATS AT ITSADIABATIC LAPSE RATE.

ADIABATICHEATING

MOIST ADIABATIC LAPSE RATE2 TO 3 F/1000FT.

MOIST ADIABATIC LAPSE RATE5.5 F/1000FT.

Figure 2-6.—Adiabatic cooling and heating process.

Atmospheric resistance to vertical motion(stability), depends upon the vertical distribution of theair’s weight at a particular time. The weight varies withair temperature and moisture content. As shown infigure 2-7, in comparing two parcels of air, hotter air islighter than colder air; and moist air is lighter than dryair. If air is relatively warmer or more moist than itssurroundings, it is forced to rise and is unstable. If theair is colder or dryer than its surroundings, it sinks untilit reaches its equilibrium level and is stable. Theatmosphere can only be at equilibrium when light air isabove heavier air—just as oil poured into water rises tothe top to obtain equilibrium. The stability of airdepends a great deal on temperature distribution and toa lesser extent on moisture distribution.

Since the temperature of air is an indication of itsdensity, a comparison of temperatures from one level toanother can indicate how stable or unstable a layer ofair might be—that is, how much it tends to resistvertical motion.

Lapse Rates

In chapter 1, it was shown that temperature usuallydecreases with altitude and that the rate at which itdecreases is called the lapse rate. The lapse rate,commonly expressed in degrees Fahrenheit per 1,000feet, gives a direct measurement of the atmospheres sresistance to vertical motion. The degree of stability ofthe atmosphere may vary from layer to layer as

2-14

STANDARDAIR

MOIST AIR ISLIGHTER THANSTANDARD AIR.

HOT AIR ISLIGHTER THANSTANDARD AIR.

- ON LOOSING ITSMOISTURE IT THENBECOMES...

DRY AIR ISHEAVIER THANSTANDARD AIR.

COLD AIR ISHEAVIER THANSTANDARD AIR.

- IF COOLED, ITTHEN BECOMES...

STANDARDAIR RELATIVE

OR TAIR WILL RISE.(UNSTABLE)

HOT MOIS

RELATIVEOR

AIR WILL SINK.(STABLE)

COLD DRY

B

A

AG5f0207

Figure 2-7.—Moisture content and temperature determines weight of air.

indicated by changes of lapse rate with height. (Seetable 2-1 and fig. 2-8.)

DRY ADIABATIC LAPSE RATE.—If a parcelof air is lifted, its pressure is DECREASED, sincepressure decreases with height, and its temperature fallsdue to the expansion. If the air is dry and the process isadiabatic, the rate of temperature fall is 1°C per100 meters of lift (10°C per Kin), or 5 l/2°F per 1,000feet of lift. If that parcel descends again to higherpressure, its temperature then INCREASES at the rateof 1°C per 100 meters or 5 1/2°F per 1,000 feet. This isknown as the dry adiabatic lapse rate.

MOIST (SATURATION) ADIABATIC LAPSERATE.—When a mass of air is lifted, it cools at the dryadiabatic lapse rate of 5 1/2°F per 1,000 feet as long asit remains unsaturated (relative humidity below 100percent). If the original moisture is being carried alongwith the mass as it ascends and it cools to its saturationtemperature, the relative humidity reaches 100 percent.Condensation takes place with further cooling. Foreach gram of water condensed, about 597 calories ofheat are liberated. This latent heat of condensation isabsorbed by the air, and the adiabatic cooling rate isdecreased to 20 to 3°F per 1,000 feet instead of 5 1/2°Fper 1,000 feet. The process during the saturatedexpansion of the air is called the saturation adiabatic,the moist adiabatic, or the pseudoadiabatic process.The pseudoadiabatic process assumes that moisturefalls out of the air as soon as it condenses.

Assume that a saturated parcel of air having atemperature of 44°F is at 5,000 feet and is forced over a

12,000-foot mountain. Condensation occurs from5,000 to 12,000 feet so that the parcel cools at the moistadiabatic rate (3°F per 1,000 ft) and reaches atemperature of approximately 23°F at the top of themountain. Assuming that the condensation in the formof precipitation has fallen out of the air during theascent, the parcel heats at the dry adiabatic rate as itdescends to the other side of the mountain. When itreaches the 5,000-foot level, the parcel has descended7,000 feet at a rate of 5 1/2°F per 1,000 feet. This resultsin an increase of 38.5°F. Adding the 38.5°F increase tothe original 12,000 feet temperature of 23°F, the parcelhas a new temperature of 61.5°F.

AVERAGE ADIABATIC LAPSE RATE.—Theaverage lapse rate lies between the dry adiabatic and themoist adiabatic at about 3.3°F per 1,000 feet.

SUPERADIABATIC LAPSE RATE.—Thesuperadiabatic lapse rate is a decrease in temperature ofmore than 5 1/2°F per 1,000 feet and less than 15°F per1,000 feet.

AUTOCONVECTIVE LAPSE RATE.—Theautoconvective lapse rate is the decrease of more than15°F per 1,000 feet. This lapse rate is rare and is usuallyconfined to shallow layers.

2-15

AG5t0201

Lapse rate Per 1,000feet

Per 100meters

Dry adiabaticSaturation (moist)

adiabaticAverageSuperadiabaticAutoconvective

5 1/2 F

2-3 F3.3 F

5 1/2-15 FMore than

15 F

1 C

.55 C

.65 C1-3.42 CMore than

3.42 C

Table 2-1.—Lapse Rates of Temperature

MOISTADIABATIC

AVERAGELAPSERATE

"DRY"ADIABATIC

SUPERADIABATIC

AUTOCONVECTIVE

AG5f0208

Figure 2-8.—Adiabatic lapse rates.

Types of Stability

In figure 2-9 a bowl is set on a flat surface with aball placed inside it. The ball rests in the bottom of thebowl; but, if you push the ball in any direction, it seeksout the bottom of the bowl again. This is referred to asABSOLUTE STABILITY (A in fig. 2-9). Turn thebowl upside down, position the ball anywhere on thebowl’s bottom surface (B in fig. 2-9) and the ball startsmoving on its own without any other force beingapplied. This is a condition of ABSOLUTEINSTABILITY. If you now remove the bowl and placethe ball on the flat surface (C in fig. 2-9), you haveNEUTRAL STABILITY—that is, if a force is appliedto the ball, it moves; but if the force is removed, the ballstops.

Air in the atmosphere reacts in a similar mannerwhen moved up or down. If it is moved up and becomesdenser than the surrounding air, it returns to its originalposition and is considered STABLE. If it becomes lessdense than the surrounding air, it continues to rise and is

considered UNSTABLE. When density remains thesame as the surrounding air after being lifted, it isconsidered NEUTRALLY STABLE, with no tendencyto rise or sink.

Equilibrium of Dry Air

The method used for determining the equilibriumof air is the parcel method, wherein a parcel of air islifted and then compared with the surrounding air todetermine its equilibrium. The dry adiabatic lapse rateis always used as a reference to determine the stabilityor instability of dry air (the parcel).

ABSOLUTE INSTABILITY.—Consider acolumn of air in which the actual lapse rate is greaterthan the dry adiabatic lapse rate. The actual lapse rate isto the left of the dry adiabatic lapse rate on the Skew-Tdiagram (fig. 2-10). If the parcel of air at point A isdisplaced upward to point B, it cools at the dryadiabatic lapse rate. Upon arriving at point B, it iswarmer than the surrounding air. The parcel therefore

2-16

AG5f0209

BALL IN BOWL FORCE MOVES BALL DISPLACED BALL OSCILLATES

(A) ABSOLUTE STABILITY

(C) NEUTRAL STABILITY

(B) ABSOLUTE INSTABILITY

BALL BALANCED ON BOWL RELEASE OF FORCEPERMITS BALL TO MOVE

BALL CONTINUESTO MOVE

BALL WILL CONTINUETO MOVE

BALL RESTING ON TABLE FORCE MOVES BALL FORCE REMOVED,BALL STOPS

BALL REMAINS INNEW POSITION

BALL EVENTUALLYRETURNS TO ORIGINAL

POSITION

Figure 2-9.—Analogy depiction of stability.

has a tendency to continue to rise, seeking air of its owndensity. Consequently the column becomes unstable.From this, the rule is established that if the lapse rate ofa column of air is greater than the dry adiabatic lapserate, the column is in a state of ABSOLUTEINSTABILITY. The term absolute is used because thisapplies whether the air is dry or saturated, as isevidenced by displacing upward a saturated parcel of

air from point A along a saturation adiabat to point B.The parce1 is more unstable than if displaced along adry adiabat.

STABILITY.—Consider a column of dry air inwhich the actual lapse rate is less than the dry adiabaticlapse rate. The actual lapse rate is to the right of the dryadiabatic lapse rate on the Skew-T diagram (fig. 2-11).

2-17

AG5f0210

A

SATURATIONADIABATIC

LAPSERATE

DRYADIABATIC

LAPSERATE

ACTUALLAPSERATE

-10 0 10

B B1

POINTS B (DRY ADIABATIC) AND B (MOIST ADIABATIC)WARMER THAN SURROUNDING AIR

1

Figure 2-10.—Absolute instability (any degree of saturation).

AG5f0211-10 0 10

DRY ADIABATICLAPSE RATE

ACTUALLAPSE RATE

B

POINT B COLDER THANTHE SURROUNDING AIR

A

Figure 2-11.—Stability (dry air).

If the parcel at point A were displaced upward to pointB, it would cool at the dry adiabatic lapse rate; and uponarriving at point B, it would be colder than thesurrounding air. It would, therefore, have a tendency toreturn to its original level. Consequently, the column ofair becomes stable. From this, the rule is establishedthat if the actual lapse rate of a column of DRY AIR isless than the dry adiabatic lapse rate, the column isstable.

NEUTRAL STABILITY.—Consider a column ofDRY AIR in which the actual lapse rate is equal to thedry adiabatic lapse rate. The parcel cools at the dryadiabatic lapse rate if displaced upward. It would at alltime be at the same temperature and density as thesurrounding air. It also has a tendency neither to returnto nor to move farther away from its original position.Therefore, the column of dry air is in a state ofNEUTRAL STABILITY.

Equilibrium of Saturated Air

When saturated air is lifted, it cools at a ratedifferent from that of dry air. This is due to release ofthe latent heat of condensation, which is absorbed bythe air. The rate of cooling of moist air is known as thesaturation adiabatic lapse rate. This rate is used as a

reference for determining the equilibrium of saturatedair.

ABSOLUTE STABILITY.—Consider a columnof air in which the actual lapse rate is less than thesaturation adiabatic lapse rate. The actual lapse rate isto the right of the saturation adiabatic lapse rate on theSkew T diagram (fig. 2-12). If the parcel of saturated airat point A is displaced upward to point B, it cools at thesaturation adiabatic lapse rate. The air upon arriving atpoint B becomes colder than the surrounding air. Thelayer, therefore, would be in a state of ABSOLUTESTABILITY. From this, the following rule isestablished: If the actual lapse rate for a column of air isless than the saturation adiabatic lapse rate, the columnis absolutely stable and the parcel would return to itsoriginal position. Dry air cools dry adiabatically and isalso colder than the surrounding air. Therefore, this ruleapplies to all air, as is evidenced when an unsaturatedparcel of air is displaced upward dry adiabatically topoint B. Here, the parcel is more stable than the parceldisplaced along a saturation adiabat.

INSTABILITY.—Consider now a column of air inwhich the actual lapse rate is greater than the saturationadiabatic lapse rate (fig. 2-13). If a parcel of moist air atpoint A is displaced upward to point B, it cools at the

2-18

AG5f0212 -10 0 10

B B

SATURATIONADIABATIC

LAPSERATE

DRYADIABATIC

LAPSERATE

ACTUALLAPSERATE

1

POINTS B (DRY ADIABATIC) AND B (MOIST ADIABATIC)WARMER THAN SURROUNDING AIR

1

A

Figure 2-12.—Absolute stability (any degree of saturation).

saturation adiabatic lapse rate. Upon arriving at point Bthe parcel is then warmer than the surrounding air. Forthis reason, it has a tendency to continue moving fartherfrom its original position. The parcel, therefore, is in astate of INSTABILITY. The following rule isapplicable. If the actual lapse rate for a column ofSATURATED (MOIST) AIR is greater than thesaturation adiabatic lapse rate, the column is unstable.

NEUTRAL STABILITY.—Consider a column ofsaturated air in which the actual lapse rate is equal to thesaturation adiabatic lapse rate. A parcel of air displacedupward cools at the saturation adiabatic lapse rate andis at all times equal in temperature to the surroundingair. On that account it tends neither to move fartheraway from nor to return to its original level. Therefore,it is in a state of NEUTRAL STABILITY. The rule forthis situation is that if the actual lapse rate for a columnof saturated air is equal to the saturation adiabatic lapserate, the column is neutrally stable.

Conditional Instability

In the treatment of stability and instability so far,only air that was either dry or saturated was considered.Under normal atmospheric conditions natural air isunsaturated to begin with, but becomes saturated iflifted high enough. This presents no problem if theactual lapse rate for the column of air is greater than the

dry adiabatic lapse rate (absolutely unstable) or if theactual lapse rate is less than the saturation adiabaticlapse rate (absolutely stable). However, if the lapse ratefor a column of natural air lies between the dryadiabatic lapse rate and the saturation adiabatic lapserate, the air may be stable or unstable, depending uponthe distribution of moisture. When the actual lapse rateof a column of air lies between the saturation adiabaticlapse rate and the dry adiabatic lapse rate, theequilibrium is termed CONDITIONALINSTABILITY, because the stability is conditioned bythe moisture distribution. The equilibrium of thiscolumn of air is determined by the use of positive andnegative energy areas as analyzed on a Skew-T, Log Pdiagram. The determination of an area as positive ornegative depends upon whether the parcel is beinglifted mechanically (by a front or orographic barriers)or by convective means and whether the environment iscolder or warmer than the ascending parcel. Positiveareas are conducive to instability. Negative areas areconducive to stability.

Conditional instability may be one of three types.The REAL LATENT type is a condition in which thepositive area is larger than the negative area (potentiallyunstable). The PSEUDOLATENT type is a condition inwhich the positive area is smaller than the negative area(potentially STABLE). The STABLE type is acondition in which there is no positive area. Figure

2-19

AG5f0213

-10 0 10

B

SATURATIONADIABATIC

LAPSE RATE

ACTUALLAPSERATE

POINT B WARMER THANTHE SURROUNDING AIR

A

Figure 2-13.—Instability (saturated air).

2-14 shows an example of analyzed positive and thenegative energy areas as they would appear on aSkew-T, Log P diagram.

Autoconvection

AUTOCONVECTION is a condition startedspontaneously by a layer of air when the lapse rate oftemperature is such that density increases withelevation. For density to increase with altitude, thelapse rate must be equal to or exceed 3.42°C per 100meters. (This is the AUTOCONVECTIVE LAPSE

RATE.) An example of this condition is found to existnear the surface of the earth in a road mirage or a dustdevil. These conditions occur over surfaces that areeasily heated, such as the desert, open fields, etc.; theyare usually found during periods of intense surfaceheating.

Convection Stability and Instability

In the discussion so far of convection stability andinstability, PARCELS of air have been considered. Letus now examine LAYERS of air. A layer of air that is

2-20

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30

20

-10

0

10

ISOBARS mb

SAT

UR

AT

ION

AD

IAB

AT

++

++++++++++++

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AREA(BLUE SHADING)

EL

POSITIVEAREA

(RED SHADING)

NEGATIVEAREA(BLUE

SHADING)

SA

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RA

TIO

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IXIN

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AT

IOLIN

E

ISO

TH

ER

MS

C

DR

YA

DIA

BAT

LFC

LCL

POINT A

T T

TT

d

d

AG5f0214

Figure 2-14.—Example of positive and negative energy areas (mechanical lifting).

originally stable may become unstable due to moisturedistribution if the entire layer is lifted.

Convective stability is the condition that occurswhen the equilibrium of a layer of air, because of thetemperature and humidity distribution, is such thatwhen the entire layer is lifted, its stability is increased(becomes more stable).

Convective instability is the condition ofequilibrium of a layer of air occurring when thetemperature and humidity distribution is such that whenthe entire layer of air is lifted, its instability is increased(becomes more unstable).

CONVECTIVE STABILITY.—Consider a layerof air whose humidity distribution is dry at the bottomand moist at the top. If the layer of air is lifted, the topand the bottom cool at the same rate until the topreaches saturation. Thereafter, the top cools at a slowerrate of speed than the bottom. The top cools saturationadiabatically (.55°C/100 meters), while the bottomcontinues to cool dry adiabatically (1°C/100 meters).The lapse rate of the layer then decreases; hence, thestability increases. The layer must be initially unstableand may become stable when lifting takes place.

CONVECTIVE INSTABILITY.—Consider alayer of air in which the air at the bottom is moist andthe air at the top of the layer is dry. If this layer of air islifted, the bottom and the top cool dry adiabaticallyuntil the lower portion is saturated. The lower part thencools saturation adiabatically while the top of the layeris still cooling dry adiabatically. The lapse rate thenbegins to increase and instability increases.

To determine the convective stability or instabilityof a layer of air, you should first know why you expectthe lifting of a whole layer. The obvious answer is anorographic barrier or a frontal surface. Next, determinehow much lifting is to be expected and at what level itcommences. Lifting of a layer of air close to the surfaceof the Earth is not necessary. The amount of lifting, ofcourse, depends on the situation at hand. Figure 2-15illustrates the varying degrees of air stability that aredirectly related to the rate at which the temperaturechanges with height.

Determining Bases of Convective Type Clouds

You have seen from our foregoing discussion thatmoisture is important in determining certain stabilityconditions in the atmosphere. You know, too, that thedifference between the temperature and the dew point isan indication of the relative humidity. When the dewpoint and the temperature are the same, the air issaturated and some form of condensation cloud may beexpected. This lends itself to a means of estimating theheight of the base of clouds formed by surface heatingwhen the surface temperature and dew point are known.You know that the dew point decreases in temperatureat the rate of 1°F per 1,000 feet during a lifting process.The ascending parcel in the convective currentexperiences a decrease in temperature of about 5 1/2°Fper 1,000 feet. Thus the dew point and the temperatureapproach each other at the rate of 4 1/2°F per 1,000 feet.As an example, consider the surface temperature to be80°F and the surface dew point 62°F, a difference of18°F. This difference, divided by the approximate ratethe temperature approaches the dew point (4 1/2°F per1,000 ft) indicates the approximate height of the base ofthe clouds caused by this lifting process (18 ÷ 4 1/2) ×

2-21

AG5f0215

STABLE

CONDITIONALLY UNSTABLE

UNSTABLE

NORMALLAPSERATE

MOISTADIABATIC

ADIABATIC ISOTHERMAL

SUPERADIABATIC

INVERSION

Figure 2-15.—Degrees of stability in relation to temperaturechanges with height.

1000 = 4,000 feet). This is graphically shown in figure2-16.

This method cannot be applied to all cloud types. Itis limited to clouds formed by convection currents, suchas summertime cumulus clouds, and only in the localitywhere the clouds form. It is not valid around maritimeor mountainous areas.

Stability in Relation to Cloud Type

The degree of stability of the atmosphere helps todetermine the type of clouds formed. For example,

figure 2-17 shows that if stable air is forced to ascend amountain slope, clouds will be layerlike with littlevertical development and little or no turbulence.Unstable air, if forced to ascend the slope, causesconsiderable vertical development and turbulence inthe clouds. The base of this type of cloud can bedetermined by mechanical lifting as analyzed on aSkew-T.

REVIEW QUESTIONS

Q2-11. What are the two basic kinds of atmosphericenergy?

2-22

MOIST ADIABATICIN CLOUD

DEWPOINTLAPSE RATE

DRY ADIABAT

40 50 60 70 80

5,000

4,000

3,000

2,000

1,000

THE AIR

DEGREES FAHRENHEITAG5f0216

HE

IGH

TIN

FE

ET

Figure 2-16.—Determine of cloud's base when the dew point and temperature are known.

AG5f0217

UNSTABLEAIR

STABLEAIR

Figure 2-17.—Illustration showing that very stable air retains its stability even when it is forced upward, forming a flat cloud. Airwhich is potentially unstable when forced upward becomes turbulent and forms a towering cloud.

Q2-12. What is the definition of lapse rate?

Q2-13. What is the rate of rise and fall dry adiabaticlapse rate?

Q2-14. What are the three types of conditionalinstability?

SUMMARY

Understanding the basic principles of atmosphericphysics is essential in order to comprehend howweather behaves. Analyzing meteorological situationsproperly depends upon what the Aerographer's Matelearns about atmospheric physics.

2-23

CHAPTER 3

ATMOSPHERIC CIRCULATION

To understand large-scale motions of theatmosphere, it is essential that the Aerographer’s Matestudy the general circulation of the atmosphere. Thesun’s radiation is the energy that sets the atmosphere inmotion, both horizontally and vertically. The rising andexpanding of the air when it is warmed, or thedescending and contracting of the air when it is cooledcauses the vertical motion. The horizontal motion iscaused by differences of atmospheric pressure; airmoves from areas of high pressure toward areas of lowpressure. Differences of temperature, the cause of thepressure differences, are due to the unequal absorptionof the Sun’s radiation by Earth’s surface. Thedifferences in the type of surface; the differentialheating; the unequal distribution of land and water; therelative position of oceans to land, forests to mountains,lakes to surrounding land, and the like, cause differenttypes of circulation of the air. Due to the relativeposition of Earth with respect to the Sun, much moreradiation is absorbed near the equator than at otherareas, with the least radiation being absorbed at or nearthe poles. Consequently, the principal factor affectingthe atmosphere is incoming solar radiation, and itsdistribution depends on the latitude and the season.

GENERAL CIRCULATION

LEARNING OBJECTIVE: Recognize howtemperature, pressure, winds, and the 3-celltheory affect the general circulation of Earth’satmosphere.

The general circulation theory attempts to explainthe global circulation of the atmosphere with someminor exceptions. Since Earth heats unequally, the heatis carried away from the hot area to a cooler one as a

result of the operation of physical laws. This globalmovement of air, which restores a balance of heat onEarth, is the general circulation.

WORLD TEMPERATURE GRADIENT

Temperature gradient is the rate of change oftemperature with distance in any given direction at anypoint. World temperature gradient refers to the changein temperature that exists in the atmosphere from theequator to the poles. The change in temperature ortemperature differential, which causes atmosphericcirculation can be compared to the temperaturedifferences produced in a pan of water placed over a gasburner. As the water is heated, it expands and its densityis lowered. This reduction in density causes thewarmer, less dense water to rise to the top of the pan. Asit rises, it cools and is forced to the edges of the pan.Here it cools further and then sinks to the bottom,eventually working its way back to the center of the panwhere it started. This process sets up a simplecirculation pattern due to successive heating andcooling.

Ideally, the air within the troposphere may becompared to the water in the pan. The most direct raysof the Sun hit Earth near the equator and cause a netgain of heat. The air at the equator heats, rises, andflows in the upper atmosphere toward both poles. Uponreaching the poles, it cools sufficiently and sinks backtoward Earth, where it tends to flow along the surface ofEarth back to the equator. (See fig. 3-1).

Simple circulation of the atmosphere would occuras described above if it were not for the followingfactors:

3-1

1. Earth rotates, resulting in an apparent forceknown as the Coriolis force (a deflecting force). Thisrotation results in a constant change to the area beingheated.

2. Earth is covered by irregular land and watersurfaces that heat at different rates.

Regions under the direct rays of the Sun absorbmore heat per unit time than those areas receivingoblique rays. The heat produced by the slanting rays ofthe Sun during early morning may be compared withthe heat that is produced by the slanting rays of the Sunduring winter. The heat produced by the more directrays at midday can be compared with the heat resultingfrom the more direct rays of summer. The length ofday, like the angle of the Sun’s rays, influences thetemperature. The length of day varies with the latitudeand the season. Near the equator there are about 12

hours of daylight with the Sun’s rays striking thesurface more directly. Consequently, equatorial regionsnormally do not have pronounced seasonal temperaturevariations.

During the summer in the Northern Hemisphere,all areas north of the equator have more than 12 hoursof daylight. During the winter the situation is reversed;latitudes north of the equator have less than 12 hours ofdaylight. Large seasonal variation in the length of theday and the seasonal difference in the angle at whichthe Sun’s rays reach Earth’s surface cause seasonaltemperature differences in middle and high latitudes.The weak temperature gradient in the subtropical areasand the steeper gradient poleward can be seen in figures3-2A and 3-2B. Note also how much steeper thegradient is poleward in the winter season of eachhemisphere as compared to the summer season.

3-2

NORTH POLE

SOUTH POLE

POLAR REGIONAREA OF LEAST HEATING

POLAR REGION AREA OFLEAST HEATING

AG5f0301

EQUATORIAL REGION

AREA OF GREATEST HEATING

Figure 3-1.—Simple circulation.

3-3

JANUARY

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OO

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75 75

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45 45

30 30

15 15

0 0

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30 30

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AG5f0302a

Figure 3-2A.—Mean world temperature for January.

JULY

JULY

40O

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80O

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70O

030 30 60 906090180 180150 150120 120

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AG5f0302b

O

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Figure 3-2B.—Mean world temperature for July.

3-4

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100 120 140 160 180 160 140 120 100 80 40 2020 060 100806040O OO OOO O

O OOOOO OO OO OO

HIGH

HIGH

H

LOW

Figure 3-3A.—Mean world pressure for January.

20

0

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40

100 120 140 160 180 160 140 120 100 80 40 2020 060 100806040

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Figure 3-3B.—Mean world pressure for July.

PRESSURE OVER THE GLOBE

The unequal heating of Earth’s surface due to itstilt, rotation, and differential insolation, results in thewide distribution of pressure over Earth’s surface.Study figures 3-3A and 3-3B. Note that a low-pressurearea lies along the intertropical convergence zone(ITCZ) in the equatorial region. This is due to thehigher temperatures maintained throughout the year inthis region. At the poles, permanent high-pressure areasremain near the surface because of the lowtemperatures in this area throughout the entire year.Mainly the "piling up" of air in these regions causes thesubtropical high-pressure areas at 30°N and S latitudes.Relatively high or low pressures also dominate otherareas during certain seasons of the year.

ELEMENTS OF CIRCULATION

Temperature differences cause pressuredifferences, which in turn cause air movements. Thefollowing sections show how air movements work andhow they evolve into the various circulations—primary,secondary, and tertiary.

To explain the observed wind circulation overEarth, three basic steps are used. The first step is toassume Earth does not rotate and is of uniform surface;that is, all land or all water. The second step is to rotateEarth, but still assume a uniform surface. The third stepis to rotate Earth and assume a non-uniform surface.For now, we deal with the first two steps, a non-rotatingEarth of uniform surface and a rotating Earth ofuniform surface.

Static Earth

The circulation on a non-rotating Earth is referredto as the thermal circulation because it is caused by the

difference in heating. The air over the equator is heatedand rises (low pressure); while over the poles the air iscooled and sinks (high pressure). This simplecirculation was shown in figure 3-1.

Rotating Earth

In thermal circulation, the assumption was madethat the Earth did not rotate, but of course this is nottrue. The rotation of Earth causes a force that affectsthermal circulation. This rotation results in thedeflection to the right of movement in the NorthernHemisphere, and to the left of the movement in theSouthern Hemisphere. This force is called the Coriolisforce. The Coriolis force is not a true force. It is anapparent force resulting from the west-to-east rotationof Earth. The effects, however, are real.

Arctic rivers cut faster into their right banks thantheir left ones. On railroads carrying only one-waytraffic, the right hand rails wear out faster than theleft-hand rails. Artillery projectiles must be aimed tothe left of target because they deflect to the right.Pendulum clocks run faster in high latitudes than inlower latitudes. All these phenomena are the result ofthe Coriolis force, which is only an apparent force. Themost important phenomena are that this force alsodeflects winds to the right in the Northern Hemisphere.Therefore, it is important to understand how this forceis produced.

As Earth rotates, points on the surface are movingeastward (from west to east) past a fixed point in spaceat a given speed. Points on the equator are moving atapproximately 1,000 miles per hour, points on the polesare not moving at all, but are merely pivoting, the pointssomewhere between are moving at speeds between1,000 and zero miles per hour depending upon theirrelative position. Refer to view A in figure 3-4.

3-5

A B C

AG5f0304

Figure 3-4.—Coriolis force.

Assume that a missile located at the North Pole islaunched at a target on the equator. The missile does nothave any eastward lateral velocity, but the target has aneastward velocity of 1,000 miles per hour. The result isthat the missile appears to be deflected to the right asthe target moves away from its initial position. Refer toview B in figure 3-4.

A similar condition assumes that a missile locatedon the equator is launched at a target at the North Pole.The missile has an eastward lateral velocity of 1,000miles per hour, while the target on the pole has nolateral velocity at all. Once again the missile appears tobe deflected to the right as a result of its initial eastwardlateral velocity. Refer to view C in figure 3-4.

Due to Earth’s rotation and the Coriolis effect, thesimple circulation now becomes more complex asshown in figure 3-5. The complex on resulting from theinterplay of the Coriolis effect with the flow of air isknown as the theory. (See fig. 3-6.)

3-CELL THEORY

According to the 3-cell theory, Earth is divided intosix circulation belts—three in the NorthernHemisphere and three in the Southern Hemisphere. Thedividing lines are the equator, latitude, and 60°N and Slatitude. The general circulation of the NorthernHemisphere is similar to those of the SouthernHemisphere. (Refer to fig. 3-6 during the followingdiscussion.)

First, note the tropical cell of the NorthernHemisphere that lies between the equator and 30°Nlatitude. Convection at the equator causes the air to heat

and rise, due to convection. When it reaches the upperportions of the troposphere, it tends to flow toward theNorth Pole. By the time the air has reached 30°Nlatitude, the Coriolis effect has deflected it so much thatit is moving eastward instead of northward. This resultsin a piling up of air (convergence) near 30°N latitudeand a descending current of air (subsidence) toward thesurface which forms a belt of high pressure. When thedescending air reaches the surface where it flowsoutward (divergence), part of it flows poleward tobecome part of the mid-latitude cell; the other partflows toward the equator, where it is deflected by theCoriolis effect and forms the northeast trades.

The mid-latitude cell is located between 30° and60°N latitude. The air, which comprises this cell,circulates poleward at the surface and equatorwardaloft with rising currents at 60° (polar front) anddescending currents at 300 (high-pressure belt).However, in general, winds both at the surface and aloftblow from the west. The Coriolis effect easily explainsthis for the surface wind on the poleward-movingsurface air. The west wind aloft is not as easilyexplained. Most authorities agree that this wind isfrictionally driven by the west winds in the two adjacentcells.

The polar cell lies between 60°N latitude and theNorth Pole. The circulation in this cell begins with aflow of air at a high altitude toward the pole. This flowcools and descends at the North Pole and forms ahigh-pressure area in the Polar Regions. After reachingthe surface of Earth, this air usually flows equatorwardand is deflected by the Coriolis effect so that it movesfrom the northeast. This air converges with thepoleward flow from the mid-latitude cell and isdeflected upward with a portion circulating polewardagain and the remainder equatorward. The outflow ofair aloft between the polar and mid-latitude cells causesa semi-permanent low-pressure area at approximately60°N latitude. To complete the picture of the world’sgeneral atmospheric circulation, we must associate thisprevailing wind and pressure belts with some basiccharacteristics.

WORLD WINDS

In the vicinity of the equator is a belt of light andvariable winds known as the doldrums. On thepoleward side of the doldrums are the trade winds; thepredominant wind system of the tropics. These easterlywinds are the most consistent on Earth, especially overthe oceans. Near 30°N and 30°S latitudes lie thesub-tropical high-pressure belts. Winds are light and

3-6

60O

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30

30

60

S

N

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S

EQUATOR

AG5f0305

Figure 3-5.—Coriolis effect on windflow.

variable. These areas are referred to as the horselatitudes. The prevailing westerlies, which are on thepoleward side of the subtropical high-pressure belt, arepersistent throughout the mid-latitudes. In the NorthernHemisphere, the direction of the westerlies at thesurface is from the southwest. In the SouthernHemisphere, westerlies are from the northwest. This isdue to the deflection area resulting from the Corioliseffect as the air moves poleward.

Poleward of the prevailing westerlies, near 60°Nand 60°S latitudes, lies the belt of low-pressure basicpressure known as the polar front zone. Here,converging winds result in ascending air currents andconsequent poor weather.

WIND THEORY

Newton’s first two laws of motion indicate thatmotion tends to be in straight lines and only deviatesfrom such lines when acted upon by another force or bya combination of forces. Air tends to move in a straight

line from a high-pressure area to a low-pressure area.However, there are forces that prevent the air frommoving in a straight line.

Wind Forces

There are four basic forces that affect thedirectional movement of air in our atmosphere:pressure gradient force (PGF), the Coriolis effect,centrifugal force, and frictional force. These forces,working together, affect air movement. The forces thatare affecting it at that particular time determine thedirection that the air moves. Also, the different namesgiven to the movement of the air (geostrophic wind,gradient wind, etc.) depends on what forces areaffecting it.

Pressure Gradient

The rate of change in pressure in a directionperpendicular to the isobars is called pressure gradient.Pressure applied to a fluid is exerted equally in all

3-7

POLAR FRONT

POLAR FRONT

WESTERLY

WESTERLY

WIND

WIND

BELT

BELT

POLAR EASTERLIES

POLAR EASTERLIES

60O

NORTHEAST TRADEWINDS

SOUTHEAST TRADEWINDS

INTERTROPICAL CONVERGENCE ZONE

DOLDRUM BELTDRUM

30O

60O

30O

SUBTROPICAL HIGH- PRESSURE BELT

SUBTROPICAL HIGH- PRESSURE BELT

DOL-

POLAR HIGH

CUMULONIMBUSTOPS AS HIGH AS60,000 FT CEILINGBELOW 1,000 FT

WINDS EASTERLY AT SURFACEWESTERLY ALOFT

WINDS WESTERLYAT ALL ELEVATIONS

WINDS EASTERLY TO 5,000 FTTHEN WESTERLY

WINDS VARIABLE

WINDS EASTERLYTO 25,000 FT,

THEN WESTERLY

60,000 FT.

TROPOPA

USE 25,000 FT.

AG5f0306

LT

Figure 3-6.—Idealized pattern of the general circulation. (The 3-cell theory).

3-8

1012

1016

1020

H

WEAK OR FLATPRESSUREGRADIENT

STRONG OR STEEPPRESSUREGRADIENT

AG5f0307

Figure 3-7.—Horizontal pressure gradient.

A. CROSS SECTION OF A VERTICAL PRESSURE GRADIENT ALONG LINE AA

HORIZONTAL PRESSURE GRADIENTB.

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L

A

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OU

GH

AG5f0308

Figure 3-8.—Cross section of a vertical pressure gradient along line AA.

directions throughout the fluid; e.g., if a pressure of1013.2 millibars is exerted downward by theatmosphere at the surface, this same pressure is alsoexerted horizontally outward at the surface. Therefore,a pressure gradient exists in the horizontal (along thesurface) as well as the vertical plane (with altitude) inthe atmosphere.

HORIZONTAL PRESSURE GRADIENT.—The horizontal pressure gradient is steep or strongwhen the isobars determining the pressure system (fig.3-7) are close together. It is flat or weak when theisobars are far apart.

VERTICAL PRESSURE GRADIENT.—Ifisobars are considered as depicting atmospherictopography, a high-pressure system represents a hill ofair, and a low-pressure system represents a depressionor valley of air. The vertical pressure gradient alwaysindicates a decrease in pressure with altitude, but therate of pressure decrease (gradient) varies directly withchanges in air density with altitude. Below 10,000 feetaltitude, pressure decreases approximately 1 inch ofmercury per 1,000 feet in the standard atmosphere. Thevertical cross section through a high and low (view A infig. 3-8) depicts the vertical pressure gradient. Asurface weather map view of the horizontal pressuregradient in the same high and low is illustrated in viewB of the figure 3-8.

Pressure Gradient Force

The variation of heating (and consequently thevariations of pressure) from one locality to another isthe initial factor that produces movement of air or wind.The most direct path from high to low pressure is thepath along which the pressure is changing most rapidly.The rate of change is called the pressure gradient.Pressure gradient force is the force that moves air froman area of high pressure to an area of low pressure. Thevelocity of the wind depends upon the pressuregradient. If the pressure gradient is strong, the windspeed is high. If the pressure gradient is weak, the windspeed is light. (See fig. 3-7.)

Figure 3-9 shows that the flow of air is from thearea of high pressure to the area of low pressure, but itdoes not flow straight across the isobars. Instead theflow is circular around the pressure systems. Pressuregradient force (PGF) causes the air to begin movingfrom the high-pressure to the low-pressure system.Coriolis (deflective) force and centrifugal force thenbegin acting on the flow in varying degrees. In thisexample, frictional force is not a factor.

Coriolis Effect

If pressure gradient force were the only forceaffecting windflow, the wind would blow at right anglesacross isobars (lines connecting points of equalbarometric pressure) from high to low pressure. Thewind actually blows parallel to isobars above anyfrictional level. Therefore, other factors must beaffecting the windflow; one of these factors is therotation of Earth. A particle at rest on Earth’s surface isin equilibrium. If the particle starts to move because ofa pressure gradient force, its relative motion is affectedby the rotation of Earth. If a mass of air from theequator moves northward, it is deflected to the right, sothat a south wind tends to become a southwesterlywind.

In the Northern Hemisphere, the result of theCoriolis effect is that moving air is deflected to the rightof its path of motion. This deflection to the right isdirectly proportional to the speed of the wind; the fasterthe wind speed, the greater the deflection to the right,and conversely, the slower the wind speed, the less thedeflection to the right. Finally, this apparent deflectiveforce is stronger at the Polar Regions than at theequator.

Centrifugal Force

According to Newton’s first law of motion, a bodyin motion continues in the same direction in a straight

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Figure 3-9.—Examples of circulation around high and lowpressure systems.

line and with the same speed unless acted upon by someexternal force. Therefore, for a body to move in acurved path, some force must be continually applied.The force restraining bodies that move in a curved pathis called the centripetal force; it is always directedtoward the center of rotation. When a rock is whirledaround on a string, the centripetal force is afforded bythe tension of the string.

Newton’s third law states that for every action thereis an equal and opposite reaction. Centrifugal force isthe reacting force that is equal to and opposite indirection to the centripetal force. Centrifugal force,then, is a force directed outward from the center ofrotation.

As you know, a bucket of water can be swung overyour head at a rate of speed that allows the water toremain in the bucket. This is an example of bothcentrifugal and centripetal force. The water is held inthe bucket by centrifugal force tending to pull itoutward. The centripetal force, the force holding thebucket and water to the center, is your arm swinging thebucket. As soon as you cease swinging the bucket, theforces cease and the water falls out of the bucket. Figure3-10 is a simplified illustration of centripetal andcentrifugal force.

High- and low-pressure systems can be comparedto rotating discs. Centrifugal effect tends to fling air outfrom the center of rotation of these systems. This forceis directly proportional to the wind speeds, the faster thewind, and the stronger the outward force. Therefore,when winds tend to blow in a circular path, centrifugaleffect (in addition to pressure gradient and Corioliseffects) influences these winds.

Frictional Force

The actual drag or slowing of air particles incontact with a solid surface is called friction. Frictiontends to retard air movement. Since Coriolis forcevaries with the speed of the wind, a reduction in thewind speed by friction means a reduction of theCoriolis force. This results in a momentary disruptionof the balance. When the new balance (includingfriction) is reached, the air flows at an angle across theisobars from high pressure to low pressure. (Pressuregradient force is the dominant force at the surface.) Thisangle varies from 10 degrees over the ocean to morethan 45 degrees over rugged terrain. Frictional effectson the air are greatest near the ground, but the effectsare also carried aloft by turbulence. Surface friction iseffective in slowing the wind to an average altitude of2,000 feet (about 600 meters) above the ground. Abovethis level, called the gradient wind level or the secondstandard level the effect of friction decreases rapidlyand may be considered negligible. Air above 2,000 feetnormally flows parallel to the isobars.

WIND TYPES

Since there is a direct relationship betweenpressure gradient and wind speed and direction, wehave a variety of wind types to deal with. We discussbelow the relationship of winds and circulations, theforces involved, and the effect of these factors on thegeneral circulation.

Geostrophic and Gradient Wind

On analyzed surface weather charts, points of equalpressure are connected by drawn lines referred to asisobars, while in upper air analysis, points of equalheights are connected and called isoheights.

The variation of these heights and pressures fromone locality to another is the initial factor that producesmovement of air, or wind. Assume that at three stationsthe pressure is lower at each successive point. Thismeans that there is a horizontal pressure gradient (adecrease in pressure in this case) for each unit distance.With this situation, the air moves from the area ofgreater pressure to the area of lesser pressure.

If the force of the pressure were the only factoracting on the wind, the wind would flow from high tolow pressure, perpendicular to the isobars. Sinceexperience shows the wind does not flow perpendicularto isobars, but at a slight angle across them and towardsthe lower pressure, it is evident that other factors are

3-10

CENTRIPEDAL

CENTRIFUGAL

AG5f0310

Figure 3-10.—Simplified illustration of centripetal andcentrifugal force.

involved. These other factors are the Coriolis effect,frictional force, and centrifugal effect. When a unit ofair moves with no frictional force involved, themovement of air is parallel to the isobars. This wind iscalled a gradient wind. When the isobars are straight, soonly Coriolis and pressure gradient forces are involved,it is termed a geostrophic wind.

Let’s consider a parcel of air from the time it beginsto move until it develops into a geostrophic wind. Assoon as a parcel of air starts to move due to the pressuregradient force, the Coriolis force begins to deflect itfrom the direction of the pressure gradient force. (Seeviews A and B of fig. 3-11). The Coriolis force is theapparent force exerted upon the parcel of air due to therotation of Earth. This force acts to the right of the pathof motion of the air parcel in the Northern Hemisphere(to the left in the Southern Hemisphere). It always actsat right angles to the direction of motion. In the absenceof friction, the Coriolis force changes the direction ofmotion of the parcel until the Coriolis force and thepressure gradient force are in balance. When the twoforces are equal and opposite, the wind blows parallelto the straight isobars (view C in fig. 3-11). The Coriolisforce only affects the direction, not the speed of themotion of the air. Normally, Coriolis force is not greaterthan the pressure gradient force. In the case ofsuper-gradient winds, Coriolis force may be greaterthan the pressure gradient force. This causes the windto deflect more to the right in the Northern Hemisphere,or toward higher pressure.

Under actual conditions, air moves around high andlow pressure centers toward lower pressure. Turn back

to figure 3-9. Here, the flow of air is from the area ofhigh pressure to the area of low pressure, but, as wementioned previously, it does not flow straight acrossthe isobars (or isoheights). Instead, the flow is circulararound the pressure systems.

The Coriolis force commences deflecting the pathof movement to the right (Northern Hemisphere) or left(Southern Hemisphere) until it reaches a point where abalance exists between the Coriolis and the pressuregradient force. At this point the air is no longerdeflected and moves forward around the systems.

Once circular motion around the systems isestablished, then centrifugal force must be considered.Centrifugal force acts outward from the center of boththe highs and the lows with a force dependent upon thevelocity of the wind and the degree of curvature of theisobars. However, the pressure gradient force is actingtowards the low; therefore, the flow in that directionpersists. When the flow is parallel to the curved portionof the analysis in figure 3-9, it is a gradient wind. Whenit is moving parallel to that portion of the analysisshowing straight flow, it is a geostrophic wind.

We defined pressure gradient as being a change ofpressure with distance. This means that if the isobarsare closely spaced, then the pressure change is greaterover a given distance; it is smaller if they are widelyspaced. Therefore, the closer the isobars, the faster theflow. Geostrophic and gradient winds are alsodependent, to a certain extent, upon the density of theatmosphere and the latitude. If the density and thepressure gradient remain constant and the latitudeincreases, the wind speed decreases. If the latitude

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AG5f0311

Figure 3-11.—Development cycle of a geostrophic wind.

decreases, the wind speed increases. If the density andthe latitude remain constant and the pressure gradientdecreases, the wind speed decreases. If the pressuregradient and the latitude remain constant and thedensity decreases, the wind speed increases. If thedensity increases, the wind speed decreases. Truegeostrophic wind is seldom observed in nature, but theconditions are closely approximated on upper-levelcharts.

Cyclostrophic Wind

In some atmospheric conditions, the radius ofrotation becomes so small that the centrifugal forcebecomes quite strong in comparison with the Coriolisforce. This is particularly true in low latitudes where theCoriolis force is quite small to begin with. In this case,the pressure gradient force is nearly balanced by thecentrifugal force alone. When this occurs, the wind issaid to be cyclostrophic. By definition, a cyclostrophicwind exists when the pressure gradient force isbalanced by the centrifugal force alone.

This exact situation rarely exists, but is so nearlyreached in some situations that the small Coriolis effectis neglected and the flow is said to be cyclostrophic.Winds in a hurricane or typhoon and the winds around atornado are considered cyclostrophic.

Movement of Wind around Anticyclones

The movement of gradient winds aroundanticyclones is affected in a certain manner by thepressure gradient force, the centrifugal force, and theCoriolis force. The pressure gradient force acts fromhigh to low pressure, and the Coriolis force actsopposite to the pressure gradient force and at rightangles to the direction of movement of the parcel of air.The centrifugal force acts at right angles to the path ofmotion and outward from the center about which theparcel is moving. (See fig. 3-12.) In the case of ahigh-pressure center, the pressure gradient force andthe centrifugal force balance the Coriolis force. Thisphenomenon may be expressed in the followingmanner:

PG + CF = D

Movement of Wind around Cyclones

As in the case of anticyclones, the pressure gradientforce, the centrifugal force, and the Coriolis force affectgradient winds around cyclones, but the balance of theforces is different. (See fig. 3-12.) In a cyclonicsituation the Coriolis force and the centrifugal forcebalance the pressure gradient force. This balance maybe expressed in the following manner:

3-12

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ANTICYCLONE CYCLONE

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AG5f0312

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Figure 3-12.—Forces acting on pressure systems.

PG = D+ CF

Centrifugal force acts with the pressure gradientforce when the circulation is anticyclonic and againstthe pressure gradient force when the circulation iscyclonic. Therefore, wind velocity is greater in ananticyclone than in a cyclone of the same isobaricspacing.

Variations

It has been determined that, given the same density,pressure gradient, and latitude, the wind is weakeraround a low-pressure cell than a high-pressure cell.This is also true for gradient and geostrophic winds.The wind we observe on a synoptic chart is usuallystronger around low cells than high cells because thepressure gradient is usually stronger around thelow-pressure cell.

Geostrophic and Gradient Wind Scales

The geostrophic wind is stronger than the gradientwind around a low and is weaker than a gradient wind

around a high. This is why the isobar spacing andcontour spacing, for a curved flow, differs from thatdetermined by a geostrophic wind scale. If the flowunder consideration is around a high-pressure cell, theisobars are farther apart than indicated by thegeostrophic wind scale. If the flow is around alow-pressure cell, the isobars are closer together thanindicated by the geostrophic wind scale.

Geostrophic and gradient wind scales are used todetermine the magnitude of these winds (based onisobar or contour spacing) and to determine the isobaror contour spacing (based on observed wind speeds).There are a number of scales available for measuringgeostrophic and gradient flow of both surface and upperair charts.

Weather plotting charts used by the NavalOceanography Command has geostrophic wind scalesprinted on them for both isobaric and contour spacing.The most common scales in general use can be used forboth surface and upper air charts. The scales are in 4mband 60m intervals. An example of a geostrophic windscale is shown in figure 3-13. Note that latitude

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Figure 3-13.—Geostrophic wind scale.

accounts for the increases in gradients. In tropicalregions, the geostrophic wind scales become lessreliable because pressure gradients are generally ratherweak.

REVIEW QUESTIONS

Q3-1. Which two factors influence the Earth'stemperature?

Q3-2. What are the major factors that result in thewide distribution of pressure over the earth'ssurface?

Q3-3. What effect does Coriolis force have onthermal circulation in the NorthernHemisphere?

Q3-4. According to the 3-cell theory, how manycirculation belts are there?

Q3-5. According to the 3-cell theory, what type ofpressure system would you normally find at30 degrees north latitude?

Q3-6. What is the predominant wind system in thetropics?

Q3-7. Name two types of pressure gradient.

Q3-8. What is the difference between centrifugalforce and centripetal force?

Q3-9. What is the difference between gradient windand geostrophic wind?

Q3-10. What is the relationship between centrifugalforce and pressure gradient force aroundanticyclones?

SECONDARY CIRCULATION

LEARNING OBJECTIVE: Determine howcenters of action, migratory systems, andseasonal variations affect secondary aircirculations.

Now that you have a picture of the generalcirculation of the atmosphere over Earth, the next stepis to see how land and water areas offset the generalcirculation. The circulations caused by the effect ofEarth’s surfaces, its composition and contour, areknown as secondary circulations. These secondarycirculations give rise to winds that often cancel out thenormal effect of the great wind systems.

There are two factors that cause the pressure beltsof the primary circulation to break up into closedcirculations of the secondary circulations. They are the

non-uniform surface of the earth and the differencebetween heating and cooling of land and water. Thesurface temperature of oceans changes very littleduring the year. However, land areas sometimesundergo extreme temperature changes with the seasons.In the winter, large high-pressure areas form over thecold land and the low-pressure areas form over therelatively warm oceans. The reverse is true in summerwhen highs are over water and lows form over thewarm land areas. The result of this difference in heatingand cooling of land and water surfaces is known as thethermal effect.

Circulation systems are also created by theinteraction of wind belts of pressure systems or thevariation in wind in combination with certaindistributions of temperature and/or moisture. This isknown as the dynamic effect. This effect rarely, if ever,operates alone in creating secondary systems, as mostof the systems are both created and maintained by acombination of the thermal and dynamic effects.

CENTERS OF ACTION

The pressure belts of the general circulation arerarely continuous. They are broken up into detachedareas of high and low pressure cells by the secondarycirculation. The breaks correspond with regionsshowing differences in temperature from land to watersurfaces. Turn back to figures 3-2A and 3-2B. Comparethe temperature distribution in views A and B of figures3-2 to the pressure distribution in views A and B offigure 3-3. Note the gradient over the Asian Continentin January. Compare it to the warmer temperature overthe ocean and coastal regions. Now look at view A offigure 3-3 and note the strong region of high-pressurecorresponding to the area. Now look at the same area inJuly. Note the way the temperature gradient flattens outand warms. Look at view B of figure 3-3 and see thelow-pressure area that has replaced the high-pressureregion of winter. These pressure cells tend to persist in aparticular area and are called centers of action; that is,they are found at nearly the same location withsomewhat similar intensity during the same month eachyear.

There is a permanent belt of relatively low pressurealong the equator and another deeper belt oflow-pressure paralleling the coast of the AntarcticContinent. Permanent belts of high pressure largelyencircle Earth, generally over the oceans in both theNorthern and Southern Hemispheres. The number ofcenters of action is at a maximum at about 30 to 35degrees from the equator.

3-14

There are also regions where the pressure ispredominantly low or high at certain seasons, but notthroughout the year. In the vicinity of Iceland, pressureis low most of the time. The water surface is warmer(due to warm ocean currents) than the surface ofIceland or the icecaps of Greenland. The Icelandic lowis most intense in winter, when the greatest temperaturecontrast occurs, but it persists with less intensitythrough the summer. Near Alaska, a similar situationexists with the Aleutian low. The Aleutian low is mostpronounced when the neighboring areas of Alaska andSiberia are snow covered and colder than the adjacentocean.

These lows are not a continuation of one and thesame cyclone. They are, however, regions of lowpressure where lows frequently form or arrive fromother regions. Here they remain stationary or movesluggishly for a time, then the lows move on or die outand are replaced by others. Occasionally these regionsof low pressure are invaded by traveling high-pressuresystems.

Two areas of semi permanent high-pressure alsoexist. There is a semi permanent high-pressure centerover the Pacific westward of California and anotherover the Atlantic, near the Azores and of the coast ofAfrica. Pressure is also high, but less persistently so,west of the Azores to the vicinity of Bermuda. Thesesubtropical highs are more intense and cover a greaterarea in summer than winter. They also extend farthernorthward summer. In winter, these systems move soultoward the equator, following the solar equator.

The largest individual circulation cells in theNorthern Hemisphere are the Asiatic high in winter andthe Asiatic low in summer. In winter, the Asiaticcontinent is a region of strong cooling and therefore isdominated by a large high-pressure cell. In summer,strong heating is present and the high-pressure cellbecomes a large low-pressure cell. (See fig. 3-3A andfig. 3-3B.) This seasonal change in pressure cells givesrise to the monsoon flow over India and Southeast Asia.

Another cell that is often considered to be a centerof action is the polar high. Both Arctic and Antarctichighs have considerable variations in pressure, andthese regions have many traveling disturbances insummer. For example, the Greenland high (due to theGreenland icecap) is a persistent feature, but it is not awell-defined high during all seasons of the year. TheGreenland high often appears to be an extension of thepolar high or vice versa. Other continental regionsshow seasonal variations, but are generally of small size

and their location is variable. Therefore, they are notconsidered to be centers of action.

An average annual pressure distribution chart(figure 3-14) reveals several important characteristics.First, along the equator there is a belt of relatively lowpressure encircling the globe with barometric pressureof about 1,012 millibars. Second, on either side of thisbelt of low pressure is a belt of high pressure. Thishigh-pressure area in the Northern Hemisphere liesmostly between latitudes 30° and 40°N with threewell-defined centers of maximum pressure. One is overthe eastern Pacific, the second over the Azores and thethird over Siberia; all are about 1,020 millibars. Thebelt of high pressure in the Southern Hemisphere isroughly parallel to 30°S. It, too, has three centers ofmaximum pressure. One is in the eastern Pacific, thesecond in the eastern Atlantic, and the third in theIndian Ocean; again, all are about 1,020 millibars. Athird characteristic to be noted from this chart is that,beyond the belt of high pressure in either hemisphere,the pressure diminishes toward the poles. In theSouthern Hemisphere, the decrease in pressure towardthe South Pole is regular and very marked. The pressuredecreases from, an average slightly above 1,016millibars along latitude 35°S to an average of 992millibars along latitude 60°S In the NorthernHemisphere, however, the decrease in pressure towardthe North Pole is less regular and not as great. This islargely due to the distribution of land and water: notethe extensive landmass in the Northern Hemisphere ascompared to those of the Southern Hemisphere.

While the pressure belts that stand out on theaverage annual pressure distribution chart representaverage pressure distribution for the year, these beltsare rarely continuous on any given day. They areusually broken up into detached areas of high or lowpressure by the secondary circulation of theatmosphere. In either hemisphere, the pressure over theland during the winter season is decidedly above theannual average. During the summer season, thepressure is decidedly below the average, with extremevariations occurring such as in the case of continentalAsia. Here the mean monthly pressure ranges fromabout 1,033 millibars during January to about 999millibars during July. Over the northern oceans, on theother hand, conditions are reversed; the summerpressure there is somewhat higher. Thus in January theIcelandic and Aleutian lows intensify to a depth ofabout 999 millibars, while in July these lows fill and arealmost obliterated.

3-15

The polar high in winter is not a cell centereddirectly over the North Pole, but appears to be anextension of the Asiatic high and often appears as awedge extending from the Asiatic continent. The cell isdisplaced toward the area of coldest temperatures—theAsiatic continent. In summer, this high appears as anextension of the Pacific high and is again displacedtoward the area of coolest temperature, which in thiscase is the extensive water area of the Pacific.

In winter over North America, the most significantfeature is the domination by a high-pressure cell. Thiscell is also due to cooling but is not as intense as theAsiatic cell. In summer, the most significant feature isthe so-called heat low over the southwestern part of thecontinent, which is caused by extreme heating in thisregion.

MIGRATORY SYSTEMS

General circulation, based on an average of windconditions, is a more or less quasi-stationarycirculation. Likewise, much of the secondarycirculation depends on more or less static conditionsthat, in turn, depend on permanent and semi permanenthigh and low-pressure areas. Changes in the circulationpatterns discussed so far have been largely seasonal.However, secondary circulation also includes wind

systems that migrate constantly, producing rapidlychanging weather conditions throughout all seasons,especially in the middle latitudes. The migratorycirculation systems are associated with air masses,fronts, cyclones, and anticyclones. These are covered indetail in the next unit.

Anticyclones

An anticyclone (high) is an area of relatively highpressure with a clockwise flow (wind circulation) in theNorthern Hemisphere and counterclockwise flow in theSouthern Hemisphere. The windflow in an anticycloneis slightly across the isobars and away from the centerof the anticyclone. (See fig. 3-15.) Anticyclones arecommonly called highs or high-pressure areas.

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Figure 3-14.—Average annual pressure distribution chart.

HIGH

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Figure 3-15.—Anticyclone.

The formation of an anticyclone or theintensification of an existing one is calledanticyclogenesis. Anticyclogenesis refers to thedevelopment of anticyclonic circulation as well as theintensification of an existing anticyclonic flow. When ahigh-pressure center is increasing in pressure, the highis BUILDING or INTENSIFYING. Although a highcan build (or intensify) without an increase inanticyclonic flow, it is rare. Normally, building andanticyclogenesis occur simultaneously. The weakeningof anticyclonic circulation is anticyclolysis. When thepressure of a high is decreasing, we say the high isweakening. Anticyclolysis and weakening can occurseparately, but usually occur together.

The vertical extent of pressure greatly depends onthe air temperature. Since density increases with adecrease in temperature, pressure decreases morerapidly vertically in colder air than in warmer air. In acold anticyclone (such as the Siberian high), thevertical extent is shallow; while in a warm anticyclone(such as the subtropical high), the vertical extentreaches high into the upper atmosphere due to the slowdecrease in temperature with elevation.

Cyclones

A cyclone (low) is a circular or nearly circular areaof low pressure with a counterclockwise flow. The flowis slightly across the isobars toward the center in theNorthern Hemisphere and clockwise in the SouthernHemisphere. (See fig. 3-16.) It is commonly called alow or a depression. This use of the word cyclone

should be distinguished from the colloquial use of theword as applied to the tornado or tropical cyclone(hurricane).

The formation of a new cyclone or theintensification of the cyclonic flow in an existing one iscalled cyclogenesis. When the pressure in the low isfalling, we say the low is deepening. Cyclogenesis anddeepening can also occur separately, but usually occurat the same time. The decrease or eventual dissipationof a cyclonic flow is called cyclolysis. When thepressure in a low is rising, we say the low is filling.Cyclolysis and filling usually occur simultaneously.Cyclones in middle and high latitudes are referred to asextratropical cyclones. The term tropical cyclone refersto hurricanes and typhoons.

VERTICAL STRUCTURE OF SECONDARYCIRCULATIONS (PRESSURE CENTERS)

To better understand the nature of the pressurecenters of the secondary circulation, it is necessary toconsider them from a three-dimensional standpoint.With the aid of surface and upper air charts, you will beable to see the three dimensions of these pressuresystems as well as the circulation patterns of thesecondary circulation as established at higher levels inthe troposphere and lower stratosphere.

In Chapter 2, the study of gas laws showed thatvolume is directly proportional to temperature. Statedanother way, we might say that the thickness of a layerbetween two isobaric surfaces is directly proportionalto the mean virtual temperature of the layer. Becausethe atmosphere is always moist to some degree, virtualtemperature is used. Mean virtual temperature isdefined as the average temperature at which dry airwould have the same pressure and density as moist air.Thus, lines representing thickness are also isotherms ofmean virtual temperature. The higher the mean virtualtemperature, the thicker the layer, or vice versa. Thethickness between layers is expressed in geopotentialmeters. The shift in location, as well as the change ofshape and intensity upward of atmospheric pressuresystems, is dependent on the temperature distribution.

An example of the effects of virtual temperaturecan be demonstrated by placing two columns of air nextto each other. One air column is cold and the other aircolumn is warm. The constant pressure surfaces in the

3-17

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CYCLONICCIRCULATION

AG5f0316

Figure 3-16.—Cyclone.

cold column are closer than the ones in the warmcolumn. Figure 3-17 shows an increase in thicknessbetween two pressure surfaces, resulting in an increasein mean virtual temperature. Note the increase in thedistance between the constant pressure surfaces; P, P1,etc., from column A to column B. Using thehypsometric equation can derive the thickness valuebetween two pressure surfaces. Thickness may also bedetermined from tables, graphs, etc.

VERTICAL STRUCTURE OF HIGH PRESSURESYSTEMS

The topographic features that indicate thecirculation patterns at 500 millibars in the atmospherecorrespond in general to those at lower and higher level.However, they may experience a shift in location aswell as a change in intensity and shape. For example, aridge aloft may reflect a closed high on a surfacesynoptic chart. In addition, upper air circulationpatterns may take on a wavelike structure in contrast tothe alternate closed lows, or closed high patterns at thesurface level. The smoothing of the circulation patternaloft is typical of atmospheric flow patterns.

Cold Core Highs

A cold core high is one in which the temperatureson a horizontal plane decrease toward the center.

Because the temperature in the center of a cold corehigh is less than toward the outside of the system, itfollows that the vertical spacing of isobars in the centerof this system is closer together than on the outside.Although the pressure at the center of these systems onthe surface may be high, the pressure decreases rapidlywith height. (See fig. 3-18.) Because these highs areoften quite shallow, it is common for an upper level lowto exist above a cold core high.

NOTE: For the purpose of illustration, figures 3-18through 3-21 are exaggerated with respect to actualatmospheric conditions.

If the cold core high becomes subjected to warmingfrom below and to subsidence from aloft, as it movessouthward from its source and spreads out, itdiminishes rapidly in intensity with time (unless somedynamic effect sets in aloft over the high to compensatefor the warming). Since these highs decrease inintensity with height, thickness is relatively low. In thevertical, cold core highs slope toward colder air aloft.Anticyclones found in Arctic air are always cold cored,while anticyclones in polar air may be warm or coldcore.

Examples of cold core highs are the NorthAmerican High, the Siberian High and the migratoryhighs that originate from these anticyclones.

Warm Core Highs

A warm core high is one in which the temperatureson a horizontal level increase toward the center.Because the temperatures in the center of a warm corehigh are higher than on the outside of the system, itfollows that the vertical spacing of isobars in the centeris farther apart than toward the outside of the high. Forthis reason, a warm core high increases in intensity withaltitude and has an anticyclonic circulation at all levels(see fig. 3-19). From a vertical view, warm core highsslope toward warmer air aloft. A warm core high isaccompanied by a high cold tropopause. Since thepressure surfaces are spaced far apart, the tropopause isreached only at great heights. The temperaturecontinues to decrease with elevation and is cold by the

3-18

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Figure 3-18.—Cold core high.

time the tropopause is reached. The subtropical highsare good examples of this type of high. Therefore,anticyclones found in tropical air are always warm core.Examples of warm core highs are the Azores orBermuda High and the Pacific High.

VERTICAL STRUCTURE OF LOW-PRESSURESYSTEMS

Low-pressure systems, like high-pressure systems,are generally a reflection of systems aloft. They, too,experience shifts in location and changes in intensityand shape with height. At times, a surface system maynot be evident aloft and a well-developed system aloftmay not reflect on a surface analysis.

Cold Core Lows

The cold core low contains the coldest air at itscenter throughout the troposphere; that is, goingoutward in any direction at any level in the troposphere,warmer air is encountered. The cold core low (figure3-20) increases intensity with height. Relativeminimums in thickness values, called cold pools, arefound in such cyclones. The temperature distribution isalmost symmetrical, and the axis of the low is nearlyvertical. When they do slope vertically, they slopetoward the coldest temperatures aloft. In the cold low,the lowest temperatures coincide with the lowestpressures.

The cold low has a more intense circulation aloftfrom 850 to 400 millibars than at the surface. Somecold lows show only slight evidence in the surfacepressure field that an intense circulation exists aloft.The cyclonic circulation aloft is usually reflected on thesurface in an abnormally low daily mean temperatureoften accompanied by instability and showeryprecipitation. A cold core low is accompanied by a lowwarm tropopause. Since the pressure surfaces are closetogether, the tropopause is reached at low altitudeswhere the temperature is relatively warm. Goodexamples of cold core lows are the Aleutian andIcelandic lows. Occluded cyclones are generally coldcore in their later stages, because polar or arctic air hasclosed in on them.

At high latitudes the cold pools and their associatedupper air lows show some tendency for location in thenorthern Pacific and Atlantic Oceans where,statistically, they contribute to the formation of theAleutian and Icelandic lows.

Warm Core Lows

A warm core low (figure 3-21) decreases intensitywith height and the temperature increases toward thecenter on a horizontal plane. The warm low isfrequently stationary, such as the heat low over thesouthwestern United States in the summer; this is aresult of strong heating in a region usually insulatedfrom intrusions of cold air that tend to fill it or cause itto move. The warm low is also found in its moving formas a stable wave moving along a frontal surface. Thereis no warm low aloft in the troposphere. The tropicalcyclone, however, is believed to be a warm low becauseits intensity diminishes with height. Because mostwarm lows are shallow, they have little slope. However,intense warm lows like the heat low over the southwestUnited States and hurricanes do slope toward warm airaloft.

In general, the temperature field is quiteasymmetrical around a warm core cyclone. Usually thesouthward moving air in the rear of the depression is

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Figure 3-19.—Warm core high.

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Figure 3-20.—Cold core low.

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Figure 3-21.—Warm core low.

not as warm as that moving northward in advance of it.A warm core low decreases intensity with height orcompletely disappears and are often replaced byanticyclones aloft. The heat lows of the southwesternUnited States, Asia, and Africa are good examples ofwarm core lows. Newly formed waves are generallywarm core because of the wide-open warm sector.

DYNAMIC LOW

Systems that retain their closed circulations toappreciable altitudes and are migratory are calleddynamic lows or highs. A dynamic low is acombination of a warm surface low and a cold upperlow or trough, or a warm surface low in combinationwith a dynamic mechanism aloft for producing a coldupper low or trough. It has an axis that slopes towardthe coldest tropospheric air. (See figure 3-22.) In thefinal stage, after occlusion of the surface warm low iscomplete, the dynamic low becomes a cold low with theaxis of the low becoming practically vertical.

DYNAMIC HIGH

The dynamic high is a combination of a surfacecold high and an upper-level warm high orwell-developed ridge, or a combination of a surfacecold high with a dynamic mechanism aloft forproducing high-level anticyclogenesis. Dynamic highshave axes that slope toward the warmest troposphericair. (See fig. 3-22.) In the final stages of warming thecold surface high, the dynamic high becomes a warmhigh with its axis practically vertical.

REVIEW QUESTIONS

Q3-11. What is the term that defines the formation ofan anticyclone or the intensification of anexisting anticyclone?

Q3-12. What is the direction of the windflow around acyclone?

Q3-13. How do temperatures change within a coldcore low?

Q3-14. Low pressure due to intense heating over thesouthwestern United States is an example ofwhich type of low-pressure system?

TERTIARY CIRCULATION

LEARNING OBJECTIVE: Define tertiarycirculation and describe how tertiarycirculations affect local weather and winddirection and speed.

Tertiary (third order) circulations are localizedcirculations directly attributable to one of the followingcauses or a combination of them: local cooling, localheating, adjacent heating or cooling, and induction(dynamics).

Many regions have local weather phenomenacaused by temperature differences between land andwater surfaces or by local topographical features. Theseweather phenomena show up as circulations. Thesetertiary circulations can result in dramatic local weatherconditions and wind flows. The most common tertiarycirculations are discussed in this lesson. However, thereare numerous other circulations and related phenomenain existence around the world.

MONSOON WINDS

The term monsoon is of Arabic origin and meansseason. The monsoon wind is a seasonal wind thatblows from continental interiors (or large land areas) tothe ocean in the winter; they blow in the oppositedirection during the summer. The monsoon wind ismost pronounced over India, although there are otherregions with noticeable monsoon winds.

Monsoon winds are a result of unequal heating andcooling of land and water surfaces. During winter amassive area of cold high pressure develops over theextensive Asiatic continent. This high pressure is dueprimarily to cold arctic air and long-term radiationcooling. To the south, the warm equatorial waters existand, in contrast, the area has relatively lower surfacepressures. The combination of high pressure over Asia

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Figure 3-22.—Vertical slope of low -pressure systems.

and low pressure over the Equatorial Belt sets up apressure gradient directed from north to south. Becauseof the flow around the massive Siberian high, northeastwinds begin to dominate the regions from India to thePhilippines. (See fig. 3-23).

During the winter months, clear skies predominateover most of the region. This is caused by the massmotion of air from a high-pressure area over land to anarea of lower pressure over the ocean. As the air leavesthe high-pressure area over land, it is cold and dry. As ittravels over land toward the ocean, there is no source ofmoisture to induce precipitation. The air is also

traveling from a higher altitude to a lower altitude;consequently, this downslope motion causes the air tobe warmed at the adiabatic lapse rate. This warmingprocess has a still further clearing effect on the skies.

During the summer the airflow over the region iscompletely reversed. The large interior of Asia isheated to the point where the continent is much warmerthan the ocean areas to the south. This inducesrelatively low pressure over Asia and higher pressureover the equatorial region. This situation produces asouthwesterly flow as shown in figure 3-24.

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Figure 3-23.—Northeast monsoon (January).

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Figure 3-24.—Southwest monsoon (July).

The weather associated with the summer monsoonwinds is thunderstorms, almost constant heavy rain,rain showers, and gusty surface winds. This condition iscaused by mass motion of air from the relativelyhigh-pressure area over the ocean to a low-pressurearea over land. When the air leaves the ocean, it is warmand moist. As the air travels over land toward thelow-pressure area, it is also traveling from a loweraltitude to a higher altitude. The air is lifted by amechanical force and cooled to its condensation pointby this upslope motion (pseudo adiabatic process).

LAND AND SEA BREEZES

There is a diurnal (daily) contrast in the heating oflocal water and land areas similar to the seasonalvariation of the monsoon. During the day, the land iswarmer than the water area; at night the land area iscooler than the water area. A slight variation in pressureis caused by this temperature contrast. At night thewind blows from land to sea and is called a land breeze.During the day, the wind blows from water areas to landareas and is called a sea breeze.

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Figure 3-25.—Circulation of land and sea breezes.

The sea breeze usually begins during midmorning(0900-1100 local time) when the land areas becomewarmer than adjacent ocean waters (see fig. 3-25). Thistemperature difference creates an area of slightly lowersurface pressures over land compared to the now coolerwaters. The result is a wind flow from water to land.The sea breeze starts with a shallow flow along thesurface; however, as maximum heating occurs, the flowincreases with height. The height varies from anaverage of 3,000 feet in moderately warm climates to4,500 (or more) in tropical regions. The effects of thesea breeze can be felt as far as 30 miles both onshoreand offshore. In extreme cases, the sea breeze is felt 100miles inland depending upon terrain. By mid afternoon(maximum heating) the sea breeze will reach itsmaximum speed and may be strong enough to beinfluenced by the Coriolis force, which causes it to flowat an angle to the shore. The sea breeze is mostpronounced in late spring, summer, and early fall whenmaximum temperature differences occur between landand water surfaces. A decrease in temperature and anincrease in humidity and wind speed mark the start of asea breeze.

The sea breeze continues until the land area coolssufficiently to dissipate the weak low pressure. Aftersunset, the land cools at a faster rate than the adjacentwaters and eventually produces a complete reversal ofthe winds. As the land continues to cool through theevening hours, a weak area of high pressure forms overthe land. The water area, with its warmer temperatures,has slightly lower pressure and again a flow isestablished; however, the flow is now from land towater (offshore). (See fig. 3-25.)

The land breezes, when compared to the seabreezes, are less extensive and not as strong (usually

less than 10 knots and less than 10 miles offshore). Thisis because there is less temperature contrast at nightbetween land and water surfaces as compared to thetemperature contrast during daytime heating. Landbreezes are at maximum development late at night, inlate fall and early winter. In the tropical land regions,the land and sea breezes are repeated day after day withgreat regularity. In high latitudes the land and seabreezes are often masked by winds of synoptic features.

WINDS DUE TO LOCAL COOLING ANDHEATING

In the next sections we discuss tertiary circulationsdue to local cooling and heating effects. Under normalcircumstances, these winds attain only light tomoderate wind speeds; however, winds often occur inand near mountain areas that have undergone dramaticchanges in normal character. At times, mountain areastend to funnel winds through valleys and mountainpasses. This funneling effect produces extremelydangerous wind speeds.

FUNNEL EFFECT

Winds blowing against mountain barriers tend toflatten out and go around or over them. If a pass or avalley breaks the barrier, the air is forced through thebreak at considerable speed. When wind is forcedthrough narrow valleys it is known as the funnel effectand is explained by Bernoulli’s theorem. According toBernoulli’s theorem, pressures are least wherevelocities are greatest; likewise, pressures are greatestwhere velocities are least. This observation is true forboth liquids and gases. (See fig. 3-26.)

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Figure 3-26.—Strong wind produced by funneling.

Bernoulli’s theorem is frequently used to forecasttertiary winds in the mountainous western UnitedStates. The famous Santa Ana winds of southernCalifornia are a prime example. Winds associated withhigh pressure situated over Utah are funneled throughthe valley leading into the town of Santa Ana near theCalifornia coast. Low pressure develops at the mouth ofthe valley and the end result is hot, dry, gusty andextremely dangerous winds. When the Santa Ana isstrong enough, the effects are felt in virtually everyvalley located along the coast of southern California.Visibility is often restricted due to blowing sand. It iscommon to see campers, trailers, and trucks turned overby the force of these winds. When funneled windsreach this magnitude, they are called jet-effect winds,canyon winds, or mountain-gap winds.

Winds Due to Local Cooling and Heating

There are two types of tertiary circulationsproduced by local cooling—glacier winds and drainagewinds.

GLACIER WINDS.—Glacier winds, or fallwinds (as they are sometimes called) occur in manyvarieties in all parts of the world where there areglaciers or elevated land masses that become coveredby snow and ice during winter. During winter, the areaof snow cover becomes most extensive. Weak pressureresults in a maximum of radiation cooling.Consequently the air coming in contact with the coldsnow cools. The cooling effect makes the overlying airmore dense, therefore, heavier than the surrounding air.When set in motion, the cold dense air flows down thesides of the glacier or plateau. If it is funneled through apass or valley, it may become very strong. This type ofwind may form during the day or night due to radiationcooling. The glacier wind is most common during thewinter when more snow and ice are present.

When a changing pressure gradient moves a largecold air mass over the edge of a plateau, this action setsin motion the strongest, most persistent, and mostextensive of the glacier or fall winds. When thishappens, the fall velocity is added to the pressuregradient force causing the cold air to rush down to sealevel along a front that may extend for hundreds ofmiles. This condition occurs in winter on a large scalealong the edge of the Greenland icecap. In some placesalong the icecap, the wind attains a velocity in excess of90 knots for days at a time and reaches more than 150nautical miles out to sea.

Glacier winds are cold katabatic (downhill) winds.Since all katabatic winds are heated adiabatically intheir descent, they are predominantly dry. Occasionally,the glacier winds pick up moisture from fallingprecipitation when they underride warm air. Even withthe adiabatic heating they undergo, all glacier or fallwinds are essentially cold winds because of the extremecoldness of the air in their source region. Contrary to allother descending winds that are warm and dry, theglacier wind is cold and dry. It is colder, level for level,than the air mass it is displacing. In the NorthernHemisphere, the glacier winds descend frequently fromthe snow-covered plateaus and glaciers of Alaska,Canada, Greenland, and Norway.

DRAINAGE WINDS.—Drainage winds (alsocalled mountain or gravity winds) are caused by thecooling air along the slopes of a mountain.Consequently, the air becomes heavy and flowsdownhill, producing the MOUNTAIN BREEZE.

Drainage winds are katabatic winds and like glacierwinds; a weak or nonexistent pressure gradient isrequired to start the downward flow. As the air near thetop of a mountain cools through radiation or contactwith colder surfaces, it becomes heavier than thesurrounding air and gradually flows downward (fig.3-27). Initially this flow is light (2 to 4 knots) and only afew feet thick. As cooling continues, the flow increasesachieving speeds up to 15 knots at the base of themountain and a depth of 200 feet or more. Winds inexcess of 15 knots are rare and only occur when themountain breeze is severely funneled.

Drainage winds are cold and dry. Adiabatic heatingdoes not sufficiently heat the descending air because ofthe relative coldness of the initial air and because thedistance traveled by the air is normally short. Drainagewinds have a very localized circulation. As the cold airenters the valley below, it displaces the warm air.Temperatures continue to fall. If the flow achievesspeeds of 8 knots or more, mixing results between thewarm valley air and the cold descending air that resultsin a slight temperature increase. Campers often preferto make summer camps at the base of mountains to takeadvantage of the cooling effect of the mountain breeze.

Funnel Effects

VALLEY BREEZES.—The valley breeze is theanabatic (uphill) counterpart of the mountain breeze.When the sun heats the valley walls and mountainslopes during the morning hours, the air next to theground is heated until it rises along the slopes. Rocky or

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sandy slopes devoid of vegetation are the most effectiveheating surfaces. If the slopes are steep, the ascendingbreeze tends to move up the valley walls. Theexpansion of the heated air next to the surface producesa slight local pressure gradient against the groundsurface. As the heating becomes stronger, convectivecurrents begin to rise vertically from the valleys (figure3-28). The updrafts along the valley walls continue tobe active, particularly at the head of the valley. Thevalley breeze usually reaches its maximum strength inthe early afternoon. It is a stronger and deeper wind

than the mountain breeze. It is difficult to isolate thevalley breeze effect because of the prevailing gradientwinds. Consequently, the valley breeze is much morelikely to be superposed as a prevailing wind than is themountain breeze, which by its very nature can developonly in the absence of any appreciable gradient wind.The valley breezes are generally restricted to slopesfacing south or the more direct rays of the sun, and theyare more pronounced in southern latitudes. They arediurnally strongest in the late afternoon and areseasonally strongest in summer.

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Figure 3-27.—Mountain breeze or katabatic wind. During the night outgoing radiation cools air along hillsides below free airtemperature. The cooled air drains to lowest point of the terrain.

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Figure 3-28.—Valley breeze or anabatic wind. During the daytime hillsides heat quickly. This heating effect causes updrafts alongslopes—downdrafts in the center.

THERMALS.—Thermals are vertical convectivecurrents that result from local heating. They stop shortof the condensation level. Thermal convection is theusual result of strong heating of the lower atmosphereby the ground surface. A superadiabatic lapse rateimmediately above the ground is necessary to thedevelopment of strong thermals. They form mostreadily over areas of bare rock or sand and in particularover sand dunes or bare rocky hills. In the presence of amoderate or fresh breeze, especially in a hilly terrain, itis impossible to distinguish between turbulent andthermal convection currents. Pure thermal convectionnormally occurs on clear summer days with very lightprevailing wind. In the eastern United States, drythermals are usually of only moderate intensity, seldomreaching an elevation in excess of 5,000 feet above thesurface. The high moisture content of the air masses inthis section in summer reduces the intensity of surfaceheating to some extent. This moisture content usuallykeeps the condensation level of the surface air near oreven below a height of 5,000 feet above the ground. Inthe dry southwestern part of the country, where groundheating during clear summer days is extreme, drythermal convection may extend to a height of 10,000feet or more. Under these conditions, extremelyturbulent air conditions can occur locally up towhatever heights the thermals extend, frequentlywithout a cloud in the sky.

One variation of the dry thermal is seen in the dustor sand whirls, sometimes called dust devils. They areformed over heated surfaces when the winds are verylight. Dust whirls are seldom more than two or threehundred feet high and they last only a few minutes atmost. Over the desert on clear hot days as many as adozen columns of whirling sand may be visible at once.The large desert sand whirls can become severalhundred feet in diameter, extend to heights of 4,000 feetor higher, and in some cases last for an hour or more.They have been observed to rotate both anticyclonicallyand cyclonically, the same as tornadoes.

An almost identical phenomenon is observed overwater in the form of the waterspout. Waterspouts occurfrequently in groups and form in relatively cool humidair over a warm water surface when the wind is light.The waterspout is visible due to the condensed watervapor, or cloud formation, within the vortex. Thecondensation is the result of dynamic cooling byexpansion within the vortex. In this respect it differsfrom the sand whirl, which is always dry. Both the sand

whirl and the waterspout represent simple thermalconvection of an extreme type. They are not to beconfused with the more violent tornado.

When dry thermal convection extends to anelevation where the dry thermals reach thecondensation level, then cumulus convection takes theplace of the dry convection. A cumulus cloud, whosebase is at the condensation level of the rising air, topseach individual thermal current. Beneath every buildingcumulus cloud a vigorous rising current or updraft isobserved. Thus the local thermal convection patternbecomes visible in the cumulus cloud pattern. Thecumulus clouds form first over the hills where thestrongest thermals develop. Under stable atmosphericconditions, little convective cloud development occurs.However, under unstable conditions these thermalsmay develop cumulonimbus clouds.

INDUCED OR DYNAMIC TERTIARYCIRCULATIONS

There are four types of induced or dynamic tertiarycirculations. They are eddies, turbulence, large-scalevertical waves, and Foehn winds.

Eddies

An eddy is a circulation that develops when thewind flows over or adjacent to rough terrain, buildings,mountains or other obstructions. They generally formon the lee (downwind or sheltered) side of theseobstructions. The size of the eddy is directlyproportional to the size of the obstruction and speed ofthe wind. Eddies may have horizontal or verticalcirculations that can be either cyclonic or anticyclonic.

Horizontal eddies form in sheltered areasdownwind of rough coastlines or mountain chains. Anexample of a horizontal eddy is the weak cycloniccirculation that develops in the channel off the coast ofSanta Barbara, California. The winds frequently blowparallel to the northern California coastline during thewinter fog and stratus season. The Santa Barbarachannel often remains fog-free because the waters areprotected from winds that transport the fog inland.However, when the winds are sufficiently strong,friction along the tough coastal range produces a weakcyclonic eddy over the channel. This cyclonic flow,though weak, is sufficient to advect fog into the region.

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Vertical eddies are generally found on the lee sideof mountains, but with low wind speeds, stationaryeddies or rotating pockets of air are produced andremain on both the windward and leeward sides ofobstructions. (See figure 3-29.) When wind speedsexceed about 20 knots, the flow may be broken up intoirregular eddies that are carried along with a wind somedistance downstream from the obstruction. Theseeddies may cause extreme and irregular variations inthe wind and may disturb aircraft landing areassufficiently to be a hazard.

A similar and much disturbed wind conditionoccurs when the wind blows over large obstructionssuch as mountain ridges. In such cases the windblowing up the slope on the windward side is usuallyrelatively smooth. On the leeward side the wind spillsrapidly down the slope, setting up strong downdraftsand causing the air to be very turbulent. This conditionis illustrated in figure 3-30. These downdrafts can bevery violent. Aircraft caught in these eddies could beforced to collide with the mountain peaks. This effect isalso noticeable in the case of hills and bluffs, but is notas pronounced.

Turbulence

Turbulence is the irregular motion of theatmosphere caused by the air flowing over an uneven

surface or by two currents of air flowing past each otherin different directions or at different speeds. The mainsource of turbulence is the friction along the surface ofEarth. This is called mechanical turbulence. Turbulenceis also caused by irregular temperature distribution.The warmer air rises and the colder air descends,causing an irregular vertical motion of air; this is calledthermal turbulence.

Mechanical turbulence is intensified in unstable airand is weakened in stable air. These influences causefluctuations in the wind with periods ranging from afew minutes to more than an hour. If these windvariations are strong, they are called wind squalls andare usually associated with convective clouds. They arean indication of approaching towering cumulus orcumulonimbus clouds.

Gustiness and turbulence are more or lesssynonymous. Gustiness is an irregularity in the windspeed that creates eddy currents disrupting the smoothairflow. Thus, the term gust is usually used inconjunction with sudden intermittent increases in thewind speed near the surface levels. Turbulence, on theother hand, is used with reference to levels above thesurface. Gustiness can be measured; turbulence,however, unless encountered by aircraft equipped witha gust probe or an accelerometer, is usually estimated.

Large-Scale Vertical Waves (Mountain Waves)

Mountain waves occur on the lee side oftopographical barriers and occur when the wind-flow isstrong, 25 knots or more, and the flow is roughlyperpendicular to the mountain range. The structure ofthe barrier and the strength of the wind determines the

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Figure 3-29.—Eddy currents formed when wind flows overuneven ground or obstructions.

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Figure 3-30.—Effect of windflow over mountains.

amplitude and the type of the wave. The characteristicsof a typical mountain wave are shown in figure 3-31.

Figure 3-31 shows the cloud formations normallyfound with wave development and illustratesschematically the airflow in a similar situation. Theillustration shows that the air flows fairly smoothlywith a lifting component as it moves along thewindward side of the mountain. The wind speedgradually increases, reaching a maximum near thesummit. On passing the crest, the flow breaks down intoa much more complicated pattern with downdraftspredominating. An indication of the possible intensitiescan be gained from verified records of sustaineddowndrafts (and also updrafts) of at least 5,000 feet perminute with other reports showing drafts well in excessof this figure. Turbulence in varying degrees can beexpected and is particularly severe in the lower levels;however, it can extend to the tropopause to a lesserdegree. Proceeding downwind, some 5 to 10 miles fromthe summit, the airflow begins to ascend in a definitewave pattern. Additional waves, generally less intensethan the primary wave, may form downwind (in somecases six or more have been reported). These are similarto the series of ripples that form downstream from asubmerged rock in a swiftly flowing river. The distancebetween successive waves usually ranges from 2 to 10miles, depending largely on the existing wind speedand the atmospheric stability. However, wavelengths upto 20 miles have been reported.

It is important to know how to identify a wavesituation. Pilots must be briefed on this condition sothey can avoid the wave hazards. Characteristic cloudforms peculiar to wave action provide the best means ofvisual identification. The lenticular (lens shaped)clouds in the upper right of figure 3-31 are smooth incontour. These clouds may occur singly or in layers atheights usually above 20,000 feet, and may be quiteragged when the airflow at that level is turbulent. Theroll cloud (also called rotor cloud) forms at a lowerlevel, generally near the height of the mountain ridge,and can be seen extending across the center of thefigure. The cap cloud, shown partially covering themountain slope, must always be avoided in flightbecause of turbulence, concealed mountain peaks, andstrong downdrafts on the lee side. The lenticular, likethe roll clouds and cap clouds, are stationary, constantlyforming on the windward side and dissipating on the leeside of the wave. The actual cloud forms can be a guideto the degree of turbulence. Smooth clouds generallyshow smoother airflow in or near them with lightturbulence. Clouds appearing ragged or irregularindicate more turbulence.

While clouds are generally present to forewarn thepresence of wave activity, it is possible for wave actionto take place when the air is too dry to form clouds. Thismakes the problem of identifying and forecasting moredifficult.

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Figure 3-31.—Schematic diagram showing airflow and clouds in a mountain wave.

Foehn Winds

When air flows downhill from a high elevation, itstemperature is raised by adiabatic compression. Foehnwinds are katabatic winds caused by adiabatic heatingof air as it descends on the lee sides of mountains.Foehn winds occur frequently in our western mountainstates and in Europe in the late fall and winter. InMontana and Wyoming, the Chinook is a well-knownphenomenon; in southern California, the Santa Ana isknown particularly for its high-speed winds that easilyexceed 50 knots. For the purpose of illustrating a Foehnwind, the Santa Ana is used.

The condition producing the Foehn wind is ahigh-pressure area with a strong pressure gradientsituated near Salt Lake City, Utah. This gradient directsthe wind flow into a valley leading to the town of SantaAna near the coast of California. As the wind enters thevalley, its flow is sharply restricted by the funnelingeffect of the mountainsides. This restriction causes thewind speed to increase, bringing about a drop inpressure in and near the valley. The Bernoulli effectcauses this pressure drop in and near a valley.

Generally speaking, when the Santa Ana blowsthrough the Santa Ana Canyon, a similar windsimultaneously affects the entire southern Californiaarea. Thus, when meteorological conditions arefavorable, this dry northeast wind blows through themany passes and canyons, over all the mountainousarea, including the highest peaks, and quite often atexposed places along the entire coast from SantaBarbara to San Diego. Therefore, the term Santa Anarefers to the general condition of a dry northeast windover southern California.

In the Rocky Mountain states, the onset of Foehnwinds have accounted for temperature rises of 50°F ormore in only a few minutes. In southern California, thetemperature, though less dramatically, also rises rapidlyand is accompanied by a rapid decrease in humidity (to20 percent or less) and a strong shift and increase inwind speeds. Although these winds may on occasionreach destructive velocities, one beneficial aspect isthat these winds quickly disperse the severe airpollutants that plague the Los Angeles Basin.

REVIEW QUESTIONS

Q3-15. What is the cause of monsoon winds?

Q3-16. What causes land and sea breezes?

Q3-17. Describe Bernoulli's theorem.

Q3-18. When does a valley breeze usually reach itsmaximum?

Q3-19. What causes eddies?

Q3-20. What causes Foehn winds?

SUMMARY

In this chapter, we studied the primary, secondaryand tertiary circulation of the atmosphere. We learnedabout large-scale circulations, worldwide locations ofmajor pressure systems, horizontal and verticalpressure systems. We studied how pressure systems,temperature, and world winds relate to each other, andfinally we studied small-scale effects, due to localfeatures. A good understanding of atmosphericcirculation is essential in order to understand thecharacteristics of air masses and fronts.

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CHAPTER 4

AIR MASSES AND FRONTS

Temperature, in the form of heating and cooling,plays a key roll in our atmosphere’s circulation.Heating and cooling is also the key in the formation ofvarious air masses. These air masses, because oftemperature contrast, ultimately result in the formationof frontal systems. The air masses and frontal systems,however, could not move significantly without theinterplay of low-pressure systems (cyclones).

Some regions of Earth have weak pressuregradients at times that allow for little air movement.Therefore, the air lying over these regions eventuallytakes on the certain characteristics of temperature andmoisture normal to that region. Ultimately, air masseswith these specific characteristics (warm, cold, moist,or dry) develop. Because of the existence of cyclonesand other factors aloft, these air masses are eventuallysubject to some movement that forces them together.When these air masses are forced together, frontsdevelop between them. The fronts are then broughttogether by the cyclones and airflow aloft. Thisproduces the classic complex frontal systems often seenon surface weather maps.

AIR MASSES

LEARNING OBJECTIVE: Determine theconditions necessary for the formation of airmasses and identify air mass source regions.

An air mass is a body of air extending over a largearea (usually 1,000 miles or more across). It isgenerally an area of high pressure that stagnates forseveral days where surface terrain varies little. Duringthis time, the air mass takes on characteristics of theunderlying surface. Properties of temperature, moisture(humidity), and lapse rate remain fairly homogeneousthroughout the air mass. Horizontal changes of theseproperties are usually very gradual.

CONDITIONS NECESSARY FOR AIR MASSFORMATION

Two primary factors are necessary to produce an airmass. First, a surface whose properties, essentiallytemperature and moisture, are relatively uniform (itmay be water, land, or a snow-covered area). Second, alarge divergent flow that tends to destroy temperature

contrasts and produces a homogeneous mass of air. Theenergy supplied to Earth’s surface from the Sun isdistributed to the air mass by convection, radiation, andconduction.

Another condition necessary for air mass formationis equilibrium between ground and air. This isestablished by a combination of the followingprocesses: (1) turbulent-convective transport of heatupward into the higher levels of the air; (2) cooling ofair by radiation loss of heat; and (3) transport of heat byevaporation and condensation processes.

The fastest and most effective process involved inestablishing equilibrium is the turbulent-convectivetransport of heat upwards. The slowest and leasteffective process is radiation.

During radiation and turbulent-convectiveprocesses, evaporation and condensation contribute inconserving the heat of the overlying air. This occursbecause the water vapor in the air allows radiation onlythrough transparent bands during radiational coolingand allows for the release of the latent heat ofcondensation during the turbulent-convectiveprocesses. Therefore, the tropical latitudes, because ofa higher moisture content in the air, rapidly form airmasses primarily through the upward transport of heatby the turbulent-convective process. The dryer polarregions slowly form air masses primarily because of theloss of heat through radiation. Since underlyingsurfaces are not uniform in thermal properties duringthe year and the distribution of land and water isunequal, specific or special summer and/or winter airmasses may be formed. The rate of air mass formationvaries more with the intensity of insolation.

EFFECTS OF CIRCULATION ON ALL AIRMASS FORMATION

There are three types of circulation over Earth.However, not all of these are favorable for air massdevelopment. They are as follows:

1. The anticyclonic systems. Anticyclonicsystems have stagnant or slow-moving air, whichallows time for air to adjust its heat and moisturecontent to that of the underlying surface. These

4-1

anticyclones have a divergent airflow that spreads theproperties horizontally over a large area; turbulence andconvection distribute these properties vertically.Subsidence (downward motion), another property ofanticyclones, is favorable for lateral mixing, whichresults in horizontal or layer homogeneity.

Warm highs, such as the Bermuda and Pacifichighs, extend to great heights because of a lesserdensity gradient aloft and thereby produce an air massof relatively great vertical extent. Cold highs, such asthe Siberian high, are of moderate or shallow verticalextent and produce air masses of moderate or shallowheight.

2. Cyclonic systems. Cyclonic systems are notconducive to air mass formation because they arecharacterized by greater wind speeds than anticyclonicsystems. These wind speeds prevent cyclonic systemsfrom stabilizing. An exception is the stationary heatlow.

3. Belts of convergence. Belts of convergence arenormally not conducive to air mass formation sincethey have essentially the same properties as cyclonicsystems. However, there are two areas of convergencewhere air masses do form. These are the areas over thenorth Pacific, between Siberia and North America, andthe Atlantic, off the coast of Labrador and

Newfoundland. These two areas act as source regionsfor maritime polar air.

AIR MASS SOURCE REGIONS

The ideal condition for the production of an airmass is the stagnation of air over a uniform surface(water, land, or ice cap) of uniform temperature andhumidity. The length of time an air mass stagnates overits source region depends upon the surroundingpressures. From the surface up through the upper levels,such air acquires definite properties and characteristics.The resulting air mass becomes virtually homogeneousthroughout, and its properties become uniform at eachlevel. In the middle latitudes, the land and sea areaswith the associated steep latitudinal temperaturegradient are generally not homogeneous enough forsource regions. These areas act as transitional zones forair masses after they have left their source regions.

The source regions for the world’s air masses areshown in figure 4-1. Note the uniformity of theunderlying surfaces; also note the relatively uniformclimatic conditions in the various source regions, suchas the southern North Atlantic and Pacific Oceans formaritime tropical air and the deep interiors of NorthAmerica and Asia for continental polar air.

4-2

SIBERIAN

SOURCE

CP

SOURCEMP

SOURCEMT

SOURCEMT

SOURCECP

SOURCEMT

SOURCEMP

SOURCECP

SOURCENUMBER

CT

SOURCECT

CT

MT

SOURCEMT

SOURCEMT

SOURCEMP

SOURCEMP

SOURCEMTSOURCE

MT

SOURCEMP

CT

60

40

20

0

20

40

100 120 140 160 160160 180180 140 120 100 80 60 40 20 0 20 40 60 80 100

AG5f0401

ARCTIC AIRMASSES

AWINTER

EQUATORIAL AIR

CT

MA

MT

Figure 4-1.—Air mass source regions.

Characteristics of Air Masses

The characteristics of an air mass are acquired inthe source region, which is the surface area over whichthe air mass originates. The ideal source region has auniform surface (all land or all water), a uniformtemperature, and is an area in which air stagnates toform high-pressure systems. The properties(temperature and moisture content) an air massacquires in its source region are dependent upon anumber of factors—the time of year (winter orsummer), the nature of the underlying surface (whetherland, water, or ice covered), and the length of time itremains over its source region.

ARCTIC (A) AIR.—There is a permanenthigh-pressure area in the vicinity of the North Pole. Inthis region, a gentle flow of air over the polar ice fieldsallows an arctic air mass to form. This air mass ischaracteristically dry aloft and very cold and stable inthe lower altitudes.

ANTARCTIC (A) AIR.—Antarctica is a greatsource region for intensely cold air masses that havecontinental characteristics. Before the antarctic airreaches other land areas, it becomes modified and isproperly called maritime polar. The temperatures arecolder than in the arctic regions. Results of OperationDeepfreeze have revealed the coldest surfacetemperatures in the world to be in the Antarctic.

CONTINENTAL POLAR (cP) AIR.—Thecontinental polar source regions consist of all landareas dominated by the Canadian and Siberianhigh-pressure cells. In the winter, these regions arecovered by snow and ice. Because of the intense coldand the absence of water bodies, very little moisture istaken into the air in these regions. Note that the wordpolar, when applied to air mass designations, does notmean air at the poles (this area is covered by the wordsarctic and antarctic). Polar air is generally found inlatitudes between 40 and 60 degrees and is generallywarmer than arctic air. The air over northern and centralAsia are exceptions to this.

MARITIME POLAR (mP) AIR.—The maritimepolar source regions consist of the open unfrozen polarsea areas in the vicinity of 60° latitude, north and south.Such areas are sources of moisture for polar air masses;consequently, air masses forming over these regions aremoist, but the moisture is sharply limited by the coldtemperature.

CONTINENTAL TROPICAL (cT) AIR.—Thecontinental tropical source regions can be any

significant land areas lying in the tropical regions;generally these tropical regions are located betweenlatitudes 25°N and 25°S. The large land areas located inthese latitudes are usually desert regions (such as theSahara or Kalahari Deserts of Africa, the ArabianDesert, and the interior of Australia). The air over theseland areas is hot and dry.

MARITIME TROPICAL (mT) AIR.—Themaritime tropical source regions are the large zones ofopen tropical sea along the belt of the subtropicalanticyclones. High-pressure cells stagnate in theseareas most of the year. The air is warm because of thelow latitude and can hold considerable moisture.

EQUATORIAL (E) AIR.—The equatorial sourceregion is the area from about latitudes 10°N to 10°S. Itis essentially an oceanic belt that is extremely warmand that has a high moisture content. Convergence ofthe trade winds from both hemispheres and the intenseinsolation over this region causes lifting of the unstable,moist air to high levels. The weather associated withthese conditions is characterized by thunderstormsthroughout the year.

SUPERIOR (S) AIR.—Superior air is a high-levelair mass found over the south central United States.This air mass occasionally reaches the surface; becauseof subsidence effects, it is the warmest air mass onrecord in the North American continent in both seasons.

Southern Hemisphere Air Masses

Air masses encountered in the SouthernHemisphere differ little from their counterparts in theNorthern Hemisphere. Since the greater portion of theSouthern Hemisphere is oceanic, it is not surprising tofind maritime climates predominating in thathemisphere.

The two largest continents of the SouthernHemisphere (Africa and South America) both taperfrom the equatorial regions toward the South Pole andhave small land areas at high latitudes. Maritime polarair is the coldest air mass observed over the middlelatitudes of the Southern Hemisphere.

In the interior of Africa, South America, andAustralia, cT air occurs during the summer. Over theremainder of the Southern Hemisphere, thepredominating air masses are mP, mT, and E air. Thestructure of these air masses is almost identical withthose found in the Northern Hemisphere.

4-3

AIR MASS CLASSIFICATION

LEARNING OBJECTIVE: Define air massclassification and describe how theclassification will change when characteristicsmodify.

Air masses are classified according to geographicsource region, moisture content, and thermodynamicprocess.

Geographic Origin

The geographical classification of air masses,which refers to the source region of the air mass,divides air masses into four basic categories: arctic orantarctic (A), polar (P), tropical (T), and equatorial (E).An additional geographical classification is thesuperior (5) air mass. The superior air mass is generallyfound aloft over the southwestern United States, but issometimes located at or near the surface.

Moisture Content

The arctic (A), polar (P), and tropical (T)classifications are further broken down by moisturecontent. An air mass is considered to be maritime (m) ifits source of origin is over an oceanic surface. If the air

mass originates over a land surface, it is consideredcontinental (c). Thus, a moist, maritime arctic air massis designated m; and a drier, continental arctic air massis designated c. Equatorial (E) air is found exclusivelyover the ocean surface in the vicinity of the equator andis designated neither c nor m but simply E.

Thermodynamic Process

The thermodynamic classification applies to therelative warmth or coldness of the air mass. A warm airmass (w) is warmer than the underlying surface; a coldair mass (k) is colder than the underlying surface. Forexample, a continental polar cold air mass over awarmer surface is classified as cPk. An mTwclassification indicates that the air mass is a maritimetropical warm air mass and overlays a cooler surface.

Air masses can usually be identified by the type ofclouds within them. Cold air masses usually showcumuliform clouds, whereas warm air masses containstratiform clouds. Sometimes, and with some airmasses, the thermodynamic classification may changefrom night to day. A particular air mass may show kcharacteristics during the day and w characteristics atnight and vice versa. The designators and descriptionsfor the classifications of air masses are listed in table4-1.

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Designator Description

cAk Continental arctic air that is colder than the surface over which it lies.

cAw Continental arctic air that is warmer than the surface over which it lies.

mAk Maritime arctic air that is colder than the surface over which it lies.

cPw Continental polar air that is warmer than the surface over which it is moving.

cPk Continental polar air that is colder than the surface over which it is moving.

mPw Maritime polar air that is warmer than the surface over which it is moving.

mPk Maritime polar air that is colder than the surface over which it is moving.

mTw Maritime tropical air that is warmer than the surface over which it is moving.

mTk Maritime tropical air that is colder than the surface over which it is moving.

cTw Continental tropical air that is warmer than the surface over which it is moving.

cTk Continental tropical air that is colder than the surface over which it is moving.

Ek Maritime equatorial air that is colder than the surface over which it is moving.

Ew Maritime equatorial air that is warmer than the surface over which it is moving.

S Superior air, found generally aloft over the southwestern United States, and occassionally at or nearthe surface.

Table 4-1.—Classification of Air Masses

AIR MASS MODIFICATION

When an air mass moves out of its source region, anumber of factors act upon the air mass to change itsproperties. These modifying influences do not occurseparately. For instance, in the passage of cold air overwarmer water surfaces, there is not only a release ofheat to the air, but also a release of some moisture.

As an air mass expands and slowly moves out of itssource region, it travels along a certain path. As an airmass leaves its source region, the first modifying factoris the type and condition of the surface over which theair travels. Here, the factors of surface temperature,moisture, and topography must be considered. The typeof trajectory, whether cyclonic or anticyclonic, also hasa bearing on its modification. The time interval sincethe air mass has been out of its source regiondetermines to a great extent the characteristics of the airmass. You must be aware of the five modifying factorsand the changes that take place once an air mass leavesits source region in order to integrate these changes intoyour analyses and briefings.

Surface Temperature

The difference in temperature between the surfaceand the air mass modifies not only the air temperature,but also the stability of the air mass. For example, if theair mass is warm and moves over a colder surface (suchas tropical air moving over colder water), the coldsurface cools the lower layers of the air mass and thestability of the air mass increases. This stability extendsto the upper layers in time, and condensation in theform of fog or low stratus normally occurs. (See fig.4-2.)

If the air mass moves over a surface that is warmer(such as continental polar air moving out from thecontinent in winter over warmer water), the warm waterheats the lower layers of the air mass, increasinginstability (decreasing in stability), and consequentlyspreading to higher layers. Figure 4-3 shows themovement of cP air over a warmer water surface inwinter.

The changes in stability of the air mass givevaluable indications of the cloud types that will form, as

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AIR TEMPERATURES

AIR TEMPERATURES

LIGHT WINDS

MOD. WINDS

STRATUS

WARM

WARM

COLD

COLD

BECOMINGOVER LAND OR WATER

BECOMINGOVER LAND OR WATER

FOG

AG5f0402

Figure 4-2.—Passage of warm air over colder surfaces.

COOL CONTINENT WARM OCEANAG5f0403

Figure 4-3.—Continental polar air moving from cool continent to warm ocean (winter).

well as the type of precipitation to be expected. Also,the increase or decrease in stability gives furtherindication of the lower layer turbulence and visibility.

Surface Moisture

An air mass may be modified in its moisturecontent by the addition of moisture as a result ofevaporation or by the removal of moisture as a result ofcondensation and precipitation. If the air mass ismoving over continental regions, the existence ofunfrozen bodies of water can greatly modify the airmass; in the case of an air mass moving from acontinent to an ocean, the modification can beconsiderable. In general (dependent upon thetemperature of the two surfaces), the movement over awater surface increases both the moisture content of thelower layers and the relative temperature near thesurface.

For example, the passage of cold air over a warmwater surface decreases the stability of the air withresultant vertical currents. The passage of warm, moistair over a cold surface increases the stability and couldresult in fog as the air is cooled and moisture is addedby evaporation.

Topography of Surface

The effect of topography is evident primarily in themountainous regions. The air mass is modified on thewindward side by the removal of moisture throughprecipitation with a decrease in stability; and, as the airdescends on the other side of the mountain, the stabilityincreases as the air becomes warmer and drier.

Trajectory

After an air mass has left its source region, thetrajectory it follows (whether cyclonic or anticyclonic)has a great effect on its stability. If the air follows acyclonic trajectory, its stability in the upper levels isdecreased; this instability is a reflection of cyclonicrelative vorticity. The stability of the lower layers is notgreatly affected by this process. On the other hand, ifthe trajectory is anticyclonic, its stability in the upperlevels is increased as a result of subsidence associatedwith anticyclonic relative vorticity.

Age

Although the age of an air mass in itself cannotmodify the air mass, it does determine (to a great

extent) the amount of modification that takes place. Forexample, an air mass that has recently moved from itssource region cannot have had time to become modifiedsignificantly. However, an air mass that has moved intoa new region and stagnated for some time is now oldand has lost many of its original characteristics.

Modifying Influences on Air Mass Stability

The stability of an air mass often determines thetype of clouds and weather associated with that airmass. The stability of an air mass can be changed byeither thermodynamic or mechanical means.

THERMODYNAMIC.—The thermodynamicinfluences are reflected in a loss or gain in heat and inthe addition or removal of moisture.

Heat Loss or Gain.—The air mass may lose heatby radiational cooling of Earth’s surface or by the airmass passing from a warm surface to a cold surface.The air mass may gain heat by solar heating of theground over which the air mass moves or by the airmass passing from a cold to a warm surface.

Moisture Increase or Decrease.—Moisture maybe added to the air mass by evaporation. One source ofevaporation may be the precipitation as it falls throughthe air; other sources may be a water surface, ice andsnow surface, or moist ground. Moisture may beremoved from the air mass by condensation andprecipitation.

MECHANICAL.—Mechanical influences on airmasses depend upon movement. The mechanicalprocess of lifting an air mass over elevation of land,over colder air masses, or to compensate for horizontalconvergence produces a change in an air mass.Turbulent mixing and the shearing action of wind alsocause air mass modifications. The sinking of air fromhigh elevations to relatively lower lands or from abovecolder air masses and the descent in subsidence andlateral spreading are also important mechanicalmodifiers of air masses.

The thermodynamic and mechanical influences onair mass stability are summarized in figure 4-4. Thefigure indicates the modifying process, what takesplace, and the resultant change in stability of the airmass. These processes do not occur independently;instead, two or more processes are usually in evidenceat the same time. Within any single air mass, theweather is controlled by the moisture content, stability,and the vertical movements of air.

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WORLD AIR MASSES

LEARNING OBJECTIVE: Describe thetrajectories and weather associated with worldair masses.

NORTH AMERICAN AIR MASSES,TRAJECTORIES, AND WEATHER(WINTER)

The shape and location of the North Americancontinent make it an ideal source region and also permitthe invasion of maritime air masses. You must be ableto identify these air masses and trace their trajectoriesto develop and present an in-depth weather briefing.

Within an air mass, weather is controlled primarilyby the moisture content of the air, the relationshipbetween surface temperature and air mass temperature,and terrain (upslope or downslope). Rising air is

cooled; descending air is warmed. Condensation takesplace when the air is cooled to its dew point. A cloudwarmed above the dew point temperature evaporatesand dissipates. Stability tends to increase if the surfacetemperature is lowered or if the temperature of the air athigher levels is increased while the surface temperatureremains the same. Stability tends to be reduced if thetemperature aloft is lowered. Smooth stratiform cloudsare associated with stable air, whereas turbulence,convective clouds, and thunderstorms are associatedwith unstable air.

cPk and cAk Air in Winter

The weather conditions with cPk and cAk air overthe United States depend primarily on the trajectory ofthe air mass after it leaves its source region.Trajectories, as observed on a surface chart, areindicated as one of the trajectories (A, B, C, D, E, F, G)

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1. Heating from below.

2. Cooling from below.

3. Addition of moisture.

4. Removal of moisture.

1. Turbulent mixing.

2. Sinking.

3. Lifting.

B. MECHANICAL

A. THERMODYNAMIC

Air mass passes from over a cold surface toa warm surface, or surface under air massis heated by sun.

Air mass passes from over a warm surface toa cold surface, OR radiational cooling ofsurface under air mass takes place.

By evaporation from water, ice, or snowsurfaces, or moist ground, or from rain-drops or other precipitation which fallsfrom overrunning saturated air currents.

By condensation and precipitation from theair mass.

Up- and down-draft.

Movement down from above colder air massesor descent from high elevations to low-lands, subsidence and lateral spreading.

Movement up over colder air masses or overelevations of land or to compensate forair at the same level converging.

Decrease in stability.

Increase in stability.

Decrease in stability.

Increase in stability.

Tends to result in athorough mixing ofthe air through thelayer where the tur-bulence exists.

Increases stability.

Decreases stability.

THE PROCESS HOW IT HAPPENS RESULTS

AG5f0404

Figure 4-4.—Air mass changes.

shown in figure 4-5. In the mid-latitudes, for an air massto be classified as arctic, the surface temperature isgenerally 0 degrees Fahrenheit (-18 degrees Celsius) orbelow.

TRAJECTORY PATHS A AND B(CYCLONIC).—Paths A and B (fig. 4-5) are usuallyindicative of a strong outbreak of cold air and surfacewinds of 15 knots or more. This wind helps to decrease

the stable conditions in the lower levels. If this modifiedair moves rapidly over rough terrain, the turbulenceresults in low stratocumulus clouds and occasionalsnow flurries (see fig. 4-6).

A particularly troublesome situation often ariseswhen the cold air flows from a cold, snow-coveredsurface to a water surface and then over a cold,snow-covered surface again. This frequently happenswith air crossing the Great Lakes. (See fig. 4-7.)

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150 O

140 O

50 O

50 O30 O

120O

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100O

60 O

60O

60O

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60O

50O

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80O

40O

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O

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40 O

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130 O

120O

110O

90O

90O

100O30 O

30 O

30O 30

O

30O

30O

80O

80O

90O

50O

60O

G B

A

C

DF

E

AG5f0405

Figure 4-5.—Trajectories of cP and cA air in winter.

CP

20 TO 30 MPHTURBULENCE

SNOWFLURRIES

CLEAR AND COLD

N S

AG5f0406

Figure 4-6.—cP air moving southward.

CP

TEMP.DIFFERENCE + 10 F

O

TEMP. 2000' 10 FO

MOISTURE ANDWARM

AIR RISING

O

SURFACE TEMP. 0 F

NW SE

WATER TEMP. 34 FO

SURFACE TEMP. 21 FO

CLEAR

SNOW FLURRIESAPPALACHIANMOUNTAINS

10 FO

3C/1000'

O

AG5f0407

Figure 4-7.—cP air moving over the Great Lakes (winter).

On the leeward side of the Great Lakes and on thewindward side of the Appalachians, you can expect arather low, broken to overcast sky condition withfrequent and widespread snow squalls. Stratocumulusand cumulus clouds with bases at 500 to 1,000 feet andtops at 7,000 to 10,000 feet form on the leeward side ofthe Great Lakes. Over the mountains, their tops extendto about 14,000 feet. Visibility ranges from 1 to 5 milesduring rain or snow showers and occasionally lowers tozero in snow flurries.

Severe aircraft icing conditions may be expectedover the mountains and light to moderate aircraft icingon the leeward side of the lakes. Moderate to severeflying conditions are the rule as long as the outflow ofcold air continues.

East of the Appalachians, skies are relatively clearexcept for scattered stratocumulus clouds. Visibility isunrestricted and the surface temperature is relativelymoderate because of turbulent mixing. In the MiddleWest, clouds associated with this type of air masscontinue for 24 to 48 hours after the arrival of the coldmass, while along the Atlantic Coast rapid passage ofthe leading edge of the air mass produces almostimmediate clearing.

TRAJECTORY PATHS C AND D (ANTI-CYCLONIC).—The weather conditions experiencedover the central United States under the influence oftrajectories similar to C and D (fig. 4-5) are quitedifferent. Unusually smooth flying conditions arefound in this region, except near the surface where aturbulence layer results in a steep lapse rate and somebumpiness. Low stratus or stratocumulus clouds in mayform at the top of the turbulence layer. As the cold airstagnates and subsides under the influence of theanticyclonic trajectory, marked the haze layers developindicating the presence of subsidence inversions. Thesurface visibility also deteriorates because of anaccumulation of smoke and dust as the air stagnates andsubsides. This is especially noticeable during the earlymorning hours when the stability in the surface layers ismost pronounced. In the afternoon, when surfaceheating has reached a maximum, the visibility usuallyimproves because of the steep lapse rate and resultantturbulence.

Movement of cPk and cAk air westward over theRocky Mountains to the Pacific coast is infrequent.However, when successive outbreaks of cold air buildup a deep layer of cP air on the eastern slopes of theRocky Mountains, relatively cold air can flow towardthe Pacific coast.

TRAJECTORY PATH E.—When the trajectoryof the cold air is similar to E in figure 4-5, rather mildtemperatures and low humidities result on the Pacificcoast because adiabatic warming of the air flowingdown the mountain slopes produces clear skies andgood visibility.

TRAJECTORY PATHS F AND G.—Occasionally, the trajectory passes out over the PacificOcean (see fig. 4-5). The air then arrives over centraland southern California as cold, convectively unstableair. This type is characterized by squalls and showers,cumulus and cumulonimbus clouds, visibility of 1 to 5miles during squalls and showers, and snow even as farsouth as southern California.

Maritime Polar (mP) Air Pacific in Winter

Maritime polar air from the Pacific dominates theweather conditions of the west coast during the wintermonths. In fact, this air often influences the weatherover most of the United States. Pacific coastal weather,while under the influence of the same general air mass,varies considerably as a result of different trajectoriesof mP air over the Pacific. Thus knowledge oftrajectories is of paramount importance in forecastingwest coast weather.

When an outbreak of polar air moves over only asmall part of the Pacific Ocean before reaching theUnited States, it usually resembles maritime arctic cold(mAk). If its path has been far to the south, it is typicallymP. Figure 4-8 shows some of the trajectories (A, B, C,D) by which mP air reaches the North American coastduring the winter.

TRAJECTORY PATH A (CYCLONIC).—Trajectory path A air originates in Alaska or northernCanada and is pulled out over the Pacific Ocean by alow center close to British Columbia in the Gulf ofAlaska. This air has a relatively short overwater pathand brings very cold weather to the Pacific Northwest.When the air reaches the coast of British Columbia andWashington after 2 to 3 days over the water, it isconvectively unstable. This instability is released whenthe air is lifted by the coastal mountain ranges. Showersand squalls are common with this condition. Ceilingsare generally on the order of 1,000 to 3,000 feet alongthe coast and generally 0 over the coastal mountainranges. Cumulus and cumulonimbus are thepredominating cloud types, and they generally extendto very high levels. Visibility is generally good becauseof turbulence and high winds commonly found withthis trajectory. Of course, in areas of precipitation, the

4-9

visibility is low. Icing conditions, generally quitesevere, are present in the clouds. After this mP air hasbeen over land for several days, it has stabilized andweather conditions improve significantly.

TRAJECTORY PATHS B AND C(CYCLONIC).—Trajectory paths B and C air with alonger overwater trajectory dominate the west coast ofthe United States during winter months. When there israpid west-to-east motion and small north-to-southmotion of pressure systems, mP air may influence theweather over most of the United States. Because of alonger overwater trajectory, this mP air is heated togreater heights, and convective instability is present upto about 10,000 feet.

This air has typical k characteristics—turbulentgusty winds, steep lapse rate, good visibility at groundexcept 0 to 3 miles in precipitation, as well as cumulusand cumulonimbus clouds with showers. Theseshowers are not as intense as those produced in theshorter trajectory mP air, but the total amount ofprecipitation is greater.

TRAJECTORY PATH D (ANTI-CYCLONIC).—This trajectory usually is over waterlong enough to permit modifications to reachequilibrium at all levels. When the air reaches the coast,it is very stable with one or two subsidence inversions.Stratus or stratocumulus clouds are frequently found.Ceilings are usually 500 to 1,500 feet and the tops ofclouds are generally less than 4,000 feet. Visibility isfair except during the early morning hours when hazeand smoke reduce the visibility to less than 1 mile. Thistype of air is found over the entire Pacific coast. It isincorrectly referred to as mT air, since it follows thenorthern boundary of the Pacific anticyclone. However,mT air does on rare occasions move into Californiaalong this path.

Gradually mP air drifts eastward with theprevailing west-east circulation. In crossing the coastalranges and the Rocky Mountains, much of the moisturein the lower layers is condensed out; the heat ofcondensation liberated is absorbed by the intermediatelayers of air. On the eastern slopes of the mountains, theair is warmed as it descends dry-adiabatically. As itflows over the cold and often snow-covered landsurface east of the mountains, the warm mP airbecomes stable in the lower layers.

The flying conditions in mP air east of the RockyMountains are in general the best that are experiencedin winter. Relatively large diurnal temperature rangesare observed. Turbulence is almost absent and visibilityis good, except for the smoke and haze in industrialareas. Ceilings are generally unlimited, since either noclouds or only a few high clouds are present. This typeof mild winter weather occasionally spreads eastwardto the Atlantic coast. When mP air crosses the RockyMountains and encounters a deep, dense dome of cP air,it is forced to overrun it and results in storm conditionsthat produce blizzards over the plains states.

Maritime Tropical (mT) Air Pacific in Winter

Maritime tropical (mT) air is observed onlyinfrequently on the Pacific coast, particularly near thesurface. Air flowing around the northern boundary ofthe Pacific anticyclone is at times mT air but is usuallymP air. This air has the weather characteristics (as wellas the low temperature) of mP air, having had a longtrajectory over the water. (See fig. 4-9.)

Occasionally the eastern cell of the Pacificanticyclone splits, and one portion moves southwardoff the coast of southern California. This portion of theanticyclone is then able to produce an influx of mT air.

4-10

50

O

AG5f0408

50

O

180

O

180

O

17

0O

40

O 17

0O

40

O

50

O

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O160

O

160

O

60

O

150 O

150 O

150 O

140 O

60 O

140 O

50 O

50 O

40 O

40 O

140 O

40 O

130 O

130 O

130 O

120 O

120 O

120 O

110 O

110 O

100O

40 O

30

O30

O

30

O

30O

30 O

30 O

130 O

30 O

20 O

AB

C

D

17

0O

Figure 4-8.—Trajectories of mP air over the Pacific Coast inwinter.

Generally the influx of mT air is carried aloft by arapidly occluding frontal system somewhere oversouthern California, producing the heaviestprecipitation recorded in that area. Occasionally mT airis seen above the surface with pronounced stormdevelopments over the Great Basin. Since large, open,warm sectors of mT air do not occur along the westcoast, representative air mass weather is notexperienced. Flying conditions are generally restrictedwhen this air is present, mainly because of low frontalclouds and reduced visibility in precipitation areas.

Maritime Polar (mP) Air Atlantic in Winter

Maritime polar air, which originates in the Atlantic,becomes significant at times along the east coast. It isnot nearly so frequent over North America as the othertypes because of the normal west-east movement of allair masses. This type of air is observed over the eastcoast in the lower layers of the atmosphere whenever acP anticyclone moves slowly off the coast of themaritime provinces and New England. (See fig. 4-10.)This air, originally cP, undergoes less heating than itsPacific counterpart because the water temperatures arecolder and also because it spends less time over thewater. This results in the instability being confined tothe lower layers of this air. The intermediate layers ofthis air are very stable. Showers are generally absent;however, light drizzle or snow and low visibility arecommon. Ceilings are generally about 700 to 1,500 feet

with tops of the clouds near 3,000 feet. Markedsubsidence above the inversion ensures that cloudscaused by convection will not exist above that level.

The synoptic weather condition favorable to mP airover the east coast is usually also ideal for the rapiddevelopment of a warm front with maritime tropical airto the south. Maritime tropical air then overruns the mPair and a thick cloud deck forms. Clouds extendingfrom near the surface to at least 15,000 feet areobserved. Ceilings are near zero and severe icingconditions exist in the cold air mass. Frequently,freezing rain and sleet are observed on the ground.Towering cumulus clouds prevail in the warm air andoften produce thunderstorms.

Flying conditions are rather dangerous with mP airbecause of turbulence and icing conditions present nearthe surface. Poor visibility and low ceilings areadditional hazards. The cloudiness associated with themP air mass usually extends as far west as theAppalachians.

Maritime Tropical (mT) Air Atlantic in Winter

Temperature and moisture content are higher in mTair masses than in any other American air mass inwinter. In the southern states, along the Atlantic coast

4-11

AG5f0409

140 O

50 O

mT

150 O

130O

120O

120O130

O

40 O

40 O

40 O

40O

110O

130O

140 O

30 O

30 O

30O

30O

120O

140 O

50 O

mP

Figure 4-9.—Trajectory of mT air over the Pacific in winter.AG5f0410

40O

cP50

O

50O

50O

50O

70O

50O 50

O

60O

80O

60O

100O

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40O

70O

40O

60O

60O

70O

80O

90O

30O 30

O

30O

mP

90O 70O

NOTE: The representation of internationalboundaries on this chart is not necessarilyauthoritative.

Figure 4-10.—Trajectory of mP air over the Atlantic in winter.

and Gulf of Mexico (fig. 4-11), mild temperatures, highhumidities, and cloudiness are found, especially duringthe night and early morning. This is the characteristicweather found in mT air in the absence of frontalconditions. The stratus and stratocumulus clouds thatform at night tend to dissipate during the middle of theday and fair weather prevails. Visibility is generallypoor when the cloudiness is present; however, itimproves rapidly because of convective activity whenthe stratus clouds dissipate. The ceilings associatedwith the stratus condition generally range from 500 to1,500 feet, and the tops are usually not higher than3,500 to 4,500 feet. Precipitation does not occur in the

absence of frontal action. With frontal activity, theconvective instability inherent in this air is released,producing copious precipitation.

If mT air is forced over mountainous terrain, as inthe eastern part of the United States, the conditionalinstability of the air is released at higher levels. Thismight produce thunderstorms or at least largecumuliform clouds. (See fig. 4-12.) Pilots must beaware that these clouds may develop out of stratiformcloud systems and therefore may occur withoutwarning. Icing may also be present. Thus, in the GreatLakes area, a combination of all three hazards (fog,thunderstorms, and icing) is possible.

Occasionally when land has been cooled along thecoastal area in winter, maritime tropical air flowinginland produces an advection fog over extensive areas.(See fig. 4-13.) In general, flying conditions under thissituation are fair. Ceilings and visibilities areoccasionally below safe operating limits; however,flying conditions are relatively smooth and icingconditions are absent near the surface layers.

As the trajectory carries the mT air northward overprogressively colder ground, the surface layers cool andbecome saturated. This cooling is greatly accelerated ifthe surface is snow or ice covered or if the trajectorycarries the air over a cold-water surface. Depending onthe strength of the air mass, fog with light winds or alow stratus deck with moderate to strong winds formsrapidly because of surface cooling. Occasionallydrizzle falls from this cloud form; and visibility, evenwith moderate winds, is poor. Frontal lifting of mT airin winter, even after the surface layers have becomestabilized, results in copious precipitation in the form ofrain or snow.

4-12

AG5f0411

mT

40O

100O

40O

90O

80O

50O 50

O

70O

40O 40

O

70O

30O

30O

80O

30O

100O

80O

70O

20O

mT20

O

Figure 4-11.—Trajectories of mT air over the Atlantic inwinter.

WATER TEMP. 20 C.O

GULF OF MEXICO DRIZZLE AND FOG

SW NE

APPALACHIANS

AG5f0412

10 C. SURFACE TEMP. 0 C.O O

STRATUSmT

CUMULONIMBUS

Figure 4-12.—mT air moving northeastward.

During the winter, air resembling mT isoccasionally observed flowing inland over the gulf andsouth Atlantic states. Generally the air that had arelatively short trajectory over the warm waters off thesoutheast coast is cP air. Clear weather usuallyaccompanies cP air in contrast to cloudy weatheraccompanying a deep current of mT air. On surfacesynoptic charts, the apparent mT air can bedistinguished from true mT air by the surface dew-pointtemperature value. True mT air always has dew-pointtemperature values in excess of 60°F. The highlymodified cP air usually has dew-point values between50°F and 60°F.

NORTH AMERICAN AIR MASSES,TRAJECTORIES, AND WEATHER(SUMMER)

During the summer most of the United States isdominated by either S or mT air, whereas Canada andthe northwestern United States are dominated by polarair. Occasionally, tropical air is transported to theCanadian tundra and Hudson Bay region.

Continental Polar (cP) Air in Summer

Continental polar (cP) air has characteristics andproperties quite different from those of its wintercounterpart. Because of the long days and the higheraltitude of the sun (as well as the absence of a snowcover over the source region), this air is usuallyunstable in the surface layers, in contrast to the markedstability found in cP air at its source in winter. By thetime this air reaches the United States, it can no longerbe distinguished from air coming in from the NorthPacific or from the Arctic Ocean. (See fig. 4-14.)

Clear skies or scattered cumulus clouds withunlimited ceilings characterize this mass at its sourceregion. Occasionally, when this air arrives over thecentral and eastern portion of the United States, it is

4-13

WARM OCEAN COLD CONTINENT

FOG OR LOW STRATUS

AG5f0413

Figure 4-13.—mT (Gulf of Mexico or Atlantic) air of wintermoving northward over cold continent.

AG5f0414

cP

70O

60O60

O

80O

60O

50O

50O

100O

50O

120 O

120 O

130 O

130 O

140 O

50 O

50 O

40 O

40 O

40O

40O

120 O

120 O

30 O

30O

30O

110O

110O

100O

100O

40O

40O90

O

80O

80O

90O

30O

70O

40O

50 O

50 O

Figure 4-14.—Continental polar (cP) air in summer.

characterized by early-morning ground fogs or lowstratus decks. Visibility is generally good except whenhaze or ground fog occurs near sunrise. Convectiveactivity, usually observed during the daytime, ensuresthat no great amounts of smoke or dust accumulate inthe surface layers. An exception to this is found understagnant conditions near industrial areas, whererestricted visibility may occur during the day and night.Pronounced surface diurnal temperature variations areobserved in cP air during summer.

The convective activity of this air is generallyconfined to the lower 7,000 to 10,000 feet. Flyingconditions are generally smooth above approximately10,000 feet except when local showers develop.Showers, when observed, usually develop in a modifiedtype of cPk over the southeastern part of the country.The base of cumulus clouds that form in this air isusually about 4,000 feet because of the relative drynessof this air mass.

Maritime Polar (mP) Air Pacific in Summer

The entire Pacific coast is usually under theinfluence of mP air in the summer. (See fig. 4-15.) Witha fresh inflow of mP air over the Pacific coast, clearskies or a few scattered cumulus are generally observedover the coastal mountains. As this air flows southwardalong the coast, a marked turbulence inversionreinforced by subsidence from aloft is observed. Stratusor stratocumulus clouds generally form at the base ofthe inversion. Ceilings are generally 500 to 1,500 feetwith tops of clouds seldom above 3,500 feet. The

formation of the stratus condition along the coast ofCalifornia is greatly enhanced by the presence of theupwelling of cold water along the coast. East of theRocky Mountains, this air has the same properties as cPair.

Maritime Polar (mP) Air Atlantic in Summer

In spring and summer, mP air is occasionallyobserved over the east coast. Marked drops intemperature that frequently bring relief from heatwaves usually accompany the influx of this air (fig.4-16). Just as in winter, there is a steep lapse rate in thelower 3,000 feet of this mass. Stratiform clouds usuallymark the inversion. Ceilings are from 500 to 1,500 feet,and the tops of the clouds are usually 1,000 to 2,500feet. No precipitation occurs from these cloud types andvisibility is usually good. This air usually does notconstitute a severe hazard to flying.

Maritime Tropical (mT) Air Pacific in Summer

Maritime tropical (mT) Pacific air has no directinfluence on the weather over the Pacific coast. Duringthe summer season, the Pacific anticyclone movesnorthward and dominates the Pacific Coast weatherwith mP air. Occasionally mT air reaches the WestCoast; for example, tropical storms or typhoonssometimes move northerly along the Baja Coast. Thissynoptic condition produces a great amount ofcloudiness and precipitation.

4-14

AG5f0415

mP

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O

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O

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110O

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130O

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O

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Figure 4-15.—Trajectories of mP air over the Pacific insummer.

AG5f0416

mP

100 O

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40O

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80O

30O

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Figure 4-16.—Trajectories of mP air over the Atlantic insummer.

Maritime Tropical (mT) Air Atlantic in Summer

The weather in the eastern half of the United Statesis dominated by mT air in summer (fig. 4-17). As inwinter, warmth and high moisture content characterizethis air. In summer, convective instability extends tohigher levels; there is also a tendency toward increasinginstability when the air moves over a warmer landmass.(See fig. 4-18.) This is contrary to winter conditions.

Along the coastal area of the southern states, thedevelopment of stratocumulus clouds during the earlymorning is typical. These clouds tend to dissipateduring the middle of the morning and immediatelyreform in the shape of scattered cumulus. Thecontinued development of these clouds leads toscattered showers and thunderstorms during the lateafternoon. Ceilings in the stratocumulus clouds aregenerally favorable (700 to 1,500 feet) for the operationof aircraft. Ceilings become unlimited with thedevelopment of the cumulus clouds. Flying conditionsare generally favorable despite the shower andthunderstorm conditions, since the convective activityis scattered and can be circumnavigated. Visibility isusually good except near sunrise when the air isrelatively stable over land.

When mT air moves slowly northward over thecontinent, ground fogs frequently form at night. Seafogs develop whenever this air flows over a relativelycold current such as that occurring off the east coast.The notorious fogs over the Grand Banks ofNewfoundland are usually formed by this process.

In late summer, the Bermuda high intensifies attimes and seems to retrograde westward. This results ina general flow of mT air over Texas, New Mexico,Arizona, Utah, Colorado, and even southern California.The mT air reaching these areas is very unstablebecause of the intense surface heating and orographiclifting it undergoes after leaving the source region in theCaribbean and Gulf of Mexico. Shower andthunderstorm conditions, frequently of cloudburstintensity, then prevail over the southwestern states.Locally this condition is termed sonora weather.

4-15

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O

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Figure 4-17.—Maritime tropical (mT) air, Atlantic, in summer.

WARM OCEAN WARMER CONTINENT

AG5f0418

Figure 4-18.—mT (Gulf of Mexico or Atlantic) air in summermoving northward over a warm continental surface.

Continental Tropical (cT) Air in Summer

Continental tropical air is found over the UnitedStates only in the summer. Its source region is therelatively small area over the northern portion ofMexico, western Texas, New Mexico, and easternArizona. High surface temperatures and very lowhumidities are the main air mass characteristics. Largediurnal temperature ranges and the absence ofprecipitation are additional properties of cT air. Flyingconditions are excellent. However, during the daytimeturbulence sometimes extends from the surfacethroughout the average flying levels.

Superior (S) Air in Summer

Superior air usually exists over the southwesternstates and is believed to be the result of strong subsidingmotions. Most frequently this air is observed above aninversion layer at high levels; it rarely descends to thesurface. Above the inversion layer, this superior air isthe warmest air mass observed in the United States at itsaltitude; but, because of its steep lapse rate, itstemperature at higher levels is less than that of tropicalair. Relative humidity is usually less than 30 percent.Quite often they are too low to measure accurately.

Superior air is observed in both summer and winter.Flying conditions are excellent in this air mass, since nocloud forms are present and visibilities are usually verygood because of the dryness. This type of air mass isvery important because superior air frequently stops allconvective activity caused by intruding maritimetropical air. This generally prevents the formation ofshowers and thunderstorms unless the mT air mass isdeep.

NOTE: Views A and B of figure 4-19 show theproperties of significant North American air massesduring the winter and summer seasons from thestandpoint of flying.

AIR MASSES OVER ASIA

The air masses commonly observed over Asia(especially eastern Asia) are continental polar,maritime tropical, and equatorial. Maritime polar andcontinental tropical air play a minor part in the air masscycle of Asia.

Continental Polar (cP) Air

Continental polar air, as observed over the interiorof Asia, is the coldest air on record in the Northern

Hemisphere. This is brought about by the fact that theinterior of Asia, made up of vast level and treelessregions, serves as an ideal source region. The Himalayamountain range, across southern Asia, aids in theproduction of cP air. It tends to keep the polar air overthe source region for a long time and to block the inflowof tropical air from the lower latitudes.

The weather conditions over eastern Asia aregoverned by this air mass throughout the winter.Successive outbreaks of this air occur over Siberia,China, and the Japanese Islands and establish the winterweather pattern. The weather conditions prevailing inthis air are similar to those found in cP air over theeastern portion of North America.

The cold air that is forced southward over theHimalaya Mountains to India and Burma arrives in ahighly modified form and is known as the wintermonsoon. The weather conditions during the wintermonsoon are dominated by the dry and adiabaticallywarmed polar air flowing equator-ward. It is whileunder the influence of these monsoon conditions thatgenerally pleasant weather prevails over most of thearea.

Maritime Tropical (mT) Air

Maritime tropical air is usually observed along thecoast of China and over the Japanese Islands during thesummer. In structure it is almost identical to the mT airobserved off the east coast of North America. Theweather conditions found in this air are similar to thoseof its North American counterpart.

Equatorial (E) Air

Equatorial air is observed over southeastern Asia.During the summer all of India and Burma are under theinfluence of E air, because of the summer monsooncirculation. In the wintertime, when offshore windsprevail, E air is not found over the landmasses but isfound some distance offshore. Equatorial air is anextremely warm and moist air mass. It has great verticaldepth, often extending beyond 20,000 feet in height.This entire column is unstable, and any slight lifting orsmall amount of surface heating tends to release theinstability and produce showers and squalls. Theequatorial air observed over India and Burma is almostidentical in structure with E air found all along theequatorial zone over the entire Earth. Unmodifiedequatorial air is observed over India and Burma duringthe summer monsoon.

4-16

4-17

cP (near sourceregion)

cP (southeast ofGreat Lakes)

mP (on Pacificcoast)

mP (east ofRockies)

mP (east coast)

mT (Pacificcoast)

mT (east ofRockies)

None

Stratocumulus and cumulustops 7,000-10,000 feet.

Cumulus tops above20,000 feet.

None

Stratocumulus and stratustops 6,000-8,000 feet.

Stratus or stratocumulus.

Stratus or stratocumulus

Unlimited

500-1,000 feet,0 over mountains.

1,000-3,000 feet,0 over mountains.

Unlimited

0-1,000 feet

500-1,500 feet

100-1,5000 feet

Excellent (except near indus-trial areas, then 1-4 miles).

1-5 miles, 0 in snowflurries.

Good except 0 over mountainsand in showers.

Excellent except near indus-trial areas, then 1-4 miles.

Fair except 0 in precipi-tation area.

Good

Good

Smooth except withhigh winds velocities.

Moderate turbulenceup to 10,000 feet.

Moderate to strongturbulence.

Smooth except in lowerlevels with high winds.

Rough in lower levels.

Smooth

Smooth

-10 to -60.

0 to 20.

45 to 55.

30 to 40.

30 to 40.

55 to 60.

60 to 70.

WINTERAIR MASSES CLOUDS CEILINGS VISIBILITIES TURBULENCE

SURFACETEMPERA-TURE F.A

cP (near sourceregion)

cP (Pacificcoast)

mP (east ofPacific)

S (MississippiValley)

mT (east ofRockies)

Scattered cumulus.

Stratus tops,2,000-5,000 feet.

None except scattered cumu-lus near mountains.

None

Stratocumulus early morning;cumulonimbus afternoon.

Unlimited

100 feet-2,500 feet, un-limited during day.

Unlimited

Unlimited

500-1,000 feet a.m.;3,000-4,000 feet p.m.

Good

1/2 - 10 miles

Excellent

Excellent

Excellent

Moderate turbulence up to10,000 feet.

Slightly rough in clouds.Smooth above.

Generally smooth except overdesert regions in afternoon.

Slightly rough up to 15,000feet.

Smooth except in thunder-strorms, then strong tur-bulence.

55-60

50-60

60-70

75-85

75-85

SUMMERAIR MASSES CLOUDS CEILINGS VISIBILITIES TURBULENCE

SURFACETEMPERA-TURE F.B

AG5f0419

Figure 4-19.—Properties of significant air masses over North America from the standpoint of flying—(A) Winter; (B) Summer.

The weather conditions during the summermonsoon consist of cloudy weather with almostcontinuous rain and widespread shower activity. Hightemperatures and high humidities further add to thediscomfort.

AIR MASSES OVER EUROPE

Although, in general, the characteristics of airmasses over Europe are much the same as those foundover North America, certain differences do exist. Onereason for this is that an open ocean extends betweenEurope and North America toward the Arctic. Thisallows an influx of mA air to reach Europe. This type ofair is not encountered over North America. Thelocation of an extensive mountain range in an east-westdirection across southern Europe is an additionalinfluence not present over North America, where theprevailing ranges are oriented in a north-southdirection.

If the trajectory of the air is observed carefully andthe modifying influences of the underlying surface areknown, it is easy to understand the weather and flyingconditions that occur in an air mass over any continentor ocean.

Maritime Arctic (mA) Air in Winter

Maritime arctic air is observed primarily overwestern Europe. Strong outbreaks of this air,originating in the Arctic between Greenland andSpitsbergen, usually follow a cyclonic trajectory intowestern Europe.

Because of their moisture content and instability,cumulus and cumulonimbus clouds are typical of thisair mass, frequently producing widespread showers andsqualls. Visibility is generally good, but moderate tosevere icing often affects aircraft operations.

With the presence of a secondary cyclonic systemover France or Belgium, mA air occasionally sweepssouthward across France to the Mediterranean, givingrise to the severe mistral winds of the Rhone Valley andthe Gulf of Lyons. Heavy shower and thunderstormconditions are typical in this situation.

Maritime Arctic (mA) Air in Summer

In summer, this air is so shallow that in movingsouthward from its source region, it modifies to thepoint where it can no longer be identified and is thenindicated as mP air.

Maritime Polar (mP) Air in Winter

Maritime polar air observed over Europe usuallyoriginates in the form of cP air over North America. Itreaches the west coast of Europe by various trajectoriesand is found in different stages of modification; itproduces weather similar to mP air over the west coastof North America.

Maritime Polar (mP) Air in Summer

Maritime polar air observed over Europe is similarto mP air observed on the west coast of North America.The weather conditions associated with this air aregenerally good. Occasionally, because of surfaceheating, a shower or thunderstorm is observed in thedaytime over land.

Continental Arctic (cA) and Continental Polar(cP) Air in Winter

The source region for cA and cP air is over northernRussia, Finland, and Lapland. The cA and cP airmasses are observed over Europe in connection with ananticyclone centered over northern Russia and Finland.Occasionally they reach the British Isles and at timesextend southward to the Mediterranean.

Because of the dryness of cA and cP air, clouds areusually absent over the continent. Fair-weathercumulus are the typical clouds when cA and cP air areobserved over the British Isles. Over theMediterranean, cA and cP air soon become unstableand give rise to cumulus and cumulonimbus cloudswith showers. Occasionally these air masses initiate thedevelopment of deep cyclonic systems over the centralMediterranean. Visibility is usually good; however,after this type becomes modified, haze layers form andreduce the visibility.

Continental Arctic (cA) and Continental Polar(cP) Air in Summer

The source region for cA and cP air is the same asfor its counterpart in winter. It is a predominantly dryair mass and produces generally fair weather over thecontinent and the British Isles. The visibility is usuallyreduced because of haze and smoke in the surfacelayers. As cA and cP air stream southward, the lowerlayers become unstable; and eventually convectiveclouds and showers develop in the later stages of theirlife cycles.

4-18

Maritime Tropical (mT) Air in Winter

Maritime tropical air that arrives over Europeusually originates over the southern portion of theNorth Atlantic under the influence of the Azoresanticyclone. Maritime tropical air is marked bypronounced stability in the lower layers and typicalwarm-mass cloud and weather conditions. Relativelyhigh temperatures accompany the influx of mT air, andthe moisture content is greater than in any other airmass observed in the middle latitudes of Europe.Visibility is, as a rule, reduced because of the presenceof fog and drizzle, which are frequently observed withan influx of mT air. Maritime tropical air in winterexists only in western Europe. By the time it reachesRussia, it is generally found aloft and greatly modified.

Maritime Tropical (mT) Air in Summer

In general, mT air has the same properties as itscounterpart in winter with the exception that it is lessstable over land because of surface heating.Additionally, this air mass loses its maritimecharacteristics soon after passing inland.

Over water, mT air is still a typical warm air mass.Sea fog frequently occurs in the approaches to theEnglish Channel during the spring and early summer.Visibility in mT air is generally better in summer thanin winter, particularly over land where convectioncurrents usually develop.

Maritime tropical air flowing over theMediterranean in summer usually changes to a coldmass, since the water temperature of the Mediterraneanis then slightly higher than that of the air. Weakconvection currents prevail, usually sufficiently strongto form cumulus clouds but seldom sufficiently strongto produce showers.

Continental Tropical (cT) Air in Winter

The continental tropical air that arrives over Europein winter originates over North Africa. By the time itreaches central Europe, it differs little from mT air. Ingeneral, a cT air mass is much more prevalent oversouthern Europe than over central or western Europe.Although the moisture content of cT air is less than thatobserved in mT air, the visibility is not much better,primarily because of the dust that cT air picks up whileover Africa. This air mass constitutes the major sourceof heat for the development of the Mediterraneancyclonic storms, most common during the winter andspring months.

Continental Tropical (cT) Air in Summer

The cT air usually develops over North Africa, AsiaMinor, and the southern Balkans. At its source region,the air is dry and warm as well as unstable. The NorthAfrican air mass is the hottest air mass on record in theworld. In its northward flow over southern Europe, cTair absorbs moisture and increases its convectiveinstability. The summer showers and thunderstormsobserved over southern Europe are often produced by amodified cT air mass. This air mass is much moreprevalent over southern Europe than is its wintercounterpart.

AIR MASSES IN THE SOUTHERNHEMISPHERE

The air masses of the Southern Hemisphere arepredominantly maritime. This is because of theoverwhelming preponderance of ocean areas. Greatmeridional (south-north and north-south) transports ofair masses, as they are known in the NorthernHemisphere, are absent because the westerlies aremuch more developed in the Southern Hemisphere thanin the Northern Hemisphere. Except for Antarctica,there are no large landmasses in the high latitudes in theSouthern Hemisphere; this prevents sizable invasionsof antarctic air masses. The large landmasses near theequator, on the other hand, permit the extensivedevelopment of warm air masses.

The maritime tropical air masses of the SouthernHemisphere are quite similar to their counterparts ofthe Northern Hemisphere. In the large area of Brazil,there are two air masses for consideration. One is theregular air mass from the Atlantic, which is composedof unmodified mT air. The other originates in theAtlantic; but by the time it spreads over the hugeAmazon River basin, it undergoes two importantchanges—the addition of heat and moisture. As a resultof strong summer heating, a warm, dry continentaltropical (cT) air mass is located from 30° south to 40°south.

The maritime polar air that invades South Americais quite similar to its counterpart in the United States.Maritime polar air occupies by far the most territory inthe Southern Hemisphere, encircling it entirely.

Australia is a source region for continental tropicalair. It originates over the vast desert area in the interior.Except along the eastern coast, maritime tropical airdoes not invade Australia to a marked degree. This air isbrought down from the north, particularly in the

4-19

summer, by the counterclockwise circulation aroundthe South Pacific high.

Antarctica is a great source region for intenselycold air masses. The temperatures are colder than in thearctic regions. These air masses have continentalcharacteristics, but before the air reaches other landareas, it becomes modified and is properly calledmaritime polar.

During the polar night the absence of insolationcauses a prolonged cooling of the snow surface, whichmakes Antarctica a permanent source of very cold air. Itis extremely dry and stable aloft. This polar air mass isreferred to as continental antarctic (cA) air. In summerthe continent is not as cold as in winter because ofconstant solar radiation but continues to function as asource for cold cA air.

In both winter and summer, the air mass isthermally modified as it flows northward throughdownslope motion and surface heating; as a result, itbecomes less stable. It assumes the characteristics ofmaritime antarctic air. The leading edge of this air massthen becomes the northern boundary of the antarcticfront.

To the north of the antarctic front is found a vastmass of maritime polar air that extends around thehemisphere between 40°S and 68°S in summer andbetween 34°S and 65°S in winter. At the northern limitof this air mass is found the Southern Hemisphere polarfront. During summer this mP air is by far the mostimportant cold air mass of the hemisphere because ofthe lack of massive outbreaks of cold continental airfrom Antarctica.

Different weather conditions occur with each typeof air mass. The cA air produces mostly clear skies. ThemA air masses are characterized generally by anextensive overcast of stratus and stratocumulus cloudswith copious snow showers within the broad zone of theantarctic front. An area of transition that extendsmainly from the coastline to the northern edge of theconsolidated pack ice is characterized by broken toovercast stratocumulus clouds with somewhat higherbases and little precipitation.

REVIEW QUESTIONS

Q4-1. What is the definition of an air mass?

Q4-2. Name the two factors that are necessary toproduce an air mass?

Q4-3. What type of air mass is mTk?

Q4-4. What are the two modifying influences on airmasses?

Q4-5. What is the warmest air mass observed in theUnited States at its altitude?

FRONTS

LEARNING OBJECTIVE: Describe thespecific parts that make up a front and identifyhow a front is classified as either cold, warm,occluded, or quasi-stationary.

A front, generally speaking, is a zone of transitionbetween two air masses of different density andtemperature and is associated with major weatherchanges, some of which can be violent. This fact aloneis sufficient reason for an in-depth study of fronts andtheir relationship to air masses and cyclones.

DEFINITIONS AND CLASSIFICATIONS

A front is not just a colorful line drawn on a surfacechart. A front is a three-dimensional phenomena with avery specific composition. Since a front is a zone oftransition between two air masses of different densities,there must be some sort of boundary between these airmasses. One of these boundaries is the frontal surface.The frontal surface is the surface that separates the twoair masses. It is the surface next to the warmer air (lessdense air). In reality, however, the point at which two airmasses touch is not a nice, abrupt separation. This areais a zone of a large density gradient. This zone is calledthe frontal zone. A frontal zone is the transition zonebetween two adjacent air masses of different densities,bounded by a frontal surface. Since the temperaturedistribution is the most important regulator ofatmospheric density, a front almost invariably separatesair masses of different temperatures.

At this point you should be aware of the varioustypes of fronts. The question in your mind should behow a front is classified. Whether it is cold, warm, orstationary. A front is classified by determining theinstantaneous movement. The direction of movementof the front for the past 3 to 6 hours is often used.Classification is based on movement relative to thewarm and cold air masses involved. The criterion is asfollows:

Cold Front

A cold front is one that moves in a direction inwhich cold air displaces warm air at the surface. Inother words the cold (or cooler) air mass is moving

4-20

toward a warmer air mass. The cooler, denser air issliding under the warmer, less dense air displacing itupward.

Warm Front

A warm front is one along which warmer airreplaces colder air. In this case, a warmer air mass ismoving toward a cooler retreating air mass. Thewarmer, less dense air moves only toward and replacesthe colder, denser air if the colder air mass is alsomoving.

Quasi-Stationary Front

This type front is one along which one air massdoes not appreciably replace the other. These fronts arestationary or nearly so (speed under 5 knots). They canmove or undulate toward either the cold or warm airmass.

Occluded Front

An occluded front is one where a cold frontovertakes a warm front, forcing the warm air upward.The occluded front may be either a warm front or a coldfront type. a warm front type is one in which the cool airbehind the cold front overrides the colder air in advanceof the warm front, resulting in a cold front aloft. A coldfront type is one in which the cold air behind the coldfront under rides the warm front, resulting in a warmfront aloft.

RELATION OF FRONTS TO AIRMASSES AND CYCLONES

LEARNING OBJECTIVE: Describe therelationship of fronts to air masses and stableand unstable wave cyclones.

RELATION OF FRONTS TO AIR MASSES

At this point you should have figured out thatwithout air masses there would be no fronts. Thecenters of action are responsible for bringing the airmasses together and forming frontal zones.

The primary frontal zones of the NorthernHemisphere are the arctic frontal zone and the polarfrontal zone. The most important frontal zone affectingthe United States is the polar front. The polar front isthe region of transition between the cold polar air andwarm tropical air. During the winter months (in theNorthern Hemisphere), the polar front pushes farthersouthward, because of the greater density of the polarair, than during the summer months. During thesummer months (in the Northern Hemisphere), thepolar front seldom moves farther south than the centralUnited States.

On a surface map a front is indicated as a lineseparating two air masses; this is only a picture of thesurface conditions. These air masses and fronts extendvertically. (See fig. 4-20.)

A cold air mass, being heavier, acts like a wedgeand tends to under run a warm air mass. Thus, the coldair is below and the warm air is above the surface ofdiscontinuity. This wedge of cold air produces a slopeof the frontal surface. This slope is usually between 1 to50 (1-mile vertical for 50 miles horizontal) for a coldfront and 1 to 300 (1-mile vertical for 300 mileshorizontal) for a warm front. For example, 100 milesfrom the place where the frontal surface meets theground, the frontal surface might be somewherebetween 2,000 feet and 10,000 feet above Earth’ssurface, depending on the slope. The slope of a front isof considerable importance in visualizing andunderstanding the weather along the front.

4-21

COLD-FRONTSLOPE

COLD AIR

WARM AIR

WARM-FRONTSLOPE

COLD AIR

AG5f0420

Figure 4-20.—Vertical view of a frontal system (clouds not shown).

RELATION OF FRONTS TO CYCLONES

There is a systemic relationship between cyclonesand fronts, in that the cyclones are usually associatedwith waves along fronts—primarily cold fronts.Cyclones come into being or intensify because pressurefalls more rapidly at one point than it does in thesurrounding area. Cyclogenesis can occur anywhere,but in middle and high latitudes, it is most likely tooccur on a frontal trough. When a cyclone (or simplylow) develops on a front, the cyclogenesis begins at thesurface and develops gradually upward as the cyclonedeepens. The reverse also occurs; closed circulationsaloft sometime work their way downward until theyappear on the surface chart. These cyclones rarelycontain fronts and are quasi-stationary or drift slowlywestward and/or equatorward.

Every front, however, is associated with a cyclone.Fronts move with the counterclockwise flow associatedwith Northern Hemisphere cyclones and clockwisewith the flow of Southern Hemisphere cyclones. Themiddle latitudes are regions where cold and warm airmasses continually interact with each other. Thisinteraction coincides with the location of the polarfront.

When the polar front moves southward, it is usuallyassociated with the development and movement ofcyclones and with outbreaks of cold polar air. Thecyclonic circulation associated with the polar fronttends to bring polar air southward and warm moisttropical air northward.

During the winter months, the warm airflowusually occurs over water and the cold air movessouthward over continental areas. In summer thesituation is reversed. Large cyclones that form on thepolar front are usually followed by smaller cyclonesand are referred to as families. These smaller cyclones

tend to carry the front farther southward. In an idealsituation these cyclones come in succession, causingthe front (in the Northern Hemisphere) to lie in asouthwest to northeast direction.

Every moving cyclone usually has two significantlines of convergence distinguished by thermalproperties. The discontinuity line on the forward side ofthe cyclone where warm air replaces cold air is thewarm front; the discontinuity line in the rear portion ofthe cyclone where cold air displaces warm air is thecold front.

The polar front is subject to cyclonic developmentalong it. When wind, temperature, pressure, and upperlevel influences are right, waves form along the polarfront. Wave cyclones normally progress along the polarfront with an eastward component at an average rate of25 to 30 knots, although 50 knots is not impossible,especially in the case of stable waves. These waves mayultimately develop into full-blown low-pressuresystems with gale force winds. The development of asignificant cyclone along the polar front depends onwhether the initial wave is stable or unstable. Waveformation is more likely to occur on slowly moving orstationary fronts like the polar front than on rapidlymoving fronts. Certain areas are preferred localities forwave cyclogenesis. The Rockies, the Ozarks, and theAppalachians are examples in North America.

Stable Waves

A stable wave is one that neither develops noroccludes, but appears to remain in about the same state.Stable waves usually have small amplitude, weak lowcenters, and a fairly regular rate and direction ofmovement. The development of a stable wave is shownin views A, B, and C of figure 4-21. Stable waves do notgo into a growth and occlusion stage.

4-22

COLDAIR MASS

AG5f0421

POLARFRONT

WARMAIR MASS

A. COLD AND WARM AIR FLOW B. FORMATION C. TYPICAL WAVE

Figure 4-21.—Life cycle of a stable wave cyclone.

Unstable Waves

The unstable wave is by far the more common wavethat is experienced with development along the polarfront. The amplitude of this wave increases with timeuntil the occlusion process occurs. The formation of adeep cyclone and an occluded front breaks up the polarfront. When the occlusion process is complete, thepolar front is reestablished. This process is shown infigure 4-22. Views A through G of figure 4-22, referredto in the next three paragraphs, show the life cycle ofthe unstable wave.

In its initial stage of development, the polar frontseparates the polar easterlies from the mid-latitudewesterlies (view A); the small disturbance caused bythe steady state of the wind is often not obvious on theweather map. Uneven local heating, irregular terrain, or

wind shear between the opposing air currents may starta wavelike perturbation on the front (view B); if thistendency persists and the wave increases in amplitude,a counterclockwise (cyclonic) circulation is set up. Onesection of the front begins to move as a warm frontwhile the adjacent sections begin to move as a coldfront (view C). This deformation is called a frontalwave.

The pressure at the peak of the frontal wave falls,and a low-pressure center is formed. The cycloniccirculation becomes stronger; the wind components arenow strong enough to move the fronts; the westerliesturn to southwest winds and push the eastern part of thefront northward as a warm front; and the easterlies onthe western side turn to northerly winds and push thewestern part southward as a cold front. The cold front ismoving faster than the warm front (view D). When the

4-23

COLD

WARM

COLD

WARM

COLD

FRONTAL

WAVE

WARM

COOL

COLD

WARMWARM

COLD

WARM

COLD

COOL

WARM

COLD

COOL

A

B

C

D

E

F

GAG5f0422

Figure 4-22.—Life cycle of an unstable frontal wave.

cold front overtakes the warm front and closes thewarm sector, an occlusion is formed (view E). This isthe time of maximum intensity of the wave cyclone.

As the occlusion continues to extend outward, thecyclonic circulation diminishes in intensity (thelow-pressure area weakens), and the frontal movementslows down (view F). Sometimes a new frontal wavemay begin to form on the westward trailing portion ofthe cold front. In the final stage, the two fronts becomea single stationary front again. The low center with itsremnant of the occlusion has disappeared (view G).Table 4-2 shows the numerical average life cycle of atypical unstable wave cyclone from initial developmentto cyclolysis. It is only intended to be used as a guide inareas where reports are sparse.

FRONTOGENESIS AND FRONTOLYSIS

LEARNING OBJECTIVE: Describe theconditions necessary for frontogenesis andfrontolysis, and identify the worldfronto-genetical zones.

CONDITIONS NECESSARY FORFRONTOGENESIS

Frontogenesis is the formation of a new front or theregeneration of an old one. Frontogenesis takes placeonly when two conditions are met. First, two air massesof different densities must exist adjacent to one another;and second, a prevailing wind field must exist to bringthem together. There are three basic situations, whichare conducive to frontogenesis and satisfy the two basicrequirements.

The windflow is cross isothermal and flowing fromcold air to warmer air. The flow must be crossisothermal, resulting in a concentration of isotherms(increased temperature gradient). The flow does nothave to be perpendicular; however, the moreperpendicular the cross isothermal flow, the greater theintensity of frontogenesis.

The winds of opposite air masses move toward thesame point or line in that cross-isothermal flow. Aclassic example of this situation is the polar front wherecold polar air moves southward toward warmer

4-24

Time (hours)

Central Pressure (mb)

Direction of Movement(toward)

Speed of Movement(knots)

NE to SE or(quad)

30-35

1,012-1,000

Wave Cyclone

1000-988

NNE to N(arc)

20-25

984-968

N to NNW(arc)

10-15

998-1,004

0-5

Occlusion Mature Occlusion Cyclolysis

0 12-24 24-36 36-72

The symbol indicates that the filling center drifts slowly in a counterclockwise direction alongan approximately circular path about a fixed point.

AG5t0402

Table 4-2.—Numerical Characteristics of the Life Cycle of an Unstable Wave Cyclone

T1

T2

T3

T4

T5

H L

XX

L H

Y

T6

Y

T1T2

T3T4

T5T6

H L

Y

XX

L H

X

T1T2T3T4T5T6

H L

Y

X

L H

A. IDEALLY FRONTOGENETIC B. CRITICALLY FRONTOGENETIC C. IDEALLY FRONTOLYTIC

AG5f0423

Y

Figure 4-23.—Perpendicular deformation field.

temperatures and warm tropical air moves northwardtoward colder temperatures.

The wind flow has formed a deformation field. Adeformation field consists basically of an area of flatpressure between two opposing highs and two opposinglows (also called a COL or saddle). It has two axes thathave their origin at a neutral point in the COL (view Ain fig. 4-23). The y axis, or axis of contraction, liesbetween the high and low that bring the air particlestoward the neutral point. (Note the flow arrows in fig.4-23.) The x axis lies between the high and low that takeair particles away from the neutral point and is knownas the axis of dilation.

The distribution and concentration of isotherms T1through T6 in this deformation field determine whetherfrontogenesis results. If the isotherms form a largeangle with the axis of contraction, frontogenesisresults. If a small angle exists, frontolysis (thedissipation of a front) results. It has been shown that in aperpendicular deformation field, isotherms must forman angle of 450 or less with the axis of dilation forfrontogenesis to occur as shown in views A and B of thefigure. In a deformation field not perpendicular, thecritical angle changes correspondingly as illustrated inviews A and B of figure 4-24. In most cases,frontogenesis occurs along the axis of dilation.Frontogenesis occurs where there is a concentration ofisotherms with the circulation to sustain thatconcentration.

CONDITIONS NECESSARY FORFRONTOLYSIS

Frontolysis, or the dissipation of a front, occurswhen either the temperature difference between the twoair masses disappears or the wind carries the air

particles of the air mass away from each other.Frontolytical processes are more common in theatmosphere than are frontogenetical processes. Thiscomes about because there is no known property of theair, which is conservative with respect to all thephysical or dynamical processes of the atmosphere.

Frontolytical processes are most effective in thelower layers of the atmosphere since surface heatingand turbulent mixing are the most intense of thenonconservative influences on temperature.

For frontolysis to occur, only one of the twoconditions stated above need be met. The simultaneoushappening of both conditions results in more rapidfrontolysis than if only one factor were operative. Theshape and curvature of the isobars also give valuableindications of frontolysis and frontogenesis, and,therefore, possible cyclolysis or cyclogenesis.

On a cold front, anticyclonically curved isobarsbehind the front indicate that the FRONT is slowmoving and therefore exposed to frontogenesis.

Cyclonically curved isobars in the cold air behindthe cold front indicate that the front is fast moving andexposed to frontolysis. On the warm fronts the converseis true.

Anticyclonically curved isobars in advance of thewarm front indicate the front is fast moving andexposed to frontolysis.

With cyclonically curved isobars the warm front isretarded and exposed to frontogenesis.

WORLD FRONTOGENETICAL ZONES

Certain regions of the world exhibit a highfrequency of frontogenesis. These regions are

4-25

T1

T2

T3

T4

T5

H

L

X

X

Y

T6

T1

T2

T3

T4T5

T6

H

L

Y

T1

T2T3

T4T5

T6

H

L

A. FRONTOGENETIC B. CRITICALLY FRONTOGENETIC C. IDEALLY FRONTOLYTIC

AG5f0424

YH

H

Y

X

X

Y

YH

X

X

L

LL

Figure 4-24.—Nonperpendicular deformation field.

coincident with the greatest temperature contrasts. Twoof the most important frontal zones are those over thenorth Pacific and north Atlantic Oceans. In winter, thearctic front, a boundary between polar and arctic air,forms in high latitudes over northwest North America,the north Pacific, and near the Arctic Circle north ofEurope (fig. 4-25). In summer, the arctic front mainlydisappears, except north of Europe. (See fig. 4-26.)

The polar front, on the other hand, is present theyear round, although it is not as intense in the summeras in the winter because of a lessening temperaturecontrast between the opposing air masses. The polarfront forms wherever the wind flow and temperaturecontrast is favorable. Usually this is the boundarybetween tropical and polar air, but it may form betweenmaritime polar and continental polar air. It also mayexist between modified polar air and a fresh outbreak ofpolar air. The polar front is common over NorthAmerica in the continental regions in winter in thevicinity of 50°N latitude.

The polar front in winter is found most frequentlyoff the eastern coasts of continents in areas of 30° to 60°

latitude. It is also found over land; but since thetemperature contrasts are greater between the continentand the oceans, especially in winter, the coastal areasare more favorable for formation and intensification ofthe polar front.

The intertropical convergence zone (ITCZ), thoughnot truly a front but a field of convergence between theopposing trades, forms a third semipermanent frontaltype. This region shows a seasonal variation just as dothe trade winds.

FRONTAL CHARACTERISTICS

LEARNING OBJECTIVE: Describe thefrontal elements and general characteristics offronts.

FRONTAL ELEMENTS

From our previous discussion and definitions offronts, it was implied that a certain geometrical andmeteorological consistency must exist between fronts

4-26

cASOURCE

ARCTIC FRONT

LOWMP

1020

1020

HIGH

HIGHP

OLA

R FRONT

cA AM

SOURCE

LOWARCTIC

FRONT

1000 MP

M CP P

POLAR FRONT (ATLANTIC)

1020HIGH

MT

CP

(I CT Z)

MT

(I CT Z)

MED POLAR FRONT

1020

CSOURCE

TH CT

1020HIGHL

LOW

1020HIGH

MT1010

SO. PACIFIC POLAR FRONT

SO. ATLANTIC POLAR FRONT

MT

1020

HIGH

MT

(ITCZ)

POLAR FRONT

CT

CTLOW

POLAR FRONT

LOWC

SOURCEP

1032

CSOURCE

A

ASIA ARCTIC FRONT

CA

180 150 120 90 60 30 0 30 60 90 120 150 180

75

60

45

30

15

0

15

30

45

60

75

60

45

30

15

0

15

30

45

60

POLAR FRONT (SO. INDIAN OCEAN)

POLARFRONT (PACIFIC)

JANUARYAG5f0425

Figure 4-25.—Chart showing world air masses, fronts and centers of major pressure systems in January.

at adjoining levels. It can also be inferred that the data atno one particular level is sufficient to locate a front withcertainty in every case. We must consider the horizontaland vertical distribution of three weather elements(temperature, wind, and pressure) in a frontal zone.

Temperature

Typical fronts always consist of warm air abovecold air. A radiosonde observation taken through afrontal surface often indicates a relatively narrow layerwhere the normal decrease of temperature with heightis reversed. This temperature inversion is called afrontal inversion; its position indicates the height of thefrontal surface and the thickness of the frontal zoneover the particular station. The temperature increasewithin the inversion layer and the thickness of the layercan be used as a rough indication of the intensity of afront. Strong fronts tend to have a distinct inversion;moderate fronts have isothermal frontal zones; and

weak fronts have a decrease in temperature through thefrontal zone.

Frontal zones are often difficult to locate on asounding because air masses become modified afterleaving their source region and because of turbulentmixing and falling precipitation through the frontalzone. Normally, however, some indication does exist.The degree to which a frontal zone appears pronouncedis proportional to the temperature difference betweentwo air masses.

The primary indication of a frontal zone on a SkewT diagram is a decrease in the lapse rate somewhere inthe sounding below 400 mb. The decrease in lapse ratemay be a slightly less steep lapse rate for a stratum in aweak frontal zone to a very sharp inversion in strongfronts. In addition to a decrease in the lapse rate, there isusually an increase in moisture (a concurrent dew-pointinversion) at the frontal zone. This is especially truewhen the front is strong and abundant cloudiness and

4-27

SOURCE

POLAR FRONTMP

1024

LOWHIGH

CT

180 150 120 90 60 30 0 30 60 90 120 150 180

75

60

45

30

15

0

15

30

45

60

75

60

45

30

15

0

15

30

45

60

POLARFRONT

JULY

MT

1010 MT

MT 1020

1024HIGH

1012

MPLOW

(NO. AM. NO. ATL)

CT

SOURCE

MP CPARCTIC FRONT

POLARFRONT

CP CT

CP

SOURCE

ASIAN POLO

AR

R FNT

CSOURCE

T

LOW

MP

PACIFIC POLAR

FRONT1018

1010

1010

(I C )T Z

CT

SOURCE

MT

E

HIGH1026 AUSTRALIAN POLARFRONT

MP

INDIAN OCEAN P TOLAR OFR N

ESOURCE

MT1020

HIGH

C TSOURCE

(I C )T Z

POLA .R LTFR O.AONT (S )

MP

(I C )T Z

1020

HIGHMT

MP

E

P

C

O(

LAR FRONT SO.PA .)

1010

1010

1005

1000

CP

AG5f0426

Figure 4-26.—Chart showing world air masses, fronts and centers of major pressure systems in July.

precipitation accompany it. View A of figure 4-27shows the height of the inversion in two different partsof a frontal zone, and view B of figure 4-27 shows astrong frontal inversion with a consequent dew-pointinversion.

A cold front generally shows a stronger inversionthan a warm front, and the inversion appears atsuccessively higher levels as the front moves past astation. The reverse is true of warm fronts. Occludedfronts generally show a double inversion. However, asthe occlusion process continues, mixing of the airmasses takes place, and the inversions are wiped out orfuse into one inversion.

It is very important in raob analysis not to confusethe subsidence inversion of polar and arctic air masseswith frontal inversions. Extremely cold continentalarctic air, for instance, has a strong inversion thatextends to the 700-mb level. Sometimes it is difficult tofind an inversion on a particular sounding, though it isknown that a front intersects the column of air over agiven station. This may be because of adiabatic

warming of the descending cold air just under thefrontal surface or excessive local vertical mixing in thevicinity of the frontal zone. Under conditions ofsubsidence of the cold air beneath the frontal surface,the subsidence inversion within the cold air may bemore marked than the frontal zone itself.

Sometimes fronts on a raob sounding, which mightshow a strong inversion, often are accompanied by littleweather activity. This is because of subsidence in thewarm air, which strengthens the inversion. The weatheractivity at a front increases only when there is a netupward vertical motion of the warm air mass.

Wind

Since winds near Earth’s surface flow mainly alongthe isobars with a slight drift toward lower pressure, itfollows that the wind direction in the vicinity of a frontmust conform with the isobars. The arrows in figure4-28 indicate the winds that correspond to the pressuredistribution.

4-28

ATMOSPHERIC SOUNDINGSIN THE COLD AIR MASS

FRONTAL SURFACE

WARM AIR MASS

COLD AIR MASSSURFACEPOSITION

OF FRONT

WARM AIR MASS

FRONTAL ZONE

FRONTAL SLOPE

DEW POINT

TEMPERATURE

ACCUMULATIONOF MOISTURE

HERE

A. HEIGHT AND THICKNESSOF INVERSION INDICATES SLOPE

OF FRONT AND INTENSITY

B. FRONTAL INVERSION

AG5f0427

Figure 4-27.—Inversions.

HIGH

AG5f0428

HIGH

LOW1004

1008

A

HIGH

HIGH

LOWB

1000

1004

HIGH

HIGH

LOWC

1000

1004HIGH

HIGH

LOWD

996

1000

Figure 4-28.—Types of isobars associated with fronts.

From this it can be seen that a front is a wind shiftline and that wind shifts in a cyclonic direction.Therefore, we can evolve the following rule: if youstand with your back against the wind in advance of thefront, the wind will shift clockwise as the front passes.This is true with the passage of all frontal types. Referback to figure 4-22.

NOTE: The wind flow associated with thewell-developed frontal system is shown in figure 4-22,view E. Try to visualize yourself standing ahead of eachtype of front depicted as they move from west to east.The terms backing and veering are often used whendiscussing the winds associated with frontal systems.

BACKING.—Backing is a change in winddirection—counterclockwise in the NorthernHemisphere and clockwise in the SouthernHemisphere. The opposite of backing is veering.

VEERING.—Veering is a change in winddirection—clockwise in the Northern Hemisphere,counterclockwise in the Southern Hemisphere. Theopposite of veering is backing.

The speed of the wind depends upon the pressuregradient. Look at figure 4-28. In view A, the speed isabout the same in both air masses; in views B and C, arelatively strong wind is followed by a weaker wind;and in view D, a weak wind is followed by a strongwind. An essential characteristic of a frontal zone is awind discontinuity through the zone. The windnormally increases or decreases in speed with heightthrough a frontal discontinuity. Backing usually occurswith height through a cold front and veering through awarm front. The sharpness of the wind discontinuity is

proportional to the temperature contrast across the frontand the pressure field in the vicinity of the front (thedegree of convergence between the two air streams).With the pressure field constant, the sharpness of thefrontal zone is proportional to the temperaturediscontinuity (no temperature discontinuity—no front;thus, no wind discontinuity). The classical picture ofthe variation in wind along the vertical through a frontalzone is shown in figure 4-29. An example of a frontalzone and the winds through the frontal zone is shown infigure 4-30.

4-29

AG5f0429

COLDAIR

CO

LD

FR

ON

T

COLDAIR

WARM AIR

WARM

FRONT

N

W

S

E

Figure 4-29.—Vertical distribution of wind direction in the vicinity of frontal surfaces.

AG5f0430

MBS500

600

700

800

900

1000

-30 -20 -10 0

TdT

FRONTALZONE

Figure 4-30.—Distribution of wind and temperature through awarm frontal zone.

On this sounding the upper winds that show thegreatest variation above the surface layer are thosebetween the 800- to 650-mb layers. This indicationcoincides closely with the frontal indications of thetemperature (T) and dew point (Td) curves (see fig.4-30). Since the wind veers with height through thelayer, the front would be warm. The vertical wind shiftthrough a frontal zone depends on the direction of theslope. In cold fronts the wind backs with height, whilein warm fronts the wind veers with height. At thesurface the wind always veers across the front, and theisobars have a sharp cyclonic bend or trough that pointstoward higher pressure. Sometimes the associatedpressure trough is not coincident with the front; in suchcases there may not be an appreciable wind shift acrossthe front—only a speed discontinuity.

Pressure

One of the important characteristics of all fronts isthat on both sides of a front the pressure is higher thanat the front. This is true even though one of the airmasses is relatively warm and the other is relativelycold. Fronts are associated with troughs of lowpressure. (A trough is an elongated area of relativelylow pressure.) A trough may have U-shaped orV-shaped isobars. How the pressure changes with thepassage of a front is of prime importance when you aredetermining frontal passage and future movement.

Friction causes the air (wind) near the ground todrift across the isobars toward lower pressure. Thiscauses a drift of air toward the front from both sides.Since the air cannot disappear into the ground, it mustmove upward. Hence, there is always a net movementof air upward in the region of a front. This is animportant characteristic of fronts, since the lifting of theair causes condensation, clouds, and weather.

GENERAL CHARACTERISTICS OF FRONTS

All fronts have certain characteristics that arecommon and usually predictable for that type of front.Cold frontal weather differs from warm frontalweather, and not every cold front has the same weatherassociated with it. The weather, intensity of theweather, and the movement of fronts are, to a largedegree, associated with the slope of the front.

Frontal Slope

When we speak of the slope of a front, we arespeaking basically of the steepness of the frontalsurface, using a vertical dimension and a horizontal

dimension. The vertical dimension used is normally 1mile. A slope of 1:50 (1 mile vertically for every 50miles horizontally) would be considered a steep slope,and a slope of 1:300 a gradual slope. Factors favoring asteep slope are a large wind velocity difference betweenair masses, small temperature difference, and highlatitude.

The frontal slope therefore depends on the latitudeof the front, the wind speed, and the temperaturedifference between the air masses. Because cold airtends to under run warm air, the steeper the slope, themore intense the lifting and vertical motion of the warmair and, therefore the more intense the weather.

Clouds and Weather

Cloud decks are usually in the warm air massbecause of the upward vertical movement of the warmair. Clouds forming in a cold air mass are caused by theevaporation of moisture from precipitation from theoverlying warm air mass and/or by vertical lifting.Convergence at the front results in a lifting of bothtypes of air. The stability of air masses determines thecloud and weather structure at the fronts as well as theweather in advance of the fronts.

Frontal Intensity

No completely acceptable set of criteria is inexistence as to the determination of frontal intensity, asit depends upon a number of variables. Some of thecriteria that may be helpful in delineating frontalintensity are discussed in the following paragraphs.

TURBULENCE.—Except when turbulence orgustiness may result, weather phenomena are not takeninto account when specifying frontal intensity, becausea front is not defined in terms of weather. A front maybe intense in terms of discontinuity of density across it,but may be accompanied by no weather phenomenaother than strong winds and a drop in temperature. Afront that would otherwise be classified as weak isconsidered moderate if turbulence and gustiness areprevalent along it, and an otherwise moderate front isclassified as strong if sufficient turbulence andgustiness exist. The term gustiness for this purposeincludes convective phenomena such as thunderstormsand strong winds.

TEMPERATURE GRADIENT.—Temperaturegradient, rather than true difference of temperatureacross the frontal surface, is used in defining the frontalintensity. Temperature gradient, when determining

4-30

frontal intensity, is defined as the difference betweenthe representative warm air immediately adjacent to thefront and the representative surface temperature 100miles from the front on the cold air side.

A suggested set of criteria based on the horizontaltemperature gradient has been devised. A weak front isone where the temperature gradient is less than 100Fper 100 miles; a moderate front is where thetemperature gradient is 10 0F to 20 0F per 100 miles;and a strong front is where the gradient is over 20 0F per100 miles.

The 850-mb level temperatures may be used in lieuof the surface temperatures if representative surfacetemperatures are not available and the terrain elevationis not over 3,000 feet. Over much of the western sectionof the United States, the 700-mb level temperatures canbe used in lieu of the surface temperatures.

Speed

The speed of the movement of frontal systems is animportant determining factor of weather conditions.Rapidly moving fronts usually cause more severeweather than slower moving fronts. For example,fast-moving cold fronts often cause severe prefrontalsquall lines that are extremely hazardous to flying. Thefast-moving front does have the advantage of movingacross the area rapidly, permitting the particularlocality to enjoy a quick return of good weather.Slow-moving fronts, on the other hand, may causeextended periods of unfavorable weather. A stationaryfront that may bring bad weather can disrupt flightoperations for several days in succession. The specificcharacteristics of each of the types of fronts is discussedin lessons 3 through 6.

Wind Component

The speed of a front is controlled by a resultantcomponent of wind behind a front. The windcomponent normal to a front is determined by the angleat which the geostrophic winds blow toward the front,resulting in a perpendicular force applied to the back ofthe front. For example, the component of the windnormal to a front that has a geostrophic wind with aperpendicular flow of 30 knots behind the front has a30-knot component. However, a 30-knot geostrophicwind blowing at a 450 angle to the front has only a15-knot component that is normal to or perpendicularto the front. The greater the angle of the wind to thefront, the greater the wind component normal to that

front. The smaller the angle, the less the windcomponent normal to the front.

REVIEW QUESTIONS

Q4-6. What is the definition of a frontal surface?

Q4-7. Where is the frontal zone located?

Q4-8. What is the difference between a stable waveand an unstable wave?

Q4-9. Where does frontogenesis occur?

Q4-10. Where is the polar front normally foundduring the winter?

THE COLD FRONT

LEARNING OBJECTIVE: Describeslow-moving cold fronts, fast-moving coldfronts, secondary cold fronts, and cold frontsaloft.

A cold front is the leading edge of a wedge of coldair that is under running warm air. Cold fronts usuallymove faster and have a steeper slope than other types offronts. Cold fronts that move very rapidly have verysteep slopes in the lower levels and narrow bands ofclouds that are predominant along or just ahead of thefront. Slower moving cold fronts have less steep slopes,and their cloud systems may extend far to the rear of thesurface position of the fronts. Both fast-moving andslow-moving cold fronts may be associated with eitherstability or instability and either moist or dry airmasses.

Certain weather characteristics and conditions aretypical of cold fronts. In general, the temperature andhumidity decrease, the pressure rises, and in theNorthern Hemisphere the wind shifts (usually fromsouthwest to northwest) with the passage of a coldfront. The distribution and type of cloudiness and theintensity and distribution of precipitation dependprimarily on the vertical motion within the warm airmass. This vertical motion is in part dependent upon thespeed of that cold front.

SLOW-MOVING COLD FRONTS (ACTIVECOLD FRONT)

With the slow-moving cold front, there is a generalupward motion of warm air along the entire frontalsurface and pronounced lifting along the lower portionof the front. The average slope of the front is

4-31

approximately 1:100 miles. Near the ground, the slopeis often much steeper because of surface friction.

Figure 4-31 illustrates the typical characteristics inthe vertical structure of a slow-moving cold front. Thelower half shows the typical upper airflow behind thefront, and the upper half shows the accompanyingsurface weather. This is only one typical case. Manyvariations to this model can and do occur in nature. Theslow-moving cold front is an active front because it haswidespread frontal cloudiness and precipitation at andbehind the front caused by actual frontal lifting.

Surface Characteristics

The pressure tendency associated with this type offrontal passage is indicated by either an unsteady or

steady fall prior to frontal passage and then weak risesbehind. Temperature and dew point drop sharply withthe passage of a slow-moving cold front. The windveers with the cold frontal passage and reaches itshighest speed at the time of frontal passage. Isobars areusually curved anticyclonically in the cold air. Thistype of front usually moves at an average speedbetween 10 and 15 knots. Slow-moving cold frontsmove with 100% of the wind component normal to thefront.

Weather

The type of weather experienced with aslow-moving cold front is dependent upon the stabilityof the warm air mass. When the warm air mass is stable,

4-32

200 150 100 50 0 50 100 MILES

PRECIP.AREA

NIMBOSTRATUS

ALTOCUMULUS

POSSIBLECUMULONIMBUS

POINT A

COLD AIR

SUBSIDENCEINVERSION

FRONTALINVERSION

TEMP.CURVE

WIND

ST ST

AS

15,000

10,000

5,000

ALTITUDE FT

CIRRO-STRATUS

CIRRUS

UPPER AIRTROUGH

UPPER

WIN

DFLO

W

AREAOF

CLOUDY

SKIESAND

RAIN

(SURFACE WINDFLOW)POINT A

AG5f0431

0 CO0 CO

WIND

Figure 4-31.—Typical vertical structure of a slow-moving cold front with upper windflow in back of the front.

a rather broad zone of altostratus and nimbostratusclouds accompany the front and extend several hundredmiles behind the front. If the warm air is unstable (orconditionally unstable), thunderstorms andcumulonimbus clouds may develop within the cloudbank and may stretch for some 50 miles behind thesurface front. These cumulonimbus clouds form withinthe warm air mass. In the cold air there may be somestratus or nimbostratus clouds formed by theevaporation of falling rain; but, generally, outside of therain areas, there are relatively few low clouds. This isbecause of the descending motion of the cold air thatsometimes produces a subsidence inversion somedistance behind the front.

The ceiling is generally low with the frontalpassage, and gradual lifting is observed after passage.Visibility is poor in precipitation and may continue tobe reduced for many hours after frontal passage as longas the precipitation occurs. When the cold air behindthe front is moist and stable, a deck of stratus cloudsand/or fog may persist for a number of hours afterfrontal passage. The type of precipitation observed isalso dependent upon the stability and moistureconditions of the air masses.

Upper Air Characteristics

Upper air contours show a cyclonic flow and areusually parallel to the front as are the isotherms. Theweather usually extends as far in back of the front asthese features are parallel to it. When the orientationchanges, this usually indicates the position of theassociated upper air trough. (A trough is an elongatedarea of relatively low pressure.)

The temperature inversion on this type of front isusually well marked. In the precipitation area therelative humidity is high in both air masses. Fartherbehind the front, subsidence may occur, giving asecond inversion closer to the ground.

The wind usually backs rapidly with height (on theorder of some 60 to 70 degrees between 950 and 400mb), and at 500 mb the wind direction is inclined atabout 15 degrees to the front. The wind componentnormal to the front decreases slightly with height, andthe component parallel to the front increases rapidly.

On upper air charts, slow-moving cold fronts arecharacterized by a packing (concentration) ofisotherms behind them. The more closely packed theisotherms and the more nearly they parallel the fronts,the stronger the front.

FAST-MOVING COLD FRONTS (INACTIVECOLD FRONT)

The fast-moving cold front is a very steep front thathas warm air near the surface being forced vigorouslyupward. At high levels, the warm air is descendingdownward along the frontal surface. This front has aslope of 1:40 to 1:80 miles and usually moves rapidly;25 to 30 knots may be considered an average speed ofmovement. They move with 80 to 90 percent of thewind component normal to the front. As a result ofthese factors, there is a relatively narrow but oftenviolent band of weather.

Figure 4-32 shows a vertical cross section of afast-moving cold front with resultant weather. Alsoindicated in the lower half of the diagram is the surfaceweather in advance of the front and the upper airflowabove the front.

If the warm air is moist and unstable, a line ofthunderstorms frequently develops along this front.Sometimes, under these conditions, a line of strongconvective activity is projected 50 to 200 miles ahead ofthe front and parallel to it. This may develop into a lineof thunderstorms called a squall line. On the other hand,when the warm air is stable, an overcast layer ofaltostratus clouds and rain may extend over a large areaahead of the front. If the warm air is very dry, little or nocloudiness is associated with the front. The frontdepicted is a typical front with typical characteristics.

The fast-moving cold front is considered aninactive front because lifting occurs only at and aheadof the front. The lifting is caused by descending airahead of the front and only in part by the frontalsurface.

Surface Characteristics

Pressure tendencies fall ahead of the front withsudden and strong rises after frontal passage. If a squallline lies some distance ahead of the front, there may bea strong rise associated with its passage and a shift inthe wind. However, after the influence of the squall linehas passed, winds back to southerly and pressures leveloff. The temperature falls in the warm air just ahead ofthe front. This is caused by the evaporation of fallingprecipitation. Rapid clearing and adiabatic warmingjust behind the front tend to keep the cold airtemperature near that of the warm air. An abrupttemperature change usually occurs far behind the frontnear the center of the high-pressure center associatedwith the cold air mass. The dew point and wind

4-33

directions are a good indication of the passage of afast-moving cold front. The wind veers with frontalpassage and is strong, gusty, and turbulent for aconsiderable period of time after passage. The dewpoint decreases sharply after frontal passage.

Weather

Cumulonimbus clouds are observed along and justahead of the surface front. Stratus, nimbostratus, andaltostratus may extend ahead of the front in advance ofthe cumulonimbus and may extend as much as 150

4-34

200 150 100 50 0 50 100

PRECIP.AREA

CUMULONIMBUS

ALTOCUMULUS

0 CO

CIRROSTRATUS

COLDAIR

SUBSIDENCEINVERSION

FRONTALINVERSION

CUMULUS

WIND

Ns

15,000

10,000

5,000

ALTITUDE FT

CIRRUS

AG5f0432

STRATOCUMULUS

20,000

25,000

WIND

0O

UPPERWIND-FLOW

OR

OR

GENERALLY

CLEAR

SKIES

Figure 4-32.—Typical vertical structure of a fast-moving cold front with upper windflow across the front.

miles ahead of the front. These clouds are all found inthe warm air. Generally, unless the cold air is unstableand descending currents are weak, there are few cloudsin the cold air behind the front. Showers andthunderstorms occur along and just ahead of the front.The ceiling is low only in the vicinity of the front.Visibility is poor during precipitation but improvesrapidly after the frontal passage.

Upper Air Characteristics

Because of the sinking motion of the cold airbehind the front and the resultant adiabatic warming,the temperature change across the front is oftendestroyed or may even be reversed. A sounding taken inthe cold air immediately behind the surface frontindicates only one inversion and an increase in moisturethrough the inversion. Farther back of the front, adouble inversion structure is evident. The lowerinversion is caused by the subsidence effects in the coldair. This is sometimes confusing to the analyst becausethe subsidence inversion is usually more marked thanthe frontal inversion and may be mistaken for thefrontal inversion.

In contrast to the slow-moving cold front, the windabove the fast-moving cold front exhibits only a slightbacking with height of about 20 degrees between 950and 400 mb; the wind direction is inclined toward thefront at an average angle of about 45 degrees. The windcomponents normal and parallel to the front increasewith height; the wind component normal to the frontexceeds the mean speed of the front at all levels abovethe lowest layers. On upper air charts, the isotherms areNOT parallel to the front. Instead they are at an angle ofabout 30 degrees to the front, usually crossing the coldfront near its junction with the associated warm front.

SECONDARY COLD FRONTS

Sometimes there is a tendency for a trough of lowpressure to form to the rear of a cold front, and asecondary cold front may develop in this trough.Secondary cold fronts usually occur during outbreaksof very cold air behind the initial outbreak. Secondarycold fronts may follow in intervals of several hundredmiles to the rear of the rapidly moving front. When asecondary cold front forms, the primary front usuallytends to dissipate and the secondary front then becomesthe primary front. Secondary fronts usually do notoccur during the summer months because there is rarelyenough temperature discontinuity.

COLD FRONTS ALOFT

There are two types of upper cold fronts. One is theupper cold front associated with the warm occlusionthat is discussed later in this unit. The other occursfrequently in the areas just east of mountains in winter.This cold front aloft is associated with mP air crossingthe mountains behind a cold front or behind a coldtrough aloft and a very cold layer of continental polarair lying next to the ground over the area east of themountains. The area east of the Rocky Mountains is onesuch area in the United States. When warm maritimetropical air has moved northward from the Gulf ofMexico and has been forced aloft by the cold cP air, andcool mP air flows over the mountains, it forces its wayunder the warm mT air aloft. The resulting front thenflows across the upper surface of the colder cP air justas if it were the surface of the ground. All frontalactivity in this case takes place above the top of the cPlayer. Figure 4-33 shows an example of this type offront and the synoptic structure. Weather from cold

4-35

FRONTALZONE

AG5f0433

WEST

COOLmP

WARMmT

cPVERY COLD

ROCKYMOUNTAINS

EAST

Figure 4-33.—Cold front aloft.

fronts aloft can produce extensive cloud decks andblizzard conditions for several hundred miles over themid western plains.

INSTABILITY AND SQUALL LINES

The terms instability line and squall line aresynonymous with violent winds, heavy rain, lightning,thunder, hail, and tornadoes. The terms are often usedinterchangeably and are incorrectly applied to anysevere weather phenomena that moves through aregion. However, there is a difference between aninstability line and a squall line.

Instability Line

An instability line is any nonfrontal line or band ofconvective activity. This is a general term and includesthe developing, mature, and dissipating stages of theline of convective activity. However, when the maturestage consists of a line of active thunderstorms, it isproperly termed a squall line. Therefore, in practice, theinstability line often refers only to the less activephases.

Squall Line

A squall line is a nonfrontal line or band of activethunderstorms (with or without squalls). It is themature, active stage of the instability line. From thesedefinitions, instability and squall lines are air massphenomenon because they are both nonfrontaloccurrences. However, they are frequently associatedwith the fast-moving cold front.

NOTE: The term instability line is the moregeneral term and includes the squall line as a specialcase.

Prefrontal Squall Lines

A prefrontal squall line is a squall line located inthe warm sector of a wave cyclone. They form about 50to 300 miles in advance of fast-moving cold fronts andare usually oriented roughly parallel to the cold front.They move in about the same direction as the cold front;however, their speed is, at times, faster than the coldfront. You can roughly compute the direction and speedby using the winds at the 500-mb level. Squall linesgenerally move in the direction of the 500-mb windflow and at approximately 40% of the wind speed.

FORMATION.—There are several theories on thedevelopment of prefrontal squall lines. A generally

accepted theory is that as thunderstorms develop alongthe fast-moving front, large quantities of cold air fromaloft descend in downdrafts along the front and form awedge of cold air ahead of the front. The wedge of coldair then serves as a lifting mechanism for the warm,moist, unstable air; and a line of thunderstormsdevelops several miles in advance of the front. Since thethunderstorms form within the air mass and not alongthe front, the squall line is considered as air massweather (fig. 4-34). In the United States, squall linesform most often in spring and summer. They arenormally restricted to the region east of the RockyMountains with a high frequency of occurrence in thesouthern states.

WEATHER.—Squall-line weather can beextremely hazardous. Its weather is usually more severethan the weather associated with the cold front behindit; this is because the moisture and energy of the warmair mass tends to be released at the squall line prior tothe arrival of the trailing cold front. Showers andthunderstorms (sometimes tornadoes) occur along thesquall line, and the wind shifts cyclonically with theirpassage (fig. 4-35). However, if the zone is narrow, thewind shift may not be noticeable on surface charts.There is generally a large drop in temperature becauseof the cooling of the air by precipitation. Pressure risesafter the passage of the squall line, and, at times, a

4-36

SQUALL LINE DEVELOPMENT

AG5f0434

COLDFRONT

DOWNDRAFTS

UNSTABLEWARM AIR

COLD FRONT

DOWNDRAFTS

UNSTABLEWARM AIR

SQUALL LINE

50 TO 100 MILES

COLD FRONT

Figure 4-34.—Prefrontal squall line development.

micro-high (small high) may form behind it. Afterpassage of the squall line, the wind backs to southerlybefore the cold frontal passage. When the squall linedissipates, severe weather may develop along thefast-moving cold front.

Turbulence is severe in the squall-linethunderstorms because of violent updrafts anddowndrafts. Above the freezing level, icing may occur.Hail is another possibility in the squall-linethunderstorm and can do extensive structural damage toan aircraft. Under the squall line, ceiling and visibilitymay be reduced because of heavy rain showers. Fog is arare occurrence because of the strong wind and gusts,but it may be found in isolated cases. Tornadoesfrequently occur with squall lines when the warm airmass is extremely unstable.

Great Plains Squall Lines

Not all instability lines that reach the mature orsquall-line stage develops in advance of a fast-movingcold front. The Great Plains region of the United Stateshas a high frequency of these squall lines. The GreatPlains type of squall lines also develop in warm, moist,unstable air masses. The necessary lifting or triggermay be supplied by intense thermal heating, orographiclifting, or convergent winds associated with alow-pressure area.

FORMATION.—The Great Plains squall lineforms when an extremely unstable conditiondevelops—normally in spring and summer. Extremelyunstable conditions exist when moist mP air cools inthe upper levels because of the evaporation of falling

precipitation. This cooler air aloft then moves overwarm moist mT air (or even warm, moist, highlymodified mP air) at the surface. If a sufficient triggersuch as a steep lapse rate of a lifting mechanism exists,this extremely unstable situation rapidly develops into asquall line.

WEATHER.—The weather associated with theGreat Plains squall line is the same as that found withthe prefrontal squall line. Because of the extremeinstability, tornadoes are a common occurrence.

REVIEW QUESTIONS

Q4-11. What is the pressure tendency with thepassage of a slow moving cold front?

Q4-12. What is the normal slope of a fast moving coldfront?

Q4-13. Where do prefrontal squall lines normallyform?

THE WARM FRONT

LEARNING OBJECTIVE: Describe thecharacteristics and weather of warm fronts atthe surface and aloft.

A warm front is the line of discontinuity where theforward edge of an advancing mass of relatively warmair is replacing a retreating relatively colder air mass.The slope of the warm front is usually between 1:100and 1:300, with occasional fronts with lesser slopes.Therefore, warm fronts have characteristically shallowslopes caused by the effect of surface friction thatretards the frontal movement near the ground.

SURFACE CHARACTERISTICS

Warm fronts move slower than cold fronts. Theiraverage speed is usually between 10 and 20 knots. Theymove with a speed of 60 to 80 percent of the componentof the wind normal to the front in the warm air mass.

The troughs associated with warm fronts are not aspronounced as those with cold fronts and sometimesmake location difficult on the surface chart. Thepressure tendency ahead of the front is usually a rapidor unsteady fall with a leveling off after frontal passage.A marked decrease in isallobaric gradient is noticed inthe warm sector except when rapid deepening is takingplace. The wind increases in velocity in advance ofwarm fronts because of an increase in pressure gradientand reaches a maximum just prior to frontal passage.The wind veers with frontal passage, usually from a

4-37

AG5f0435

Figure 4-35.—Typical isobaric pattern associated with aprefrontal squall line.

southeasterly direction to a southwesterly directionbehind the front. This shift is not as pronounced as withthe cold front.

Temperature generally is constant or slowly risingin advance of the front until the surface front passes, atwhich time there is a marked rise. This rise is dependentupon the contrast between the air masses. Dew pointusually increases slowly with the approach of the frontwith a rapid increase in precipitation and fog areas. Ifthe warm sector air is maritime tropical, the dew pointshows a further increase.

WEATHER

A characteristic phenomenon of a typical warmfront is the sequence of cloud formations (fig. 4-36).They are noticeable in the following order: cirrus,

cirrostratus, altostratus, nimbostratus, and stratus. Thecirrus clouds may appear 700 to 1,000 miles or moreahead of the surface front, followed by cirrostratusclouds about 600 miles ahead of the surface front andaltostratus about 500 miles ahead of the surface front.

Precipitation in the form of continuous orintermittent rain, snow, or drizzle is frequent as far as300 miles in advance of the surface front. Surfaceprecipitation is associated with the nimbostratus in thewarm air above the frontal surface and with stratus inthe cold air. However, when the warm air isconvectively unstable, showers and thunderstorms mayoccur in addition to the steady precipitation. This isespecially true with a cyclonic flow aloft over the warmfront. Fog is common in the cold air ahead of a warmfront.

4-38

AG5f0436

STABLEWARM

AIR30,000

20,000

10,000

SFC

0 CO

S C

N S

A S

C S

C I

A S

S TF O G , R A I N A N D L O W N I M B O S T R A T U S

200 100 0 100 200 300 400 500 600

SCSC 0 CO

C IC S

A S

N S

C B

UNSTABLEWARM

AIR

30,000

20,000

10,000

SFC

0 CO

200 100 0 100 200 300 400 500 600

L I G H T R A I N A N D S H O W E R S

N SS TS H O W E R S

A C

A S

SC

0 CO

Figure 4-36.—Vertical cross section of a warm front with stable and unstable air.

Clearing usually occurs after the passage of a warmfront, but under some conditions drizzle and fog mayoccur within the warm sector. Warm fronts usuallymove in the direction of the isobars of the warm sector;in the Northern Hemisphere this is usually east tonortheast.

The amount and type of clouds and precipitationvary with the characteristics of the air masses involvedand depending on whether the front is active or inactive.Generally, with warm fronts, an increase of the windcomponent with height perpendicular to the front givesan active front. This produces strong overrunning andpronounced prefrontal clouds and precipitation.Inactive fronts, characterized by broken cirrus andaltocumulus, are produced by a decrease with height ofthe wind component perpendicular to the front.

When the overrunning warm air is moist and stable,nimbostratus clouds with continuous light to moderateprecipitation are found approximately 300 miles aheadof the front. The bases of the clouds lower rapidly asadditional clouds form in the cold air under the frontalsurface. These clouds are caused by evaporation of thefalling rain. These clouds are stratiform when the coldmass is stable and stratocumulus when the cold air isunstable.

When the overrunning air is moist and unstable,cumulus and cumulonimbus clouds are frequentlyimbedded in the nimbostratus and altostratus clouds. Insuch cases, thunderstorms occur along with continuousprecipitation. When the overrunning warm air is dry, itmust ascend to relatively high altitudes beforecondensation can occur. In these cases only high andmiddle clouds are observed. Visibility is usually goodunder the cirrus and altostratus clouds. It decreasesrapidly in the precipitation area. When the cold air isstable and extensive, fog areas may develop ahead ofthe front, and visibility is extremely reduced in thisarea.

UPPER AIR CHARACTERISTICS

Warm fronts are usually not as well defined as coldfronts on upper air soundings. When the front is strongand little mixing has occurred, the front may show awell-marked inversion aloft. However, mixing usuallyoccurs and the front may appear as a rather broad zonewith only a slight change in temperature. Quitefrequently there may be two inversions—one caused bythe front and the other caused by turbulence. Isothermsare parallel to the front and show some form of packingahead of the front. The stronger the packing, the more

active the front. The packing is not as pronounced aswith the cold front.

WARM FRONTS ALOFT

Warm fronts aloft seldom occur, but generallyfollow the same principles as cold fronts aloft. One casewhen they do occur is when the very cold airunderneath a warm front is resistant to displacementand may force the warm air to move over a thinningwedge with a wave forming on the upper surface. Thisgives the effect of secondary upper warm fronts andmay cause parallel bands of precipitation at unusualdistances ahead of the surface warm front. Warm airadvection is more rapid and precipitation is heaviestwhere the steeper slope is encountered. Pressure fallsrapidly in advance of the upper warm front and levelsoff underneath the horizontal portion of the front. Whena warm front crosses a mountain range, colder air mayoccur to the east and may move along as a warm frontaloft above the layer of cold air. This is common when awarm front crosses the Appalachian Mountains inwinter.

REVIEW QUESTIONS

Q4-14. What is the average speed of a warm front?

Q4-15. What cloud types, and in what order usuallyform in advance of a warm front?

THE OCCLUDED FRONTS

LEARNING OBJECTIVE: Describe theformation, structure, and characteristics of coldand warm air occluded fronts.

An occluded front is a composite of two fronts.They form when a cold front overtakes a warm frontand one of these two fronts is lifted aloft. As a result, thewarm air between the cold and warm front is shut off.An occluded front is often referred to simply as anocclusion. Occlusions may be either of the cold type orwarm type. The type of occlusion is determined by thetemperature difference between the cold air in advanceof the warm front and the cold air behind the cold front.

A cold occlusion forms when the cold air inadvance of a warm front is warmer than the cold air tothe rear of the cold front. The overtaking cold airundercuts the cool air in advance of the warm front.This results in a section of the warm front being forcedaloft. A warm occlusion forms when the air in advanceof the warm front is colder than the air to the rear of thecold front. When the cold air of the cold front overtakes

4-39

the warm front, it moves up over this colder air in theform of an upper cold front.

The primary difference between a warm and a coldtype of occlusion is the location of the associated upperfront in relation to the surface front (fig. 4-37). In awarm type of occlusion, the upper cold front precedesthe surface-occluded front by as much as 200 miles. Inthe cold type of occlusion the upper warm front followsthe surface-occluded front by 20 to 50 miles.

Since the occluded front is a combination of a coldfront and a warm front, the resulting weather is that ofthe cold front’s narrow band of violent weather and thewarm front’s widespread area of cloudiness andprecipitation occurring in combination along theoccluded front. The most violent weather occurs at theapex or tip of the occlusion. The apex is the point on thewave where the cold front and warm front meet to startthe occlusion process.

COLD OCCLUSIONS

A cold occlusion is the occlusion that forms when acold front lifts the warm front and the air masspreceding the front (fig. 4-38). The vertical andhorizontal depiction of the cold occlusion is shown infigure 4-39. Cold occlusions are more frequent thanwarm occlusions. The lifting of the warm front as it isunderrun by the cold front implies existence of an upperwarm front to the rear of the cold occlusion; actuallysuch a warm front aloft is rarely discernible and isseldom delineated on a surface chart.

Most fronts approaching the Pacific coast of NorthAmerica from the west are cold occlusions. In winterthese fronts usually encounter a shallow layer ofsurface air near the coastline (from about Oregonnorthward) that is colder than the leading edge of coldair to the rear of the occlusion. As the occluded frontnears this wedge of cold air, the occlusion is forcedaloft and soon is no longer discernible on a surfacechart. The usual practice in these cases is to continue todesignate the cold occlusion as though it were a surfacefront because of the shallowness of the layer over whichit rides. As the occlusion crosses over the mountains, iteventually shows up again on a surface analysis.

The passage of the cold type of occlusion over thecoastal layer of colder air presents a difficult problem ofanalysis in that no surface wind shift ordinarily occursat the exact time of passage. However, a line of stations

4-40

OCCLUD

ED

FR

ON

T

WA

RM

FRONTCOLDFRONT

COLDAIR

COOLAIR

APEX

WARMAIR

UPPER WARM FRONT

O

N

C

O

C

R

L

F

UD

ED

T

WARM

FRONTCOLDFRONT

COOLAIR

APEX

COLDAIR

UPPER COLD FRONT

WARMAIR

A. COLD TYPE OF OCCLUSION B. WARM TYPE OF OCCLUSION

AG5f0437

Figure 4-37.—Sketch of occlusions (in the horizontal) and associated upper fronts.CO

LD-FRONT

SUR

FACE

WARM-FRONT

SURFACE

COLD AIR COOL AIR

WARM AIR

COLD TYPE OCCLUSIONAG5f0438

Figure 4-38.—Vertical cross section of a cold type of occlusion.

reporting surface-pressure rises is the best criterion ofits passage. This should be verified by reference toplotted raob soundings where available. When a Pacificcold occlusion moves farther inland, it sometimesencounters colder air of appreciable depth over thePlateau or Western Plains areas; in this case, it shouldbe redesignated as an upper cold front.

Surface Characteristics

The occlusion lies in a low-pressure area; and in thelatter stages, a separate low center may form at the tip ofthe occlusion, leaving another low-pressure cell nearthe end of the occlusion. The pressure tendency acrossthe cold occluded front follows closely with thoseoutlined for cold fronts; that is, they level off, or moreoften, rapid rises occur after the passage of theoccluded front.

Weather

In the occlusion’s initial stages of development, theweather and cloud sequence ahead of the occlusion isquite similar to that associated with warm fronts;however, the cloud and weather sequence near thesurface position of the front is similar to that associatedwith cold fronts. As the occlusion develops and thewarm air is lifted to higher and higher altitudes, thewarm front and prefrontal cloud systems disappear. Theweather and cloud systems are similar to those of a coldfront. View A of figure 4-39 shows the typical cloudand weather pattern associated with the cold occlusion.Most of the precipitation occurs just ahead of theocclusion. Clearing behind the occlusion is usuallyrapid, especially if the occlusion is in the advancedstage. Otherwise, clearing may not occur until after thepassage of the warm front aloft.

4-41

WARM AIR

COLD FRONT

COLD AIR

FLOW

WARM FRONT

COOL AIR

ALTOSTRATUS

CIRRUS

STRATOCUMULUS

NIMBOSTRATUS

STRATOCUMULUS

RAIN

250 200 150 100 150 0 50 100 150

DISTANCE MILES

COLD AIR COOL AIR

AA'

ISO

BA

RS

B

A

WARM AIR

AG5f0439

ALTOCUMULUS

Figure 4-39.—Cold front type of occlusion. (A) Vertical structure through points A and A'; (B) horizontal structure.

Upper Air Characteristics

If only one upper air sounding were taken so that itintersected either the cold or warm front, the soundingwould appear as a typical warm or cold front sounding.However, if the sounding were taken so that itintersected both the cold and warm air, it would showtwo inversions.

The occlusion may appear on some upper aircharts. It usually appears on the 850-mb chart, butrarely on the 700-mb chart. As the two air masses arebrought closer together and as the occlusion processbrings about gradual disappearance of the warm sector,the isotherm gradient associated with the surface frontweakens. The degree of weakening depends on thehorizontal temperature differences between the cold airto the rear of the cold front and that ahead of the warmfront. The angle at which the isotherms cross thesurface position of the occluded fronts becomes greateras the temperature contrast between the two cold airlasses decreases. A typical illustration of the isothermsshows a packing of isotherms in the cold mass behindthe cold front and less packing in the cool mass inadvance of the warm front. A warm isotherm ridgeprecedes the occlusion aloft.

WARM OCCLUSIONS

A warm occlusion is the occlusion that forms whenthe overtaking cold front is lifted by overrunning thecolder retreating air associated with the warm front.This is shown in figure 4-40. The warm occlusionusually develops in the Northern Hemisphere whenconditions north and of ahead of the warm front aresuch that low pa temperatures exist north of the warmfront. This usually occurs along the west coasts ofcontinents when a relatively cool maritime cold front

overtakes a warm front associated with a very coldcontinental air mass of high pressure situated over thewestern portion of the continent. The cold front thencontinues as an upper cold front above the warm frontsurface. The occlusion is represented as a continuationof the warm front. The cold front aloft is usuallyrepresented on all surface charts. Figure 4-41 depicts atypical warm type of occlusion in both the vertical andhorizontal.

Surface Characteristics

The warm type of occlusion has the same type ofpressure pattern as the cold type of occlusion. The mostreliable identifying characteristics of the upper frontare a line of marked cold frontal precipitation andclouds ahead of the occluded front, a slight but distinctpressure trough and a line of pressure-tendencydiscontinuities.

NOTE: The pressure tendency shows a steady fallahead of the upper cold front and, with passage, aleveling off for a short period of time. Another slightfall is evident with the approach of the surface positionof the occlusion. After passage the pressure shows asteady rise.

The pressure trough is often more distinct with theupper front than with the surface front.

Weather

The weather associated with warm front occlusionshas the characteristics of both warm and cold fronts.The sequence of clouds ahead of the occlusion issimilar to the sequence of clouds ahead of a warm front;the cold front weather occurs near the upper cold front.If either the warm or cool air that is lifted is moist andunstable, showers and sometimes thunderstorms maydevelop. The intensity of the weather along the upperfront decreases with distance from the apex. Weatherconditions change rapidly in occlusions and are usuallymost severe during the initial stages. However, whenthe warm air is lifted to higher and higher altitudes, theweather activity diminishes. When showers andthunderstorms occur, they are found just ahead and withthe upper cold front. Normally, there is clearingweather after passage of the upper front, but this is notalways the case.

Upper Air Characteristics

Upper air soundings taken through either frontshow typical cold or warm front soundings. Those

4-42

COLD-FRONT

SURFACE WARM-FRONT

SURFACE

COLD AIR COLDER AIR

WARM AIR

WARM TYPE OF OCCLUSION AG5f0440

Figure 4-40.—Vertical cross-section of a warm type ofocclusion.

taken that intersect both fronts show two inversions.The warm type of occlusion (like the cold type) appearson upper air charts at approximately the same pressurelevel. However, one distinct difference does appear inthe location of the warm isotherm ridge associated withocclusions. The warm isotherm ridge lies just to the rearof the occlusion at the peak of its development.

REVIEW QUESTIONS

Q4-16. What is the difference between warm and coldocclusions?

Q4-17. Where does the most violent weather occurwith the occlusion?

THE QUASI-STATIONARY FRONT

LEARNING OBJECTIVE: Describe thecharacteristics of stable and unstablequasi-stationary fronts.

A quasi-stationary front, or stationary front as it isoften called, is a front along which one air mass is notappreciably replacing another air mass. A stationaryfront may develop from the slowing down or stoppingof a warm or a cold front. When this front forms, theslope of the warm or cold front is initially very shallow.The dense cold air stays on the ground, and the warmair is displaced slowly upward. The front slows or stopsmoving because the winds behind and ahead of thefront become parallel to the stationary front. It is quite

4-43

WARM AIR

COOL AIR

FLOW

COLD AIR

CUMULONIMBUS

CIRRUS

STRATOCUMULUS OR STRATUS

NIMBOSTRATUS

STRATOCUMULUS

RAIN AND FOG

250200150100

A

DISTANCE MILES

COLD AIR

COOL AIR

A

A'

ISO

BA

RS

B

A

WARM AIR

AG5f0441

OCCLUDED FRONT

500

A'

Figure 4-41.—Illustration of warm type of occlusion. (A) Vertical structure through points A and A'; (B) Horizontal structure.

unusual for two masses of different properties to be sideby side without some movement, so the term stationaryis a misnomer. Actually the front, or dividing linebetween the air masses, is most likely made up of smallwaves undulating back and forth; hence the termquasi-stationary. The important thing is that the front isnot making any appreciable headway in any onedirection. A front moving less than 5 knots is usuallyclassified as a stationary front.

CHARACTERISTICS

When a front is stationary, the whole cold air massdoes not move either toward or away from the front. Interms of wind direction, this means that the wind abovethe friction layer blows neither toward nor away fromthe front, but parallel to it. The wind shift across thefront is usually near 180 degrees. It follows that theisobars, too, are nearly parallel to a stationary front.This characteristic makes it easy to recognize astationary front on a weather map.

STABLE STATIONARY FRONT

There is frictional inflow of warm air toward astationary front causing a slow upglide of air on the

frontal surface. As the air is lifted to and beyondsaturation, clouds form in the warm air above the front.If the warm air in a stationary front is stable and theslope is shallow, the clouds are stratiform. Drizzle maythen fall; and as the air is lifted beyond the freezinglevel, icing conditions develop and light rain or snowmay fall. At very high levels above the front, ice cloudsare present. (See fig. 4-42).

If, however, the slope is steep and significant warmair is being advected up the frontal slope, stratiformclouds with embedded showers result (view B of fig.4-42). Slight undulation or movement of thequasi-stationary front toward the warm air mass adds tothe amount of weather and shower activity associatedwith the front.

UNSTABLE STATIONARY FRONT

If the warm air is conditionally unstable, the slopeis shallow, and sufficient lifting occurs, the clouds arethen cumuliform or stratiform with embedded toweringcumulus. If the energy release is great (warm, moist,unstable air), thunderstorms result. Within the cold airmass, extensive fog and low ceiling may result if thecold air is saturated by warm rain or drizzle falling

4-44

STRATUS

COLD AIR SCUD

WARM AIR

A. SHALLOW STATIONARY FRONT

0 CO

NIMBOSTRATUS

STRATUS ORSTRATOCUMULUS

WARMAIR

0 CO

0 CO

SCUD

COLDAIR

0 CO

B. STEEP STATIONARY FRONTAG5f0442

Figure 4-42.—Types of stable stationary fronts.

through it from the warm air mass above. If thetemperature is below 0°C, icing may occur; butgenerally it is light (view A of fig. 4-43). The shallowslope of an unstable stationary front results in a verybroad and extensive area of showers, fog, and reducedvisibility.

If the slope of an unstable stationary front is steepand sufficient warm air is advected up the slope or thefront moves slowly toward the warm air mass, violentweather can result (view B of fig. 4-43). Heavy rain,severe thunderstorms, strong winds, and tornadoes areoften associated with this front. The width of the bandof precipitation and low ceilings vary from 50 miles toabout 200 miles, depending upon the slope of the frontand the temperatures of the air masses. One of the mostannoying characteristics of a stationary front is that itmay greatly hamper and delay air operations bypersisting in the area for several days.

REVIEW QUESTIONS

Q4-18. When a quasi-stationary front moves, if itdoes, what is the normal speed?

Q4-19. What type of weather is normally associatedwith an unstable stationary front?

MODIFICATIONS OF FRONTS

LEARNING OBJECTIVE: Describe howfronts are modified by their movement,orographic features, and underlying surfaces.

The typical fronts we have just covered can and doundergo modifications that strengthen or weaken them.Such things as frontal movement, orographic effects,and the type of surface the fronts encounter contributeto the modification of fronts.

4-45

SCATTEREDTHUNDERSTORMS

COLD AIR

A. SHALLOW STATIONARY FRONT

0 CO

AG5f0443

WARM AIR

TOWERINGCUMULUS

STRATOCUMULUS

0 CO

SCUD

LINE SQUALLALTOCUMULUS

STRATUS ORSTRATOCUMULUS

COLD AIRWARM

AIR

0 CO

0 CO

SCUD

B. STEEP STATIONARY FRONT

Figure 4-43.—Types of unstable stationary fronts.

EFFECTS CAUSED BY MOVEMENT

The weather is greatly affected by the movement offrontal systems. From the time the front develops untilit passes out of the weather picture, it is watchedclosely. The speed of the movement of frontal systemsis an important determining factor of weatherconditions. Rapidly moving fronts usually cause moresevere weather than slower moving fronts. Fast-movingcold fronts often cause severe prefrontal squall linesthat are extremely hazardous to flying. The fast-movingfront does have the advantage of moving across the arearapidly, permitting the particular locality to enjoy aquick return of good weather. Slow-moving fronts, onthe other hand, may cause extended periods ofunfavorable weather. A stationary front may bring badweather and can disrupt flight operations for severaldays if the frontal weather is sitting over your station.

Knowledge of the speed of the frontal system isnecessary for accurate forecasting. If the front has asomewhat constant speed, it makes your job and theforecaster’s job comparatively easy. However, if thespeed is erratic or unpredictable, you may err as far as

time and severity are concerned. If a front wasultimately forecast to pass through your station andinstead becomes stationary or dissipates, the stationforecast will be a total bust.

OROGRAPHIC EFFECTS

Mountain ranges affect the speed, slope, andweather associated with a front. The height andhorizontal distance of the mountain range along withthe angle of the front along the mountain range are theinfluencing factors. Mountain ranges can affect coldfronts, warm fronts, and occluded fronts differently.

Cold Fronts

As a cold front approaches a mountain range, thesurface portion of the front is retarded and the upperportion pushes up and over the mountain. On thewindward side of the mountain, warm air is pushed upalong the mountain slope because of the additional liftof a now steeper frontal slope and the mountain itself(view A of fig. 4-44). After the front passes the crest ofthe mountain, the air behind the front commences to

4-46

COLD

A

WARM

CLEARING

B

COLD WARM

COLDER

C

EXTREMEINSTABILITY

STAGNANT WARM AIR

AG5f0444

Figure 4-44.—Orographic effects on a cold front.

flow down the leeward side of the range. The warmerair on the leeward side of the mountain is displaced bythe colder air mass. As this cold air descends theleeward side of the mountain, the air warmsadiabatically (view A of fig. 4-44) and clearing occurswithin it. However, since the cold air is displacingwarm air, typical cold frontal clouds and precipitationmay occur within the warm air if the warm air issufficiently moist and conditionally unstable. In somecases maritime polar air that has crossed the Rockies isless dense than maritime tropical air from the Gulf ofMexico that may lie just east of the mountains. If themaritime polar air is moving with a strong westerlywind flow and the maritime tropical air is moving witha strong southerly wind flow, the maritime polar airmay overrun the maritime tropical air. This results inextremely heavy showers, violent thunderstorms, andpossible tornadoes.

If COLDER stagnant air lies to the leeward side ofthe mountain range, the cold front passing over themountain range does not reach the surface but travels as

an upper cold front (view B of fig. 4-44). Under thiscondition, frontal activity is at a minimum. Thissituation does not continue indefinitely; either thestagnant air below mixes with the air above or the uppercold front breaks through to the ground when thestagnant surface air has warmed sufficiently. Then thefront returns to a normal classic front and begins to liftthe now warm air. This ultimately results in thedevelopment of thunderstorms and squall lines (view Cof fig. 4-45). In the summer, this occurs frequently inone form along the eastern United States. When a coldsea breeze occurs and a cold front crosses theAppalachian Mountains, the associated cold wedge ofon-shore flow forces the warm air in advance of thecold front aloft, producing intense thunderstormactivity.

Generally, the area of precipitation is widened as acold front approaches a mountain range. There is anincrease in the intensity of the precipitation and cloudarea on the windward side of the range and a decreaseon the leeward side.

4-47

A.

WARM

FRONTAL SURFACE

B.

COLD

WARM

C.

AG5f0445

FRONTALSURFACE

FRONTALSURFACE

WARM

COLD

COLD

COLD

WARM

WARM

FRONTAL SURFACE

D.

Figure 4-45.—Orographic effects on a warm front.

Warm Fronts

When a warm front approaches a mountain range,the upper section of the frontal surface is above theeffects of the mountain range and does not come underits influence (view A of fig. 4-45). As the lower portionof the frontal surface approaches the range, theunderlying cold wedge is cut off, forming a more or lessstationary front on the windward side of the range. Theinclination of the frontal surface above the rangedecreases and becomes more horizontal near themountain surfaces, but the frontal surface maintains itsoriginal slope at higher altitudes (view B of fig. 4-45).While the stationary front on the windward side of therange may be accompanied by prolonged precipitation,the absence of ascending air on the leeward side of therange causes little or no precipitation. The warm airdescending the leeward side of the range causes thecloud system to dissipate and the warm front to travel asan upper front.

Frontogenesis (the formation of a new front or theregeneration of an old front) may occur in thepressure-trough area that accompanies the front. Thefrontal surface then gradually forms downward as thefrontal system moves away from the mountain andextends to the earth’s surface again (views C and D offig. 4-45). The effect of the mountain range on a warmfront is to widen and prolong the precipitation on thewindward side of the range, while on the leeward sidethe precipitation band is narrowed and weakened, or isnonexistent.

Occluded Fronts

Mountain ranges have much the same effect onoccluded fronts as they do on warm and cold fronts.Cold type occlusions behave as cold fronts, and warmtype occlusions behave as warm fronts. The occlusionprocess is accelerated when a frontal wave approachesa mountain range. The warm front is retarded; but thecold front continues at its normal movement, quicklyovertaking the warm front (views A and B of fig. 4-46).

4-48

A B

C

AG5f0446

Figure 4-46.—Acceleration of the occlusion process and development of a frontal wave cyclone.

When a cold front associated with an occludedfrontal system passes a mountain range, the cold frontmay develop a bulge or wave. In the case of anocclusion, a new and separate low may form at the peakof the warm sector as the occluded front is retarded by amountain range (view C of fig. 4-46). The low developson the peak of the wave because of induced lowpressure that results when air descends on the leewardside of the mountain and warms adiabatically.

The development of a new low on a frontal waveand ultimate separation from its original cyclone is afairly common occurrence. This can occur over openoceans but occurs more frequently along the west coastof mountainous continents and along the west coast ofJapan. The typical stages of this type of frontalmodification are shown in figure 4-47. Orographicfeatures play a great role in certain preferred areas ofthis phenomena, but over the ocean some other factors

must be operative. In some cases, a rapidly movingwave overtakes the slow moving occlusion and may bethe triggering mechanism for this cyclogenesis.

Whatever the exact nature of its causes, this type ofcyclogenesis proceeds with great rapidity. Initially, theold occlusion in view A of figure 4-47 either movesagainst a mountain range or is overtaken by anothercyclone. The occlusion then undergoes frontolysis(view B of fig. 4-47). The new occlusion formsimmediately and soon overshadows its predecessor inboth area and intensity (view C of fig. 4-47). However,the cold occlusion, having greater vertical extension,exerts a certain control on the movement of the newcenter, which at first follows the periphery of the oldcenter. Later, the two centers pivot cyclonically (view Dof fig. 4-47) about a point somewhere on the axisjoining them until the old center has filled and loses itsseparate identity.

4-49

-10

-30

-15

-30

-15

-15

C D

A B

AG5f0447

Figure 4-47.—Stages in the development of a secondary wave cyclone.

This can take place with either a warm or coldocclusion. If it occurs near a west coast in winter, thereis a good chance the new occlusion is warm. Thisformation of a secondary wave cyclone, the dissipationof the original occluded front, and the rapiddevelopment of a new occlusion is sometimes calledskagerraking, pressure jump, or bent-back occlusion.

EFFECTS OF UNDERLYING SURFACES

The migration of a frontal system from one areaand type of underlying surface to another often has agreat modifying effect. It may cause the front to beregenerated in some instances or to dissipate in others.This transition affects cyclones, air masses, and fronts.

Movement Over Land Surfaces

So far, we have established that frontal systemsgenerally weaken when moving from water to landsurfaces. Once these systems are over land, furthermodification can be expected. A front that has justcrossed the mountains and has weakened remains weakor dissipates unless something occurs to strengthen thecontrast between the air masses. If a cold front has justmoved onshore in winter and encounters ice and snowcover over the western half of the United States, themaritime air behind the front quickly takes on coldercontinental properties. The cold underlying surfacemay totally destroy the cold front, especially if theassociated air mass is moving slowly. On the otherhand, if the front is moving quickly enough that it is nottotally destroyed or modified by the colder surface, itmay quickly regenerate as it approaches a warmerunderlying surface and air mass. These normally existover the eastern half of the United States. In thisparticular situation, the air behind the front is muchcolder than when it started. As the front arrives at theedge of the snow field, it probably will encounterwarmer moist air from the gulf or the ocean. Thissituation quickly results in frontogenesis because of asharp air mass contrast. Strong lifting by the wedge ofapproaching cold air results in severe thunderstormsand abundant precipitation along the frontal surface.

If the ice and snow field does not exist over thewestern half of the United States, then the weakenedfront gradually strengthens as it approaches the warmereastern United States. The weather will not be asintense; however, the cold front will have a much widerband of clouds and precipitation. With this situation, airmass contrast is not strong. If the air masses behind andahead of the front are weak, the front becomes

stationary over the extreme southeast United States.The frontal systems are usually oriented in anortheast-southwest direction and occur mostly duringthe summer and autumn months. Frequently, stablewaves develop and travel along this frontal system,causing unfavorable weather conditions. When thesewaves move out to sea and warmer moist air is broughtinto them, they become unstable waves and areregenerated as they move across the ocean.

As the cold fronts cross the AppalachianMountains, they normally weaken once again becausewarm moist air is cut off. After passage over themountains, warm Gulf Stream waters quickly resupplythe frontal surface with the moisture and warm airneeded for the front to strengthen.

Land to Water Migration

Once a cold front moves offshore, most forecastersand analysts forget about them and concentrate on thenext approaching weather. When a front moves into theAtlantic, the weather generally becomes more intense,especially during fall and winter. While your stationmay be relaxing to some degree and enjoying the clearskies after frontal passage, Bermuda and ships at seaare most likely bracing for gale force wind and severethunderstorm activity.

In middle latitudes, ocean currents carry warmwater away from the equator along the eastern coasts ofcontinents and carry cold water toward the equatoralong the western coasts of continents. The most activefrontal zones of the winter season are found where coldcontinental air moves over warm water off easterncoasts. This situation is noticeable off the eastern coastof the United States over the Atlantic Ocean. As a coldfront moves off the coast and over the Gulf Stream, itintensifies, and frequently wave development occursnear the Cape Hatteras area. This gives the eastern coastof the United States much cloudiness and precipitation.This system and its newly intensified front eventuallyreaches Bermuda. A similar situation occurs off theeastern coast of Japan. That area in the Pacificgenerates more cyclones than any other area in theworld.

REVIEW QUESTIONS

Q4-20. What two effects cause the modification offronts?

Q4-21. What normally happens to a cold front thatmoves off the eastern coast of the UnitedStates in the winter?

4-50

CHAPTER 5

ATMOSPHERIC PHENOMENA

Atmospheric phenomena include all hydrometeors,lithometeors, photo-meteors, and electrometeors andtheir associated effects. As an observer, you have theopportunity to observe and record some of thesephenomena on a daily basis; however, as an analyst youmust understand how and why these phenomena occurand what effects they can have on naval operations.Some phenomena have little effect on naval operations,but others such as extensive sea fogs and thunderstormactivity can delay or cancel operations.

HYDROMETEORS

LEARNING OBJECTIVE: Identify thecharacteristics of hydrometeors (precipitation,clouds, fog, dew, frost, rime, glaze, drifting andblowing snow, spray, tornadoes, andwaterspouts).

Hydrometeors consist of liquid or solid waterparticles that are either falling through or suspended inthe atmosphere, blown from the surface by wind, ordeposited on objects. Hydrometeors comprise all formsof precipitation, such as rain, drizzle, snow, and hail,and such elements as clouds, fog, blowing snow, dew,frost tornadoes, and waterspouts.

PRECIPITATION

Precipitation includes all forms of moisture that fallto Earth’s surface, such as rain, drizzle, snow, and hail,etc. Dew, frost, clouds, fog, rime, glaze, spray,tornadoes, and waterspouts are not forms ofprecipitation, although they are hydrometeors.Precipitation is classified according to both form(liquid, freezing, and solid) and size (rate of fall). Thesize of precipitation drops determines their rate of fallto a large extent.

Rain

Precipitation that reaches Earth’s surface as waterdroplets with a diameter of 0.02-inch (0.5 mm) or moreis classified as rain. If the droplets freeze on contactwith the ground or other objects, the precipitation isclassified as freezing rain. Rain falling from convectiveclouds is referred to as rain showers. Showers are

usually intermittent in character, are of large dropletsize, and change rapidly in intensity.

Drizzle

Drizzle consists of very small and uniformlydispersed droplets that appear to float while followingair currents. Sometimes drizzle is referred to as mist.Drizzle usually falls from low stratus clouds and isfrequently accompanied by fog and reduced visibility.A slow rate of fall and the small size of the droplets(less than 0.5-mm) distinguish drizzle from rain. Whendrizzle freezes on contact with the ground or otherobjects, it is referred to as freezing drizzle. Drizzleusually restricts visibility.

Snow

Snow consists of white or translucent ice crystals.In their pure form, ice crystals are highly complexhexagonal branched structures. However, most snowfalls as parts of crystals, as individual crystals, or morecommonly as clusters and combinations of these. Snowoccurs in meteorological conditions similar to those inwhich rain occurs, except that with snow the initialtemperatures must be at or below freezing. Snow fallingfrom convective clouds is termed snow showers.

Snow Pellets

Snow pellets are white, opaque, round (oroccasionally conical) kernels of snow-like consistency,0.08 to 0.2 inch in diameter. They are crisp, easilycompressible, and may rebound or burst when strikinghard surfaces. Snow pellets occur almost exclusively insnow showers.

Snow Grains

Snow grains consist of precipitation of very small,white, opaque grains of ice similar in structure to snowcrystals. They resemble snow pellets somewhat, but aremore flattened and elongated. When the grains hit hardground, they do not bounce or shatter. Snow grainsusually fall in small quantities, mostly from stratusclouds, and never as showers.

5-1

Ice Pellets

Ice pellets are transparent or translucent pellets ofice that are round or irregular (rarely conical) and havea diameter of .02 inch (.5 mm) or less. They usuallyrebound upon striking hard ground and make a soundon impact. Ice pellets are generally subdivided into twogroups, sleet and small hail. Sleet is composed of hardgrains of ice, which has formed from the freezing ofraindrops or the refreezing of largely meltedsnowflakes; it falls as continuous precipitation. Smallhail is composed of pellets of snow encased in a thinlayer of ice that has formed from the freezing of eitherdroplets intercepted by the pellets or water resultingfrom the partial melting of the pellets; small hail falls asshowery precipitation.

Hail

Ice balls or stones, ranging in diameter from that ofa medium-size raindrop to two inches or more arereferred to as hail. They may fall detached or frozentogether into irregular, lumpy masses. Hail is composedeither of clear ice or of alternating clear and opaquesnowflake layers. Hail forms in cumulonimbus cloud,and is normally associated with thunderstorm activityand surface temperatures above freezing.Determination of size is based on the diameter (ininches) of normally shaped hailstones.

Ice Crystals (Ice Prisms)

Ice crystals fall as unbranched crystals in the formof needles, columns, or plates. They are often so tinythey seem to be suspended in the air. They may fallfrom a cloud or from clear air. In a synopticobservation, ice crystals are called ice prisms. They arevisible mainly when they glitter in the sunlight or otherbright light; they may even produce a luminous pillar orother optical phenomenon. This hydrometeor iscommon in Polar Regions and occurs only at lowtemperatures in stable air masses.

PRECIPITATION THEORY

Several valid theories have been formulated inregard to the growth of raindrops. The theories mostwidely accepted today are treated here in combinedform.

Raindrops grow in size primarily because waterexists in all three phases in the atmosphere and becausethe air is supersaturated at times (especially withrespect to ice) because of adiabatic expansion and

radiation cooling. This means that ice crystals coexistwith liquid water droplets in the same cloud. Thedifference in the vapor pressure between the waterdroplets and the ice crystals causes water droplets toevaporate and then to sublimate directly onto the icecrystals. Sublimation is the process whereby watervapor changes into ice without passing through theliquid stage. Condensation alone does not causedroplets of water to grow in size. The turbulence incloud permits and aids this droplet growth processes.After the droplets become larger, they start to descendand are tossed up again in turbulent updrafts within thecloud. The repetition of ascension and descensioncauses the ice crystals to grow larger (by water vaporsublimating onto the ice crystals) until finally they areheavy enough to fall out of the cloud as some form ofprecipitation. It is believed that most precipitation inthe mid-latitudes starts as ice crystals and that mostliquid precipitation results from melting during descentthrough a stratum of warmer air. It is generally believedthat most rain in the tropics forms without goingthrough the ice phase.

In addition to the above process of droplet growth,simple accretion is important. In this process, thecollision of ice crystals with super-cooled waterdroplets causes the droplets to freeze on contact withthe ice crystals. As the droplets freeze on the icecrystals, more layers accumulate. This process isespecially effective in the formation of hail. There areother factors that explain, in part, the growth ofprecipitation, but the above processes are the primaryones.

OTHER HYDROMETEORS

The hydrometeors that follow, are notprecipitation; however, they are equally important.

Clouds

A cloud is a visible mass of minute water droplets(or ice particles) suspended in the atmosphere. It differsfrom fog in that it does not reach the surface of Earth.Clouds are a direct expression of the physical processestaking place in the atmosphere. An accurate descriptionof both type and amount plays an important part in theanalysis of the weather and in forecasting the changesthat take place in the weather.

CLOUD FORMATION.—To be able tothoroughly understand clouds, the Aerographer’s Matemust know the physical processes that form clouds.Three conditions must be met before clouds can form as

5-2

a result of condensation—presence of sufficientmoisture, hygroscopic or sublimation nuclei in theatmosphere, and a cooling process. Moisture issupplied to the atmosphere by evaporation and isdistributed horizontally and vertically by the winds andvertical currents. The first task is to consider thehygroscopic and sublimation nuclei.

Hygroscopic nuclei are particles of any nature onwhich condensation of atmospheric moisture occurs. Itcan be said that hygroscopic nuclei have an affinity forwater or that they readily absorb and retain water. Themost effective hygroscopic nuclei are the products ofcombustion (sulfuric and nitric acids) and salt sprays.Some dust particles are also hygroscopic, but noteffectively so. The presence of hygroscopic nuclei is amust; water vapor does not readily condense withouttheir presence. Air has been supersaturated inlaboratories to over 400 percent before condensationbegan when there were no hygroscopic nuclei present.On the other hand, condensation has been induced withrelative humidity of only 70 percent when there was anabundance of hygroscopic nuclei.

The condensation, which results when all threementioned conditions are fulfilled, is usually in theform of mist, clouds, or fog. Fogs are merely clouds onthe surface of Earth.

In our industrial cities, where byproducts ofcombustion are abundant, the distinction betweensmoke, fog, and haze is not easily discernible. Acombination of smoke and fog gives rise to theexistence of the so-called smog characteristic of theseindustrial areas.

Little is known about the properties of sublimationnuclei, although it is believed they are essential forsublimation to occur at all. It is assumed sublimationnuclei are very small and very rare, possibly of a quartzor meteoric dust origin. All cirriform clouds arecomposed of ice crystals and are believed to be formedas a result of direct sublimation. In the atmosphere,water clouds, water and ice crystal clouds, and pure icecrystal clouds may coexist at the same time.

Next under consideration is the cooling processthat may induce condensation. There are severalprocesses by which the air is cooled: convective coolingby expansion, mechanical cooling by expansion, andradiation cooling. Any of the three methods may workin conjunction with another method, making it evenmore effective. The methods are as follows:

1. Convective cooling. The ascent of a limitedmass of air through the atmosphere because of surface

heating is called thermal convection. If a sample of airis heated, it rises (being less dense than the surroundingair) and decreases in temperature at the dry adiabaticlapse rate until the temperature and dew point are thesame. This is the saturation point at which condensationbegins. As the parcel of air continues to rise, it cools at alesser rate—called the moist/saturation adiabatic lapserate. The parcel of air continues to rise until thesurrounding air has a temperature equal to, or higherthan, the parcel of air. At this point convection ceases.Cumuliform clouds are formed in this way. Cloud basesare at the altitude of saturation and tops are at the pointwhere the temperature of the surrounding air is thesame as, or greater than, the temperature of the parcel ofair.

2. Mechanical cooling. Orographic and frontalprocesses are considered mechanical means of coolingwhich result in cloud formation.

a. Orographic processes. If air iscomparatively moist and is lifted over mountains orhills, clouds may be formed. The type of cloud dependsupon the lapse rate (the rate of decrease in temperaturewith increase in height, unless otherwise specified) ofthe air. If the lapse rate is weak (that is, a low rate ofcooling with an increase in altitude), the clouds formedare of the stratiform type. If the lapse rate of the air issteep (that is, a high rate of cooling with increasingaltitude), the clouds formed are of the cumuliform type.

b. Frontal processes. In the previous unit, youlearned that, at frontal surfaces, the warmer, less denseair is forced to rise along the surfaces of the colder airmasses. The lifted air undergoes the same type ofadiabatic cooling as air lifted orographically. The typeof cloud formed depends on the lapse rate and moistureof the warm air and the amount of lifting. The slope ofthe front determines lifting; when the slope is shallow,the air may not be lifted to its saturation point andclouds do not form. When the slope steep, as with afast-moving cold front, and the warm air is unstable,towering cumuliform cloud form.

3. Radiation cooling. At night Earth releaseslong-wave radiation, thereby cooling rapidly. The air incontact with the surface is not heated by the outgoingradiation, but rather is cooled by contact with the coldsurface. This contact cooling lowers the temperature ofthe air near the surface, causing a surface inversion. Ifthe temperature of the air is cooled to its dew point, fogand/or low clouds form. Clouds formed in this mannerdissipate during the day because of surface heating.

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CLOUD CLASSIFICATION.—The inter-national classification of clouds adopted by mostcountries is a great help to both meteorologicalpersonnel and pilots. It tends to make cloudobservations standard throughout the world, and pilotsthat can identify cloud types will normally take thenecessary steps to avoid those types dangerous to theiraircraft.

Clouds have been divided into etages, genera,species, and varieties. This classification is basedprimarily on the process that produces the clouds.Although clouds are continually in a process ofdevelopment and dissipation, they do have manydistinctive features that make this classificationpossible.

Etages.—Observations have shown that cloudsgenerally occur over a range of altitudes varying fromsea level to about 60,000 feet in the tropics, to about45,000 feet in middle latitudes, and to about 25,000 feetin Polar Regions. By convention, the part of theatmosphere in which clouds are usually present hasbeen vertically divided into three etages—high, middle,and low. The range of levels at which clouds of certaingenera occur most frequently defines each etage.

Cirrus, cirrocumulus, and cirrostratus are alwaysfound in the high etage. Altocumulus and altostratus arefound in the middle etage, but altostratus may oftenextend into the high etage. Nimbostratus is alwaysfound in the middle etage but may extend into the high,and especially the low etage. Cumulus, cumulonimbus,stratus, and stratocumulus are always associated withthe low etage, but the tops of cumulus or cumulonimbusmay extend into one or both of the two other etages.

The HIGH ETAGE extends from about 10,000 to25,000 feet in polar regions, 16,500 to 45,000 feet intemperate regions, and 20,000 to 60,000 feet in tropicalregions.

The MIDDLE ETAGE extends from about 6,500 to13,000 feet in polar regions, 6,500 to 23,000 feet intemperate regions, and 6,500 to 25,000 feet in tropicalregions.

The LOW ETAGE extends from near Earth’ssurface to 6,500 feet in all regions of Earth.

Genera (Types).—As a weather analyst,interpreter, and briefer, you will be viewing the state ofthe sky with distinctly different objectives in mind. Areview of the various cloud types can help you toassociate past observer experiences with synopticconditions and trends.

High clouds. High clouds are described as follows:

1. Cirrus (CI). Cirrus are detached clouds ofdelicate and fibrous appearance, are generally white(cirrus are the whitest clouds in the sky), and arewithout shading. They appear in the most varied forms,such as isolated tufts, lines drawn across the sky,branching feather-like plumes, and curved lines endingin tufts. Since cirrus is composed of ice crystals, theirtransparent character depends upon the degree ofseparation of the crystals. Before sunrise and aftersunset, cirrus may still be colored bright yellow or red.Being high altitude clouds, they light up before lowerclouds and fade out much later. Cirrus often indicatesthe direction in which a storm lies.

2. Cirrocumulus (CC). Cirrocumulus, commonlycalled mackerel sky, looks like rippled sand or likecirrus containing globular masses of cotton, usuallywithout shadows. Cirrocumulus is an indication that astorm is probably approaching. The individual globulesof cirrocumulus are rarely larger than 1 degree asmeasured by an observer on the surface of Earth at ornear sea level.

3. Cirrostratus (CS). Cirrostratus form a thin,whitish veil, which does not blur the outlines of theSun, or the Moon but does give rise to halos. A milkyveil of fog, thin stratus, and altostratus aredistinguished from a veil of cirrostratus of similarappearance by the halo phenomenon, which the Sun orMoon nearly always produces in a layer of cirrostratus.The appearance of cirrostratus is a good indication ofrain. In the tropics, however, cirrostratus is quite oftenobserved with no rain following.

Middle clouds. Middle clouds are described asfollows:

1. Altocumulus (AC). Altocumulus appear as alayer (or patches) of clouds composed of flattenedglobular masses, the smallest elements of the regularlyarranged layer being fairly small and thin, with orwithout shading. The balls or patches usually arearranged in groups, lines, or waves. This cloud formdiffers from cirrocumulus by generally having largermasses, by casting shadows, and by having noconnection with cirrus forms. Corona and irisation arefrequently associated with altocumulus.

2. Altostratus (AS). Altostratus looks like thickcirrostratus, but without halo phenomena; altostratusforms a fibrous veil or sheet, gray or bluish in color.Sometimes the Sun or Moon is completely obscured.Light rain or heavy snow may fall from an altostratuscloud layer. Altostratus can sometimes be observed at

5-4

two different levels in the sky and sometimes inconjunction with altocumulus, which may also exist astwo different layers in the sky.

3. Nimbostratus (NS). Nimbostratus appears as alow, amorphous, and rainy layer of clouds of a darkgray color. They are usually nearly uniform and feeblyilluminated, seemingly from within. Whenprecipitation occurs, it is in the form of continuous rainor snow. However, nimbostratus may occur withoutrain or snow reaching the ground. In cases in which theprecipitation does not reach the ground, the base of thecloud is usually diffuse and looks wet. In most cases,nimbostratus evolve from altostratus layers, whichgrow thicker and whose bases become lower until theybecome a layer of nimbostratus.

Low clouds. Low clouds are described as follows:

1. Stratocumulus (SC). Stratocumulus appear as alayer (or patches) of clouds composed of globularmasses or rolls. The smallest of the regularly arrangedelements is fairly large. They are soft and gray withdarker spots.

2. Stratus (ST). Stratus appears as a low, uniformlayer of clouds, resembling fog, but not resting on theground. When a layer of stratus is broken up intoirregular shreds, it is designated as stratus fractus. Aveil of stratus gives the sky a characteristically hazyappearance. Usually, drizzle is the only precipitationassociated with stratus. When there is no precipitation,the stratus cloud form appears drier than other similarforms, and it shows some contrasts and some lightertransparent parts.

3. Cumulus (CU). Cumulus is dense clouds withvertical development. Their upper surfaces are domeshaped and exhibit rounded protuberances, while theirbases are nearly flat. Cumulus fractus or fractocumulusresemble ragged cumulus in which the different partsshow constant change.

4. Cumulonimbus (CB). Cumulonimbi are heavymasses of cumulus-type clouds with great verticaldevelopment whose cumuliform summits resemblemountains or towers. Tops may extend higher than60,000 feet. Their upper parts are composed of icecrystals and have a fibrous texture; often they spreadout in the shape of an anvil.

Cumulonimbi are the familiar thunderclouds, andtheir precipitation is of a violent, intermittent, showerycharacter. Hail often falls from well-developedcumulonimbus. On occasion, cumulonimbus cloudsdisplay several readily apparent supplementary

features. Examples are (1) mamma or hangingpouch-like protuberances on the under surface of thecloud; (2) tuba (commonly called the funnel cloud),resembling a cloud column or inverted cloudcone/pendant from the cloud base; and (3) virga, wispsor streaks of water or ice particles falling out of a cloudbut evaporating before reaching Earth’s surface asprecipitation.

The Aerographer’s Mate must learn to recognizethe various cloud types and associated precipitation asseen from Earth’s surface. Figure 5-1 shows the varioustypes of clouds in a tier with each cloud type at itsaverage height. Although one never sees all cloud typesat once, quite frequently two or three layers of clouds ofdifferent types may be present simultaneously.

Species.—The following species of clouds arereferred to frequently; others may be found in theInternational Cloud Atlas or in the newer publication,Cloud Types for Observers.

Castellanus. Clouds which present, in at least someportion of their upper part, cumuliform protuberancesin the form of turrets. The turrets, which are generallytaller than they are wide, are connected to a commonbase. The term applies mainly to cirrocumulus,altocumulus, and stratocumulus, but especiallyaltocumulus.

Stratiformis. Clouds which are spread out in anextensive horizontal sheet or layer. The term applies toaltocumulus, stratocumulus, and occasionally tocirrocumulus.

Lenticularis. Clouds having the shape of lenses oralmonds, often elongated and having well-definedoutlines. The term applies mainly to cirrocumulus,altocumulus, and stratocumulus.

Fractus. Clouds in the form of irregular shreds,which have a clearly ragged appearance. The termapplies only to stratus and cumulus.

Humilis. Cumulus clouds of only a slight verticalextent; they generally appear flattened.

Congestus. Cumulus clouds which are markedlysprouting and are often of great vertical extent. Theirbulging upper part frequently resembles cauliflower.

Varieties and Supplementary Features.—Cloudvarieties are established mainly on the basis of thecloud’s transparency or its arrangement in the sky. Adetailed description of the nine varieties can be found inthe International Cloud Atlas.

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Supplementary features and accessory clouds, likethe varieties, aid in the clear identification of clouds.The most common supplementary features are mamma,tuba, and virga. They are defined and associated withthe parent clouds in the general section.

Fog

Fog is a cloud on Earth’s surface. It is visiblecondensation in the atmosphere. Fog varies in depth

from a few feet to many hundreds of feet. Its density isvariable resulting in visibility from several miles tonear zero. It differs from rain or mist in that its water orice particles are more minute and suspended and do notfall earthward.

The forecasting of fog is frequently a difficult task.In addition to knowledge of the meteorological causesof fog formation, it is necessary to have a thoroughknowledge of local geography and topography. A slight

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CIRRUS

CIRROSTRATUS

20,000

6,500

CIRROCUMULUS

CUMULONIMBUS

ALTOSTRATUS

ALTOCUMULUS

NIMBOSTRATUS

STRATOCUMULUS CUMULUS

STRATUS

AG5f0501

Figure 5-1.—Layer diagram of clouds at various levels.

air drainage (gravity induced, downslope flow ofrelatively cold air) may be enough to prevent fogformation, or a sudden shift in the wind direction maycause fog to cover an airfield.

The temperature to which air must be cooled, at aconstant pressure and a constant water vapor content, inorder for saturation to occur is the dew point. This is avariable, based upon the amount of water vapor presentin the atmosphere. The more water vapor present, thehigher the dew point. Thus, the dew point is really anindex of the amount of water vapor present in the air at agiven pressure.

Temperature and dew point may be made tocoincide either by raising the dew point until it equalsthe temperature of by lowering the temperature to thedew point. The former results from the addition ofwater vapor to the air by evaporation from watersurfaces, wet ground, or rain falling through the air. Thelatter results from the cooling of the air by contact witha cold surface underneath. There are severalclassifications of fog: radiation fog, advection fog,upslope fog, and frontal fog.

RADIATION FOG.—Radiation fog, whichgenerally occurs as ground fog, is caused by theradiation cooling of Earth’s surface. It is primarily anighttime occurrence, but it often begins to form in thelate afternoon and may not dissipate until well aftersunrise. It never forms over a water surface. Radiationfog usually covers a wide area.

After sunset, Earth receives no heat from the Sun,but its surface continues to reradiate heat. The surfacebegins to cool because of this heat loss. As Earth cools,the layer of air adjacent to the surface is cooled byconduction (the transfer of heat from warmer to coldermatter by contact). This causes the layer near Earth tobe cooler than the air immediately above it, a conditioncalled an inversion. If the air beneath the inversion layeris sufficiently moist and cools to its dew point, fogforms. (See fig. 5-2.) In case of a calm wind, thiscooling by conduction affects only a very shallow layer(a few inches deep), since air is a poor conductor ofheat. Wind of low speed (3 to 5 knots) causes slight,turbulent currents. This turbulence is enough to spreadthe fog through deeper layers. As the nocturnal coolingcontinues, the air temperature drops further, moremoisture is condensed, and the fog becomes deeper anddenser. If winds increase to 5 to 10 knots, the fog willusually thicken vertically. Winds greater than 10 knotsusually result in the formation of low scud, stratus, orstratocumulus.

After the Sun rises, Earth is heated. Radiation fromthe warming surface heats the lower air, causingevaporation of the lower part of the fog, thereby givingthe appearance of lifting. Before noon, heat radiatedfrom the warming surface of Earth destroys theinversion and the fog evaporates into the warmed air.Radiation fog is common in high-pressure areas wherethe wind speed is usually low (less than 5 knots) andclear skies are frequent. These conditions permitmaximum radiation cooling.

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AG5f0502

RADIATION

LAND COOLING

Figure 5-2.—Radiation fog.

ADVECTION FOG.—Advection fog is the namegiven to fog produced by air in motion or to fog formedin one place and transported to another. This type of fogis formed when warmer air is transported over colderland or water surfaces. Cooling from below takes placeand gradually builds up a fog layer. The cooling ratedepends on the wind speed and the difference betweenthe air temperature and the temperature of the surfaceover which the air travels.

Advection fog can form only in regions wheremarked temperature contrasts exist within a shortdistance of each other, and only when the wind blowsfrom the warm region toward the cold region. (See fig.5-3.) It is easy to locate areas of temperature contrast onthe weather map, as they are usually found alongcoastlines or between snow-covered and bare ground.

Sea Fog.—Sea fog is always of the advection typeand occurs when the wind brings moist, warm air over acolder ocean current. The greater the differencebetween the air temperature and the ocean temperature,the deeper and denser the fog. Sea fog may occurduring either the day or night. Some wind is necessary,not only to provide some vertical mixing, but also tomove the air to the place where it is cooled. Mostadvection fogs are found at speeds between 4 and 13knots. Sea fogs have been maintained with wind speedas high as 26 knots. They persist at such speeds becauseof the lesser frictional effect over a water surface.

Winds of equal speed produce less turbulence overwater than over land.

Sea fogs, which tend to persist for long periods oftime, are quite deep and dense. Since the temperature ofthe ocean surface changes very little during the day, it isnot surprising to hear of sea fogs lasting for weeks. Agood example of sea fog is that found off the coast ofNewfoundland.

Land Advection Fog.—Land advection fog isfound near large bodies of water; that is, alongseacoasts and large lakes. Onshore breezes bringmaritime air over a land surface, which has cooled byradiation at night. (See fig. 5-4.) Also, fogs may formover the ocean and be blown over the land during eitherthe day or the night. Another situation favorable to fogformation is one in which air flows from warm, bareground to snow-covered ground nearby.

Land advection fog cannot exist with as high windspeed as the sea type because of the greater turbulence.It dissipates in much the same fashion as radiation fog.However, since it is usually deeper, it requires a longertime to disperse.

Steam Fog.—Steam fog occurs within air masses;but, unlike other air-mass fogs, which are formed by thecooling of the air temperature to the dew point, steamfog is caused by saturation of the air throughevaporation of water. It occurs when cold air moves

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AG5f0503

WARM AIR

COOLING BY CONTACT

COLD GROUND

Figure 5-3.—Advection fog.

over warm water. Evaporation from the surface of thewarm water easily saturates the cold air, causing fog,which rises from the surface like smoke. It should benoted that the actual process of heating cold air over awarm surface tends to produce instability. The presenceof an inversion above the surface prevents steam fogfrom rising very high; it is usually fairly dense andpersistent.

This type of fog forms on clear nights over inlandlakes and rivers in late fall before they freeze. It isprevalent along the Mississippi River and Ohio River atthat time of year. Arctic sea smoke is the name given tosteam fogs in the arctic region. It forms when cold airmoves over a warmer water surface, which is mostoften found in breaks of the surface ice. It may alsooccur over the ocean surface following a cold frontalpassage when the water is approximately 40°F warmerthan the air passing over it.

Upslope Fogs.—Upslope fog is caused byadiabatic cooling of rising air. It is formed when moist,warm air is forced up a slope by the wind. The coolingof the air is almost entirely adiabatic, since there is littleconduction taking place between the air and surface ofthe slope. The air must be stable before it starts itsmotion so that the lifting does not cause convection, orvertical currents, which would dissipate the fog.

Some wind speed is needed, of course, to cause theupslope motion. Upslope fog is usually found where theair moves up a gradual slope. This type of fog is deepand requires considerable time to dissipate. The mostcommon fog of this type is called Cheyenne fog and iscaused by the westward flow of air from the Missouri

Valley, which produces fog on the eastern slope of theRockies.

Frontal Fog.—Frontal fog is another hazard,which must be added to the list of weather problemsassociated with fronts. The actual fog is due to theevaporation of falling rain and occurs under the frontalsurface in the cold air mass. This additional water vaporgradually saturates the air. Precipitation falls from thelifted warm air through the cold air. Evaporation fromthe rain continues as long as the temperature of theraindrops is higher than the temperature of the air, eventhough the cold air is already saturated. Naturally, theupper regions become saturated first because thetemperature and dew point are lower at the higheraltitude. As the evaporation from the rain continues, alayer of clouds begins to build down from the frontalsurface. Eventually, this cloud layer extends to theground and becomes fog.

During the day, there may be enough turbulencecaused by solar heating to keep this cloud off theground. However, after dark, because of dyingconvection currents and the nocturnal cooling of the air,the ceiling drops suddenly. It is this sudden closing inafter dark that makes frontal fog so dangerous.

Cold fronts usually move so rapidly and have suchnarrow bands of precipitation and high wind speeds thatcold-front fog is comparatively rare and short lived.warm-front fog, on the other hand, is fairly common.Since warm frontal systems are quite extensive,warm-front fog may cover a wide area. This type fog isalso deep because it extends from the ground to thefrontal surface. The clouds above the frontal surfacealso slow down the dissipating effect of solar heating.

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AG5f0504

WIND

WARM OCEAN

FOG OR LOWSTRATUS

COLD COASTAL WATER COLD CONTINENT

Figure 5-4.—Land advection fog caused by an onshore flow over cold coastal water.

These factors make the warm-front fog among the mostdangerous. (See fig. 5-5.)

Dew

Dew does not actually fall; rather the moisturecondenses from air that is in direct contact with the coolsurface. During clear, still nights, vegetation oftencools by radiation to a temperature at or below the dewpoint of the adjacent air. Moisture then collects on theleaves just as it does on a pitcher of ice water in a warmroom. Heavy dew is often observed on grass and plantswhen there is none on the pavements or on large, solidobjects. These objects absorb so much heat during theday or give up heat so slowly, they may not cool belowthe dew point of the surrounding air during the night.

Another type of dew is white dew. White dew is adeposit of white, frozen dew drops. It first forms asliquid dew, then freezes.

Frost

Frost, or hoarfrost, is formed by the process ofsublimation. It is a deposit of ice having a crystallineappearance and generally assumes the form of scales,needles, feathers, or fans. Hoarfrost is the solidequivalent of dew and should not be confused withwhite dew, which is dew frozen after it forms.

Rime (Rime Icing)

Rime is a white or milky opaque granular deposit ofice. It occurs when supercooled water droplets strike anobject at temperatures at or below freezing. Factorsfavoring the formation of rime are small drop size, slowaccretion, a high degree of supercooling, and rapiddissipation of latent heat of fusion. Rime is a result offreezing drizzle and looks like frost in a freezer. Rimeicing, which forms on aircraft, can seriously distortairfoil shape, therefore diminishing lift andperformance. Rime icing is more likely to form in

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AG5f0505

A WEATHER MAPWARM-FRONT FOG

B VERTICAL CROSS SECTIONALONG A A

PRECIPITATION AREA

WARM FRONT

FOGWARM AIR

FOG

COLD FRONT

COLD AIR

A A

WARM-F

RONTSURFACE

Figure 5-5.—Warm-front fog.

stratus-type clouds with temperatures between 0°C andminus 22°C. When formed in cumuliform-type clouds,temperatures range from minus 9°C to minus 15°C andare accompanied by clear icing which is then termedmixed icing.

Glaze (Clear Icing)

Glaze is a coating of ice, generally clear andsmooth. It occurs when supercooled water dropletsdeposited by rain, drizzle, fog, or condensed watervapor strike an exposed object at temperatures at orbelow freezing. Factors favoring formation of glaze arelarge drop size, rapid accretion, slight supercooling,and slow dissipation of the latent heat of fusion. Glazeis denser, harder, and more transparent than rime andlooks similar to an ice cube. Clear icing forms onaircraft and adds appreciably to the weight of the craft.This additional weight has an even greater effect inreducing the performance of the aircraft than does rimeicing. Clear icing occurs in cumuliform-type clouds attemperatures between 0°C and a minus 9°C. It alsooccurs with rime icing in cumuliform clouds attemperatures between minus 9°C and minus 15°C.

Drifting and Blowing Snow

Drifting and blowing snow are the result of snowparticles being raised from the ground by the wind. Toclassify wind-driven snow as drifting snow, theparticles will only be lifted to shallow heights (less than6 feet) and the horizontal visibility will remain at 7miles or more at eye level (6 feet). When the winddrives snow to levels 6 feet or higher and the visibility isrestricted to 6 miles or less, it is classified as blowingsnow.

Spray and Blowing Spray

Spray and blowing spray occurs when the wind isof such force that it lifts water droplets from the watersurface (normally the wave crests) and carries them intothe air. To be classified as spray, the wind-driven waterdroplets will not obstruct visibility at eye level (6 feeton shore and generally 33 feet at sea). Blowing sprayoccurs when the water droplets are lifted in suchquantities that they reduce visibility to 6 miles or less ateye level.

TORNADOES

A tornado is an extremely violent whirling stormwith a small diameter, usually a quarter of a mile or

less. The length of the track of a tornado on the groundmay be from a few hundred feet to 300 miles; theaverage is less than 25 miles. When not touching theground, it is termed a funnel cloud or tuba. Thevelocities of tornado winds are in the general range of125 to 250 knots. A large reduction of pressure in thecenter due to the spiraling of the air seems to causebuildings in the path of the storm to explode. The speedof the storm over Earth’s surface is comparativelyslow—usually 22 to 34 knots.

Most of the tornadoes in the United States occur inthe late spring and early summer in middle and lateafternoon, and they are associated with thunderstormactivity and heavy rain. Tornadoes occur on allcontinents but are most common in Australia and theUnited States. They can occur throughout the year andat any time of day. Tornadoes have been observed withvarious synoptic situations but are usually associatedwith overrunning cold air. Statistics show that themajority of tornadoes appear about 75 to 180 milesahead of a cold front along the prefrontal squall line.Figure 5-6 shows the various stages of development of atornado.

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AG5f0506

A B

C DFigure 5-6.—Stages of development of a tornado.

A situation that is noticeably favorable to tornadoactivity is cold air advection aloft. When mP air movesacross the United States, it becomes heated in the lowlevels in the western plateaus. The resulting density ofthe now warm mP air is then equal to or less than that ofmT air moving northward over the Mississippi Valley.The mP air rides up over the mT air. The mP air stillmaintains low temperatures at higher altitudes causingextreme instability.

The following conditions may indicate possibletornado activity:

1. Pronounced horizontal wind shear. (Windshear is the rate of change of wind velocity withdistance.)

2. Rapidly moving cold front.

3. Strong convergent flow at the surface.

4. Marked convective instability.

5. Dry air mass superimposed on a moist air massand abrupt change in moisture content, usually below10,000 feet.

6. Marked convection up to the minus 10°Cisotherm.

WATERSPOUTS

Waterspouts are tornadoes that form over oceanareas. This phenomenon consists of two types: tornadoin origin and locally induced. The difference betweenthe two types is significant in that the tornado type haspotential for inducing substantial damage and injuryover a broad area, while the local type has potential forcausing only minor damage in a small area. Thefollowing information is provided to help you to betterunderstand the two types of waterspouts.

Tornado Type

These waterspouts form at the cloud and extenddown to the surface. They originate from severeconvective cells associated with a cold front, squallline, or large convective cluster. Whenever theconditions for tornado development are present overcoastal areas and the triggering mechanism extends intothe adjacent maritime area, then potential forwaterspout development is high. The tornadowaterspout has a relatively short life span and usuallystays over water. However, when one does comeashore, there is potential for it to assume thecharacteristics of a tornado; although its life span is

limited, the initial intensity is sufficient to causeproperty damage and injury to personnel.

Local Type

These waterspouts originate from convectiveclouds of moderate vertical extent which form a line ora small cluster. Their existence is sensitive to wind andtemperature in that surface winds of 20 knots or greater,or a cooling of the atmosphere by precipitation,dissipates them. Additionally, when local waterspoutscome ashore, the friction induced by the land rapidlydissipates them. The biggest threat posed by thesewaterspouts is to small craft, recreational boating, andto support facilities such as harbor operations andmarinas.

REVIEW QUESTIONS

Q5-1. Describe the major difference between rainand drizzle.

Q5-2. What altitude range do clouds occur in thetropics?

Q5-3. What is the altitude range of middle clouds inthe temperate regions?

Q5-4. Describe the difference between sea fog andsteam fog.

Q5-5. What criteria must be met for a hydrometeorto be classified as blowing spray?

LITHOMETEORS

LEARNING OBJECTIVE: Identify thecharacteristics of lithometeors (haze, smoke,dust, sand, and dust devils).

Lithometeors comprise a class of atmosphericphenomena of which dry haze and smoke are the mostcommon examples. In contrast to hydrometeors, whichconsist largely of water, lithometeors are composed ofsolid dust or sand particles, or the ashy products ofcombustion.

HAZE

Haze is composed of suspended dust or saltparticles that are so small that they cannot beindividually felt or seen by the unaided eye. Theyreduce visibility and lend a characteristic opalescentappearance to the air. Haze resembles a uniform veilover the landscape that subdues all colors. This veil hasa bluish tinge when viewed against a dark background

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and a dirty yellow or orange tinge when viewed againsta bright background. Differences in air temperaturemay cause a shimmering veil over the landscape calledoptical haze.

SMOKE

Smoke is fine ash particles suspended in theatmosphere. When smoke is present, the disk of the Sunat sunrise and sunset appears red, and during thedaytime has a reddish tinge. Smoke at a distance, suchas from forest fires, usually has a light grayish or bluishcolor and is evenly distributed in the upper air.

DUST

Dust is finely divided solid matter uniformlydistributed in the air. It imparts a tan or grayish hue todistant objects. The Sun’s disk is pale and colorless orhas a yellow tinge during the day. Blowing dust consistsof dust raised by the wind to moderate heights abovethe ground and restricting horizontal visibility to lessthan 7 miles. When visibility is reduced to less thanfive-eighths of a mile but not less than five-sixteenths ofa mile, it is classified as a dust storm and, if less thanfive-sixteenths of a mile, as a severe dust storm.

SAND

Fine particles of sand picked up from the surface bythe wind and blown about in clouds or sheets constitutea troublesome lithometeor in some regions. Blowingsand consists of sand raised by the wind to moderateheights above the ground, which reduces horizontalvisibility to less than 7 miles. When the visibility isreduced to less than five-eighths of a mile but not lessthan five-sixteenths of a mile, it is classed as asandstorm and, if less than five-sixteenths of a mile, as asevere sandstorm.

DUST DEVILS

Dust devils, or whirling, dust-laden air, are causedby intense solar radiation, which sets up a steep lapserate near the ground. They are best developed on calm,hot, clear afternoons and in desert regions. As theintense surface heating sets up a steep lapse rate, a smallcirculation is formed when the surrounding air rushesin to fill the area of the rising warm air. This warmascending air carries dust, sand, leaves, and other smallmaterial to a height of a few hundred feet.

REVIEW QUESTIONS

Q5-6. Name one way that smoke is distinguishedfrom haze.

Q5-7. When and where are dust devils usuallyobserved?

PHOTOMETEORS

LEARNING OBJECTIVE: Identify thecharacteristics of photometeors and describethe characteristics of light, reflection, andrefraction.

Photometeors are any of a number of atmosphericphenomena that appear as luminous patterns in the sky.While they constitute a variety of fascinating opticalphenomena, photometeors are not active elements; thatis, they generally do not cause adverse weather.However, many are related to clouds that do causeadverse weather. Therefore, they help in describing thestate of the atmosphere.

LIGHT

Light, acting in conjunction with some of theelements of the atmosphere, produces a variety ofatmospheric phenomena, such as halos, coronas,mirages, rainbows, and crepuscular rays. This lessondiscusses the theories of light and the resultingphotometeors.

Light is the portion of the electromagneticspectrum that can be detected by the human eye. Ittravels at the same speed as all other electromagneticradiation (186,000 miles per second). However, thecharacteristics of light are considerably different fromother regions of the electromagnetic spectrum becauseof the differences in wavelength and frequency.

Sources of Light

There are two sources of light—natural andartificial. Nearly all natural light is received from theSun. Artificial light is light such as that produced byelectric lamps, fires, or fluorescent tubes. Luminousbodies are those bodies that which produce their ownlight, such as the Sun and the stars. Illuminated or nonluminous bodies are those bodies which merely reflectthe light they receive and are therefore visible becauseof this reflection. The Moon is an example of anilluminated body.

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Theory

When light is emitted from a source, waves ofradiation travel in straight lines and in all directions.Dropping a pebble into a pool of water can see a simpleexample of motion, similar to that of radiation waves.The waves spread out in expanding circles; similarly,light waves spread out in all directions to form a sphere.The boundary formed by each wave is called a wavefront. Lines, or rays, drawn from the light source to anypoint on one of these waves indicate the direction inwhich the wave fronts are moving. Light radiates fromits source in all directions until absorbed or diverted bycoming in contact with some substance or object.

Wavelength

The wavelength of a light wave is the distance fromthe crest of one wave to the crest of the following wave.Wavelength, frequency (the number of waves whichpass a given point in a unit of time), and speed arerelated by the simple equation:

C = λF

Where:

C = speedλ = wavelengthF = frequency

Because the speed of electromagnetic energy isconstant, the frequency must increase if the wavelengthdecreases and vice versa.

Wavelength is measured in angstrom units (A).They may also be measured in millimicrons, ormillionths of millimeters (mA). Figures 5-7 and 5-8show the visible and invisible spectrum’s colors inrelation to their wavelengths. Figure 5-8 shows that thevisible spectrum occupies only a small portion of thecomplete electromagnetic spectrum extending between4,000 and 7,000 angstroms only.

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L

L

L

L

L

L

L

L

ULTRAVIOLET

VIOLET

BLUE

GREEN

YELLOW

ORANGE

RED

INFRARED

VIS

IBL

E

AG5f0507

Figure 5-7.—Wavelength of various visible and invisiblecolors.

AG5f0508

(INVISIBLE)

VI S

I BL E

SPECTRUM

(INVISIBLE)

RED

700 mµ600 mµ

400 mµ 700 mµ

500 mµ

400 mµ

INFRA-RED

ULTRA-VIOLET

ORANGEYELLOW

GREENBLUE-GREEN

BLUEINDIGOVIOLET

PRISM

WHITE LIGHT

REFRACTION OF LIGHT BY A PRISM. THE LONGEST RAYS ARE INFRARED; THE SHORTEST, ULTRAVIOLET.

10 10 10 10 10 10 10 10 10 10 10-6 -4 -2 2 4 6 8 10 12 14 161

COSMICRAYS

GAMMARAYS

X RAYS ULTRA-VIOLETRAYS

INFRA-REDRAYS

HERTZIANWAVES

RADIOWAVES

LONGELECTRICAL

OSCILLATIONS

VISIBLE SPECTRUM

WAVELENGTHS IN MILLIMICRONS

Figure 5-8.—Wavelengths and refraction.

Characteristics

When light waves encounter any substance, theyare either reflected, absorbed, or refracted. (See fig.5-9.) Substances that permit the penetration of clearvision through them and transmit almost all the lightfalling upon them, such as glass and air, are transparent.There is no known substance that is perfectlytransparent, but many are nearly so. Those substancesthat allow the passage of part of the light but appearclouded and impair vision substantially, such as frostedlight bulbs, are considered translucent. Thosesubstances that do not transmit any light are termedopaque.

All objects that are not light sources are visibleonly because they reflect all or some part of the lightreaching them from a luminous source. If light isneither refracted nor reflected, it is absorbed or taken upby the medium. When light strikes a substance, someabsorption and some reflection always takes place. Nosubstance completely refracts (transmits), reflects, orabsorbs all the light that reaches its surface. Figure 5-9illustrates this refraction, absorption, and reflection oflight using a flat pane of glass.

Candlepower and Foot-candles

Illumination is the light received from a lightsource. The intensity of illumination is measured infoot-candles. A foot-candle is the amount of lightfalling upon a 1-square-foot surface, which is 1 footaway from a 1-candlepower light source.

REFLECTION

The term reflected light refers to those light wavesthat are neither transmitted nor absorbed but are thrownback from the surface of the medium they encounter. Ifa ray of light is directed against a mirror, the light raythat strikes the surface is called the incident ray; the onethat bounces off is the reflected ray (see fig. 5-10). Theimaginary line perpendicular to the mirror at the point

where the ray strikes is the normal. The angle betweenthe incident ray and the normal is the angle ofincidence. The angle between the reflected ray and thenormal is the angle of reflection.

If the surface of the medium contacted by theincident light ray is smooth and polished, such as amirror, the reflected light is thrown back at the sameangle to the surface as the incident light. The path of thelight reflected from the surface forms an angle exactlyequal to the one formed by its path in reaching themedium. This conforms to the law of reflection, whichstates that the angle of incidence is equal to the angle ofreflection.

Reflection from a smooth-surfaced object presentsfew problems. It is a different matter, however, when arough surface reflects light. The law of reflection stillholds but because the surface is uneven, the angle ofincidence is different for each ray of light. The reflectedlight is scattered in all directions as shown in figure5-11 and is called irregular or diffused light.

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INCIDENT LIGHT RAYS

REFLECTEDRAYS

ABSORBEDRAYS

REFRACTEDRAYS

AG5f0509

Figure 5-9.—Light rays reflected, absorbed, and refracted.

INCIDENTRAY

PERPENDICULAR

MIRRORREFLECTED

RAY

ANGLE OF INCIDENCE

ANGLE OF REFLECTION

NORMAL OR

AG5f0510

Figure 5-10.—Terms used to describe the reflection of light.

AG5f0511

INCIDENT LIGHT

INCIDENT LIGHT

REFLECTED LIGHT

SPECULAR

DIFFUSED

REFLECTED LIGHT SCATTERED

(A)

(B)

Figure 5-11.—Reflected light. (A) Regular (specular); (B)Irregular (diffused).

REFRACTION

The change of direction that occurs when a ray oflight passes at an oblique angle (less than 90°) from onetransparent substance into another substance ofdifferent density is called refraction. Refraction occursbecause light travels at various speeds in differenttransparent substances of different densities. Thegreater the density of a substance, the slower the lighttravels through it.

Refraction (or change of direction) always followsa simple rule: when the light ray passes from onetransparent substance into another of greater density,refraction is toward the normal. In this context, thenormal means a line perpendicular to the surface of themedium at the point of entrance of the light ray. (Seefig. 5-12.) In passing from one transparent substanceinto another of lesser density, refraction is away fromthe normal. (See fig. 5-13.)

When a ray of light enters a denser medium at anangle of 90°, as shown in figure 5-14, the wave frontsslow down but remain parallel. When this same lightray enters a denser medium at an oblique angle, theportion of the wave front that first enters the watermoves slower than the other part of the wave front thatis still in the air. Consequently, the ray bends toward thenormal. (See fig. 5-12).

If the light ray enters a less dense medium at anoblique angle, the ray bends away from the normal asshown in figure 5-13. The portion of the wave front that

enters the less dense substance travels faster than theother part of the wave front. Consequently, the raybends away from the normal.

When a beam of white light is passed through aprism, as shown in figure 5-8, it is refracted anddispersed into its component wavelengths. Each ofthese wavelengths reacts differently on the eye, whichthen sees the various colors that compose the visiblespectrum.

The visible spectrum ranges in color from violet atone end to red at the other end. (See fig. 5-8.) There are

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AG5f0512

NORMAL

AIR

WATER

Figure 5-12.—Wave front diagram illustrating refraction oflight at an air-water boundary. Ray is entering a moredense substance.

AG5f0513

WATER

AIR

NORMAL

Figure 5-13.—Wave front diagram illustrating refraction oflight at an air-water boundary. Ray is entering a less densesubstance.

AG5f0514

AIR

WATER

DIRECTION OFLIGHT RAY

Figure 5-14.—Wave front diagram illustrating the differencein the speed of light in air and water.

six distinct colors in the spectrum: red, orange, yellow,green, blue, and violet. However, a mixture of thesecolors is also present.

ATMOSPHERIC OPTICALPHENOMENA

LEARNING OBJECTIVE: Identify thecharacteristics of atmospheric opticalphenomena (halos, coronas, rainbows,fogbows, mirages, looming, scintillation andcrepuscular rays).

ATMOSPHERIC LAWS

Atmospheric optical phenomena are thosephenomena of the atmosphere that can be explained interms of optical laws. Some of the atmosphericelements, such as moisture, serve as a prism to break alight source down into its various component colors.The resulting phenomena can be spectacular as well asdeceptive.

Halos

A halo is a luminous ring around the Sun or Moon.When it appears around the Sun, it is a solar halo; whenit forms around the Moon, it is a lunar halo. It usuallyappears whitish (caused by reflection), but it may showthe spectral colors, from refraction (red, orange, yellow,green, blue, indigo, and violet) with the red ring on theinside and the violet ring on the outside. The sky isdarker inside the ring than outside. Halos are formed byrefraction of light as it passes through ice crystals. Thismeans that halos are almost exclusively associated withcirriform clouds. Refraction of light means that thelight passes through prisms; in this case, ice crystals actas prisms. Some reflection of light also takes place.

Halos appear in various sizes, but the mostcommon size is the small 22-degree halo. The size ofthe halo can be determined visually with ease.Technically, the radius of the 22-degree halo subtendsan arc of 22°. This simply means that the anglemeasured from the observation point between theluminous body and the ring is 22°. Halos of other sizesare formed in the same manner.

Coronas

A corona is a luminous ring surrounding the Sun(solar) or Moon (lunar) and is formed by diffraction oflight by water droplets. It may vary greatly in size but isusually smaller than a halo. All the spectral colors may

be visible, with red on the outside, but frequently theinner colors are not visible. Sometimes the spectralcolors or portions of them are repeated several timesand are somewhat irregularly distributed. Thisphenomenon is called iridescence.

Rainbows

The rainbow is a circular arc seen opposite the Sun,usually exhibiting all the primary colors, with red onthe outside. Diffraction, refraction, and reflection oflight cause it from raindrops or spray, often with asecondary bow outside the primary one with the colorsreversed.

Fogbows

A fogbow is a whitish circular arc seen opposite theSun in fog. Its outer margin has a reddish tinge; its innermargin has a bluish tinge; and the middle of the band iswhite. An additional bow, with the colors reversed,sometimes appears inside the first.

Mirages

Mirages are images of objects that are made toappear displaced from their normal positions becauseof refraction. These images may be only a partial imageof the object, and they may appear in either an uprightor an inverted position, depending upon theatmospheric condition that exists at the time ofobservation. Mirages occur when adjacent layers of airhave vastly different densities because of greattemperature differences. Whether these layers existside by side and horizontally or vertically determinesthe type of mirage.

Mirages are often seen in desert areas where airnear the surface becomes very hot. Cool air overlies thishot layer resulting in a large difference in the densitiesof the two layers. Three types of mirages result from therefraction of light rays through layers of air with vastlydifferent densities.

INFERIOR MIRAGE.—The inferior mirage, themost common of the three, appears as a mirrored imagebelow the object being viewed by the observer. In thiscase, you can associate the word inferior with beneathor below.

SUPERIOR MIRAGE.—In the superior mirage,the mirrored image appears above the object beingviewed. In this case, associate the word superior withabove or over.

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LATERAL MIRAGE.—Since the positions ofabove and below represent superior and inferiormirages respectively, the lateral mirage then appears tothe side of the object being viewed.

Looming

Looming is similar to a mirage in that it brings intoview objects that are over a distant horizon. Loomingoccurs when there is superrefraction in the loweratmosphere, which makes reflected light travel a pathsimilar to the curvature of Earth. Objects over thehorizon may be seen when light reflected from themtakes this path. Looming is somewhat rare and isnormally observed over flat surfaces, such as oceansand deserts.

Scintillation

Scintillation is caused by variations in atmosphericdensity near the horizon. It produces the appearance ofrapid changes in the position, brightness, and color ofdistinct luminous objects, such as stars. Stars flickeringand changing color near the horizon shortly after sunsetare good examples of scintillation and are a reasonablycommon phenomenon.

Crepuscular Rays

Crepuscular rays are another common phenomena.They are simply sunbeams that are rendered luminousby haze and dust particles suspended in the atmosphere.They are seen before and after sunrise and sunset asthey pass through small breaks or openings in or aroundclouds. The sunbeams are actually parallel but appearto diverge from the Sun.

REVIEW QUESTIONS

Q5-8. Name the two sources of light.

Q5-9. What is the difference between natural andartificial light?

Q5-10. What is the difference between reflection andrefraction?

Q5-11. What is a mirage?

ELECTROMETEORS

LEARNING OBJECTIVE: Identify thecharacteristics of electrometeors(thunderstorms, lightning, auroras, andairglow).

An electrometeor is a visible or audiblemanifestation of atmospheric electricity. The moreimportant electrometeors are thunderstorms, lightning,and auroras.

THUNDERSTORMS

The thunderstorm represents one of the mostformidable weather hazards in temperate and tropicalzones. Though the effects of the thunderstorm tend tobe localized, the turbulence, high winds, heavy rain,and occasional hail accompanying the thunderstorm area definite threat to the safety of flight operations and tothe security of naval installations. The Aerographer’sMate must be acquainted with the structure ofthunderstorms and the types of weather associated withthem.

Formation

The thunderstorm represents a violent andspectacular atmospheric phenomenon. Lightning,thunder, heavy rain, gusty surface wind, and frequenthail usually accompany it. A certain combination ofatmospheric conditions is necessary for the formationof a thunderstorm. These factors are conditionallyunstable air of relatively high humidity and some typeof lifting action. Before the air actually becomesunstable, it must be lifted to a point where it is warmerthan the surrounding air. When this condition is broughtabout, the relatively warmer air continues to rise freelyuntil, at some point aloft, its temperature has cooled tothe temperature of the surrounding air. Some type ofexternal lifting action must be introduced in order toraise the warm surface air to a point where it can risefreely. Many conditions satisfy this requirement;heating, terrain, fronts, or convergence may lift an airmass.

Structure

The fundamental structural element of thethunderstorm is the unit of convective circulationknown as the convective cell. A mature thunderstormcontains several of these cells, which vary in diameterfrom 1 to 6 miles. By radar analysis and measurementof drafts, it has been determined that, generally, eachcell is independent of surrounding cells of the samestorm. Each cell progresses through a cycle, which lastsfrom 1 to 3 hours. In the initial stage (cumulusdevelopment), the cloud consists of a single cell, but asthe development progresses, new cells form and oldercells dissipate. The life cycle of the thunderstorm cell

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consists of three distinct stages; they are the cumulus.stage, the mature stage, and the dissipating or anvilstage. (See fig. 5-15.)

CUMULUS STAGE.—Although most cumulusclouds do not become thunderstorms, the initial stage ofa thunderstorm is always a cumulus cloud. The chiefdistinguishing feature of this cumulus or building stageis an updraft, which prevails throughout the entire cell.Such updrafts vary from a few feet per second in theearly cells to as much as 100 feet per second in maturecells.

MATURE STAGE.—The beginning of surfacerain, with adjacent updrafts and downdrafts, initiatesthe mature stage. By this time the top of the average cellhas attained a height of 25,000 feet or more. As theraindrops begin to fall, the frictional drag between theraindrops and the surrounding air causes the air to begina downward motion. Since the lapse rate within athunderstorm cell is greater than the moist adiabaticrate, the descending saturated air soon reaches a levelwhere it is colder than its environment; consequently,its rate of downward motion is accelerated, resulting ina downdraft. (See fig. 5-16.)

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AG5f0515

40,000

35,000

30,000

25,000

20,000

15,000

10,000

5,000

FREEZINGLEVEL

45,000

RAIN

SNOW

ICE CRYSTAL

CUMULUSSTAGE

MATURESTAGE

ANVIL ORDISSIPATING STAGE

FIRST RAIN AT SURFACERAIN DECREASING

AT SURFACE

0 C 0 C 0 C

Figure 5-15.—Life cycle of a thunderstorm cell.

AG5f0516 WIDTH UP TO ABOUT SIX MILES

COLD AIR SINKING DIRECTION

OF STORM

OUTWARD FLOW OFCOLD AIR THAT

PRECEDES THE RAIN

5000 FT.

SURFACE

Figure 5-16.—Downdraft beneath a thunderstorm cell in the mature stage. Arrows represent wind flow. Dashed lines indicaterainfall.

A short time after the rain starts its initial fall, theupdraft reaches its maximum speed. The speed of theupdraft increases with altitude. Downdrafts are usuallystrongest at the middle and lower levels, although thevariation in speed from one altitude to another is lessthan in the case of updrafts. Downdrafts are not asstrong as updrafts; downdraft speeds range from a fewfeet per second to 40 feet per second or more.Significant downdrafts seldom extend to the top of thecell because, in most cases, only ice crystals andsnowflakes are present, and their rate of fall isinsufficient to cause appreciable downdrafts.

The mature cell, then, generally extends far above25,000 feet, and the lower levels consist of sharpupdrafts and downdrafts adjacent to each other. Largewater droplets are encountered suspended in theupdrafts and descending with the downdrafts as rain.

DISSIPATING (ANVIL) STAGE.—Throughoutthe life span of the mature cell, the falling raindrops aredragging down more and more air aloft. Consequently,the downdraft spreads out to take the place of thedissipating updraft. As this process progresses, theentire lower portion of the cell becomes an area ofdowndraft. Since this is an unbalanced situation andsince the descending motion in the downdraft effects adrying process, the entire structure begins to dissipate.The high winds aloft have now carried the upper sectionof the cloud into the anvil form, indicating that gradualdissipation is overtaking the storm cell.

Vertical Development

Thunderstorms have been accurately measured ashigh as 67,000 feet and some severe thunderstormsattain an even greater height. More often the maximumheight is from 40,000 to 45,000 feet. In general,air-mass thunderstorms extend to greater heights thando frontal storms.

Rising and descending drafts of air are, in effect,the structural bases of the thunderstorm cell. A draft is alarge-scale vertical current of air that is continuous overmany thousands of feet of altitude. Downdraft speedsare either relatively constant or gradually varying fromone altitude to the next. Gusts, on the other hand, aresmaller scaled discontinuities associated with the draftproper. A draft may be compared to a great riverflowing at a fairly constant rate, whereas a gust iscomparable to an eddy or any other random motion ofwater within the main current.

Thunderstorm Weather

The hydrometeors and turbulence of athunderstorm that we observe and record at the surfaceare easily recognized. The weather within thethundercloud itself is another story. Visual observationsfrom aircraft are difficult because of the speed withwhich they pass through the thunderclouds, and manhas yet to devise an instrument that will measure allhydrometeors in the cloud. Let us look at those forms ofprecipitation turbulence and icing occurring with andwithin thunderclouds as we know them today.

RAIN.—Liquid water in a storm may be ascendingif encountered in a strong updraft; it may be suspended,seemingly without motion, yet in an extremely heavyconcentration; or it may be falling to the ground. Rain,as normally measured by surface instruments, isassociated with the downdraft. This does not precludethe possibility of a pilot entering a cloud and beingswamped, so to speak, even though rain has not beenobserved from surface positions. Rain is found inalmost every case below the freezing level. In instancesin which no rain is encountered, the storm probably hasnot developed into the mature stage. Statistics showthat although heavy rain is generally reported at alllevels of a mature storm, the greatest incidence of heavyrain occurs in the middle and lower levels of a storm.

HAIL.—Hail, if present, is most often found in themature stage. Very seldom is it found at more than oneor two levels within the same storm. When it isobserved, its duration is short. The maximumoccurrence is at middle levels for all intensities of hail.

SNOW.—The maximum frequency of moderateand heavy snow occurs several thousand feet above thefreezing level. Snow, mixed, in many cases, withsupercooled rain, may be encountered in updraft areasat all altitudes above the freezing level. This presents aunique icing problem: wet snow packed on the leadingedge of the wing of the aircraft resulting in theformation of rime ice.

TURBULENCE.—There is a definite correlationbetween turbulence and precipitation. The intensity ofassociated turbulence, in most cases, varies directlywith the intensity of the precipitation.

ICING.—Icing may be encountered at any levelwhere the temperature is below freezing. Both rime andclear ice occur, with rime predominating in the regionsof snow and mixed rain and snow. Since the freezinglevel is also the zone of greatest frequency of heavy

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turbulence and generally heavy rainfall, this particularaltitude appears to be the most hazardous for aircraft.

SURFACE WIND.—A significant hazardassociated with thunderstorm activity is the rapidchange in surface wind direction and speedimmediately before storm passage. The strong winds atthe surface accompanying thunderstorm passage arethe result of the horizontal spreading out of downdraftcurrent from within the storm as they approach thesurface of Earth.

The total wind speed is a result of the downdraftdivergence plus the forward velocity of the storm cell.Thus, the speeds at the leading edge, as the stormapproaches, are greater than those at the trailing edge.The initial wind surge, as observed at the surface, isknown as the first gust.

The speed of the first gust is normally the highestrecorded during storm passage, and the direction mayvary as much as 180° from the previously prevailingsurface wind. First-gust speeds increase to an averageof about 16 knots over prevailing speeds, althoughgusts of over 78 knots (90 mph) have been recorded.The average change of wind direction associated withthe first gust is about 40°.

In addition to the first gust, other strong, violent,and extremely dangerous downdraft winds areassociated with the thunderstorm. These winds arereferred to as downbursts. Downbursts are subdividedinto macrobursts and microbursts.

Macrobursts.—Macrobursts are larger scaledownbursts. Macrobursts can cause widespreaddamage similar to tornadoes. These damaging windscan last 5 to 20 minutes and reach speeds of 130 knots(150 mph) or more.

Microbursts.—Microbursts are smaller scaledownbursts. A microburst can last 2 to 5 minutes andcan also reach wind speeds in excess of 130 knots.Microbursts produce dangerous tailwinds orcrosswinds and wind shear for aircraft and are difficultto observe or forecast.

Downbursts are not the same as first gusts. Firstgusts occur in all convective cells containing showersand are predictable and expected. Downbursts,however, do not occur in all convective cells andthunderstorms.

Classifications

All thunderstorms are similar in physical makeup,but for purposes of identification, they may be dividedinto two general groups, frontal thunderstorms andair-mass thunderstorms.

FRONTAL.—Frontal thunderstorms arecommonly associated with both warm and cold fronts.The warm-front thunderstorm is caused when warm,moist, unstable air is forced aloft over a colder, densershelf of retreating air. Warm-front thunderstorms aregenerally scattered; they are usually difficult to identifybecause they are obscured by other clouds.

The cold-front thunderstorm is caused by theforward motion of a wedge of cold air, into a body ofwarm, moist unstable air. Cold-front storms arenormally positioned aloft along the frontal surface inwhat appears to be a continuous line.

Under special atmospheric conditions, a line ofthunderstorms develops ahead of a cold front. This lineof thunderstorms is the prefrontal squall line. Itsdistance ahead of the front ranges from 50 to 300 miles.Prefrontal thunderstorms are usually intense andappear menacing. Bases of the clouds are very low.Tornadoes sometimes occur when this type of activityis present.

AIR MASS.—Air-mass thunderstorms aresubdivided into several types. In this text, however,only two basic types are discussed, the convectivethunderstorm and the Orographic thunderstorm.

Convective.—Convective thunderstorms mayoccur over land or water almost anywhere in the world.Their formation is caused by solar heating of variousareas of the land or sea, which, in turn, provides heat tothe air in transit. The land type of convectivethunderstorm normally forms during the afternoonhours after Earth has gained maximum heating from theSun. If the circulation is such that cool, moist,convective, unstable air is passing over the land area,heating from below causes convective currents andresults in towering cumulus or thunderstorm activity.Dissipation usually begins during the early eveninghours. Storms that occur over bodies of water form inthe same manner, but at different hours. Sea stormsusually form during the evening after the Sun has setand dissipate during the late morning.

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Both types of convective thunderstorms occur inFlorida. The anticyclonic circulation around theBermuda high advects moist air over the land surface ofFlorida in its easterly flow. Thunderstorms off the EastCoast of Florida at night occur when this easterly flowpasses over the warm axis of the Florida current. Inthose areas where the air is cooler than the water belowit, the air is heated and convective currents (lifting)begin. Any nocturnal cooling of the easterly flow aloftaids in establishing the unstable lapse rate necessary forthunderstorm development. After sunrise, the air isheated and becomes warmer than the water, therebydestroying the balance necessary to sustain or buildsimilar storms. As the day progresses, the land surfacebecomes considerably warmer than the air. Convectivecurrents again result, and Florida’s common afternoonthunderstorms are observed. After sunset the landcools, convective currents cease, and the thunderstormsdissipate. The apparent movement of the storms to seaat night, and to shore during the day, is in reality thereformation of storms in their respective areas. As ageneral rule, convective thunderstorms are scatteredand easily recognized. They build to great heights, andvisibility is generally excellent in the surrounding area.

Orographic.—Orographic thunderstorms form inmountainous regions, particularly adjacent toindividual peaks. A good example of this type of stormoccurs in the northern Rocky Mountain region. When

the circulation of the air is from the west, moist air fromthe Pacific Ocean is transported to the mountains whereit is forced aloft by the upslope of the terrain. If the air isconditionally unstable, this upslope motion causesthunderstorm activity on the windward side of themountains. This activity may form a long, unbrokenline of storms similar to a cold front. The storms persistas long as the circulation causes an upslope motion.They tend to be more frequent during afternoon andearly evening when convective lifting coincides withthe mechanical lifting of the terrain.

LIGHTNING

Lightning is obviously the most spectacular ofelectrometeors and is directly related to thethunderstorm even though classified independently. Itis the bright flash of light accompanying a suddenelectrical discharge. Most lightning has its beginning inclouds; however, it generates from high structures onthe ground and mountains, although much lessfrequently.

The thunderstorm changes the normal electricfield, in which the ground is negatively charged withrespect to the air above it. Because the upper portion ofthe thunderstorm cloud is positive and the lower part isnegative, the negative charge induces a positive chargeon the ground. The distribution of the electric chargesin a typical thunderstorm is shown in figure 5-17. The

5-22

AG5f0517

DIRECTION

OF MOVEMENT

15 F

32 F

SMALL CENTER OFPOSITIVE CHARGE

AREA OFNEGATIVE RAIN

POSITIVECHARGECENTER

NEGATIVECHARGECENTER

SURFACE

Figure 5-17.—Location of electric charges inside a typical thunderstorm cell.

lightning first occurs between the upper positive chargearea and the negative charge area immediately below it.Lightning discharges are considered to occur mostfrequently in the area bracketed roughly by the 32°Fand the 15°F temperature levels. However, this does notmean that all discharges are confined to this region; asthe thunderstorm develops, lightning discharges mayoccur in other areas and from cloud to cloud, as well asfrom cloud to ground.

There are four main types of lightning. All can doconsiderable damage to aircraft, especially to radioequipment.

1. Cloud To Ground Lightning (CG). Lightningoccurring between cloud and ground.

2. Cloud Discharges (IC). Lightning taking placewithin the cloud.

3. Cloud To Cloud Discharges (CC). Streaks oflightning reaching from one cloud to another.

4. Air Discharges (CA). Streaks of lightningpassing from a cloud to the air that do not strike theground.

AURORAS

Auroras are luminous phenomena, which appear inthe high atmosphere in the form of arcs, bands,draperies, or curtains. These phenomena are usuallywhite but may have other colors. The lower edges of the

arcs or curtains are usually well defined while the upperedges are not. Polar auroras are caused by electricallycharged particles, ejected from the Sun, which act onthe rarefied (select) gases of the higher atmosphere.The particles are channeled by Earth’s magnetic field,so auroras are observed mainly near the magnetic poles.In the Northern Hemisphere they are known as auroraborealis; in the Southern Hemisphere they are known asaurora australis.

AIRGLOW

Airglow is similar in origin and nature to theaurora; it, too, is an upper atmospheric electricalphenomenon. The main differences between airglowand aurora are that airglow is quasi-steady (quasi meansseemingly) in appearance, is much fainter than aurora,and appears in the middle and lower altitudes.

REVIEW QUESTIONS

Q5-12. What is the diameter range of a maturethunderstorm cell?

Q5-13. During what stage of a thunderstorm is rainobserved at the surface?

Q5-14. What is the difference between a macroburstand a microburst?

Q5-15. Describe the two different types ofthunderstorms?

5-23

CHAPTER 6

CLIMATOLOGY AND WORLD WEATHER

One of the major tasks of the Aerographer’s Mateand the Naval Meteorology and OceanographyCommand is providing long-range weather informationand predictions based on recognized meteorologicaloccurrences in a particular area or region of the world.Naval exercises both at sea and ashore is plannedmonths and sometimes years in advance. To carry outthese exercises successfully, we must have an idea ofthe normal weather conditions for the operational area(OPAREA) at that time of year. It is both dangerous andunwise to conduct costly training exercises if theweather conditions for the OPAREA are known to beadverse at that time of year.

During wartime, an extensive knowledge ofweather conditions can be a decisive advantage. Navaland land forces can use their knowledge of weather tosurprise the enemy and predict when the enemy willstrike. Historically, man wages war when the weatherpermits. When Napoleon invaded Russia, his defeatwas not due to the wisdom of his opponents, but ratherto his lack of knowledge of the severe Russian winters.He was beaten by the weather.

As you gain more experience, your job will includethe preparation of long-range weather forecasts basedon climatological studies. You must prepare charts,tables and/or graphs that include sky cover,temperatures, winds, sea conditions, etc. Thisclimatological information is needed for long-rangenaval exercises, ship deployments overseas, and actualcombat operations.

CLIMATE AND CLIMATOLOGY

LEARNING OBJECTIVE: Define climate,climatology, and related terminology.

Before starting any discussion about climate andclimatology, we must become familiar with these andother related terms. In this lesson, we define climate,various types of climatology, and climatology as itrelates to other sciences such as ecology.

CLIMATE

Climate is the average or collective state of Earth’satmosphere at any given location or area over a long

period of time. While weather is the sum total of theatmosphere’s variables for a relatively short period oftime, the climate of an area is determined over periodsof many years and represents the general weathercharacteristics of an area or locality. The term climateapplies to specific regions and is therefore highlygeographical.

CLIMATOLOGY

Climatology is the scientific study of climate and isa major branch of meteorology. Climatology is the toolthat is used to develop long-range forecasts. There arethree principal approaches to the study of climatology:physical, descriptive, and dynamic.

Physical Climatology

The physical climatology approach seeks toexplain the differences in climate in light of thephysical processes influencing climate and theprocesses producing the various kinds of physicalclimates, such as marine, desert, and mountain.Physical climatology deals with explanations ofclimate rather than with presentations.

Descriptive Climatology

Descriptive climatology typically orients itself interms of geographic regions; it is often referred to asregional climatology. A description of the various typesof climates is made on the basis of analyzed statisticsfrom a particular area. A further attempt is made todescribe the interaction of weather and climaticelements upon the people and the areas underconsideration. Descriptive climatology is presented byverbal and graphic description without going intocauses and theory.

Dynamic Climatology

Dynamic climatology attempts to relatecharacteristics of the general circulation of the entireatmosphere to the climate. Dynamic climatology isused by the theoretical meteorologist and addressesdynamic and thermodynamic effects.

6-1

Climatology as Related to Other Sciences

Three prefixes can be added to the wordclimatology to denote scale or magnitude. They aremicro, meso, and macro and indicate small, medium,and large scales, respectively. These terms (micro,meso, and macro) are also applied to meteorology.

MICROCLIMATOLOGY.—Microclimatological studies often measure small-scale contrasts, such asbetween hilltop and valley or between city andsurrounding country. They may be of an extremelysmall scale, such as one side of a hedge contrasted withthe other, a plowed furrow versus level soil, or oppositeleaf surfaces. Climate in the microscale may beeffectively modified by relatively simple human efforts.

MESOCLIMATOLOGY.—Mesoclimatologyembraces a rather indistinct middle ground betweenmacroclimatology and microclimatology. The areas aresmaller than those of macroclimatology are and largerthan those of microclimatology, and they may or maynot be climatically representative of a general region.

MACROCLIMATOLOGY.—Macroclimatologyis the study of the large-scale climate of a large area orcountry. Climate of this type is not easily modified byhuman efforts. However, continued pollution of theEarth, its streams, rivers, and atmosphere, caneventually make these modifications.

Climate has become increasingly important inother scientific fields. Geographers, hydrologists, andoceanographers use quantitative measures of climate todescribe or analyze the influence of our atmosphericenvironment. Climate classification has developedprimarily in the field of geography. The basic role of theatmosphere in the hydrologic cycle is an essential partof the study of hydrology. Both air and watermeasurements are required to understand the energyexchange between air and ocean (heat budget) asexamined in the study of oceanography.

ECOLOGY

Ecology is the study of the mutual relationshipbetween organisms and their environment. Ecology isbriefly mentioned here because the environment ofliving organisms is directly affected by weather andclimate, including those changes in climate that aregradually being made by man.

During our growing years as a nation, ourinterference with nature by diverting and damming

rivers, clearing its lands, stripping its soils, and scarringits landscape has produced changes in climate. Thesechanges have been on the micro and meso scale andpossibly even on the macro scale.

REVIEW QUESTIONS

Q6-1. What is the definition of climate?

Q6-2. What type of climatology is typically orientedto a geographic region?

Q6-3. What type of climatology applies to a smallarea such as a golf course or a plowed field?

CLIMATIC ELEMENTS

LEARNING OBJECTIVE: Describe theclimatic elements of temperature,precipitation, and wind.

The weather elements that are used to describeclimate are also the elements that determine the type ofclimate for a region. This lesson presents a briefexplanation of the importance of these elements. Theclimatic elements of temperature, precipitation, andwind are not the only parameters included in aclimatology package; however, they are the mostsignificant elements used to express the climate of aregion.

TEMPERATURE

Temperature is undoubtedly the most importantclimatic element. The temperature of an area isdependent upon latitude or the distribution of incomingand outgoing radiation; the nature of the surface (landor water); the altitude; and the prevailing winds. The airtemperature normally used in climatology is thatrecorded at the surface.

Moisture, or the lack of moisture, modifiestemperature. The more moisture in a region, the smallerthe temperature range, and the drier the region, thegreater the temperature range. Moisture is alsoinfluenced by temperature. Warmer air can hold moremoisture than can cooler air, resulting in increasedevaporation and a higher probability of clouds andprecipitation.

Moisture, when coupled with condensation andevaporation, is an extremely important climaticelement. It ultimately determines the type of climate fora specific region.

6-2

PRECIPITATION

Precipitation is the second most important climaticelement. In most studies, precipitation is defined aswater reaching Earth’s surface by falling either in aliquid or a solid state. The most significant forms arerain and snow. Precipitation has a wide range ofvariability over the Earth’s surface. Because of thisvariability, a longer series of observations is generallyrequired to establish a mean or an average. Two stationsmay have the same amount of annual precipitation, butit could occur in different months or on different daysduring these months, or the intensity could vary.Therefore, it often becomes necessary to include suchfactors as average number of days with precipitationand average amount per day. Precipitation is expressedin most studies in the United States in inches, butthroughout the rest of the world, millimeters arenormally used.

Since precipitation amounts are directly associatedwith amount and type of clouds, cloud cover must alsobe considered with precipitation. Cloud climatologyalso includes such phenomena as fog andthunderstorms.

WIND

Wind is the climatic element that transports heatand moisture into a region. The climate of an area isoften determined by the properties of temperature andmoisture that are found upstream of that region.

Climatologists are mostly interested in wind withregard to its direction, speed, and gustiness. Wind istherefore usually discussed in terms of prevailingdirection, average speeds, and maximum gusts. Someclimatological studies use resultant wind, which is thevectorial average of all wind directions and speeds for agiven level, at a specific place, and for a given period.

REVIEW QUESTIONS

Q6-4. What is the most important climatic element?

Q6-5. Which climatic element transports heat andmoisture into a region?

EXPRESSION OF CLIMATICELEMENTS

LEARNING OBJECTIVE: Define the termsused to express climatic elements and themethods used to derive these terms.

Climatic elements are observed over long periodsof time; therefore, specific terms must be used toexpress these elements so they have definite meaning.This lesson defines the most commonly used terms anddiscusses how they are used to express climaticelements.

MEAN (AVERAGE)

The mean is the most commonly usedclimatological parameter. The term mean normallyrefers to a mathematical averaging obtained by addingthe values of all factors or cases and then dividing bythe number of items. For example, the average dailytemperature would be the sum of the hourlytemperatures divided by 24.

Other methods are used for computing variousmeteorological elements. For example, the meantemperature for 1 day has been devised by simplyadding the maximum and minimum values for that dayand dividing by 2. Assume the maximum temperaturefor a certain day is 75°F and the minimum temperatureis 57°F. The mean temperature for the day is 66°F.

Unfortunately, the term mean has been used inmany climatological records without clarification as tohow it was computed. In most cases, the difference inresults obtained is slight. In analyzing weather data, theterms average and mean are often usedinterchangeably.

NORMAL

In climatology, the term normal is applied to theaverage value over a period of time, which serves as astandard with which values (occurring on a date orduring a specified time) may be compared. Theseperiods of time may be a particular month or otherportion of the year. They may refer to a season or to ayear as a whole. The normal is usually determined overa 20- or 30-year period.

For example, if the average temperature for yourstation on 10 June has been 80°F over a specified periodof time, the normal temperature for your station on 10June is 80°F. If the temperature on 10 June this year wasonly 76°F, then the temperature for that day is 4°Fbelow normal.

ABSOLUTE

In climatology, the term absolute is usually appliedto the extreme highest and lowest values for any givenmeteorological element recorded at the place of

6-3

observation and are most frequently applied totemperature. Assume, for example, that the extremehighest temperature ever recorded at a particular stationwas 106°F and the lowest recorded was -15°F. Thesevalues are called the absolute maximum and absoluteminimum, respectively.

EXTREME

The term extreme is applied to the highest andlowest values for a particular meteorological elementoccurring over a period of time. This period of time isusually a matter of months, seasons, or years. The termmay be used for a calendar day only, for which it isparticularly applicable to temperature. For example, thehighest and lowest temperature readings for a particularday are considered the temperature extremes for thatday. At times the term is applied to the average of thehighest and lowest temperatures as mean monthly ormean annual extremes.

RANGE

Range is the difference between the highest andlowest values and reflects the extreme variations ofthese values. This statistic is not recommended forprecise work, since it has a high variability. Range isrelated to the extreme values of record and can be usefulin determining the extreme range for the recordsavailable. For example, if the highest temperaturerecorded yesterday was 76°F and the lowest was 41°F,then the range for the day was 35°F.

FREQUENCY

Frequency is defined as the number of times acertain value occurs within a specified period of time.When a large number of various values need to bepresented, a condensed presentation of data may beobtained by means of a frequency distribution.

MODE

Mode is defined as the value occurring with thegreatest frequency or the value about which the mostcases occur.

MEDIAN

The median is the value at the midpoint in an array.In determining the median, all values are arranged inorder of size. Rough estimates of the median may beobtained by taking the middle value of an ordered

series; or, if there are two middle values, they may beaveraged to obtain the median. The position of themedian may be found by the following formula:

Median =n +1

2

where n is the number of items.

The median is not widely used in climatologicalcomputations. However, some sources recommend theuse of the median instead of the mean or average forsome climatic elements to present more representativepictures of distribution and probability. A longer periodof record might be required to formulate an accuratemedian.

DEGREE-DAY

A degree-day is the number of degrees the meandaily temperature is above or below a standardtemperature base. The base temperature is usually65°F; however, any temperature, Celsius or Fahrenheit,can be used as a base. There is one degree-day for eachdegree (°C or °F) of departure above or below thestandard.

Degree-days are accumulated over a season. At anypoint in the season, the total can be used as an index ofpast temperature effect upon some quantity, such asplant growth, fuel consumption, power output, etc. Thisconcept was first used in connection with plant growth,which showed a relationship to cumulative temperatureabove a standard of 41°F. Degree-days are frequentlyapplied to fuel and power consumption in the form ofheating degree-days and cooling degree-days.

AVERAGE AND STANDARD DEVIATIONS

In the analysis of climatological data, it may bedesirable to compute the deviation of all items from acentral point. This can be obtained from a computationof either the mean (or average) deviation or the standarddeviation. These are termed measures of dispersion andare used to determine whether the average is trulyrepresentative or to determine the extent to which datavary from the average.

Average Deviation

Average deviation is obtained by computing thearithmetic average of the deviations from an average ofthe data. First we obtain an average of the data, then thedeviations of the individual items from this average aredetermined, and finally the arithmetic average of these

6-4

deviations is computed. The plus and minus signs aredisregarded. The formula for computation of theaverage deviation is as follows:

Average deviation =Σd

n

where the Greek letter Σ (sigma) means the sum of d(the deviations) and n is the number of items.

Standard Deviation

The standard deviation, like the average deviation,is the measure of the scatter or spread of all values in aseries of observations. To obtain the standard deviation,square each deviation from the arithmetic average ofthe data. Then, determine the arithmetic average of thesquared deviations. Finally, derive the square root ofthis average. This is also called the root-mean-squaredeviation, since it is the square root of the mean of thedeviations squared.

The formula for computing standard deviation isgiven as follows:

Standard deviation =Σd

n

2

where d2 is the sum of the squared deviations from thearithmetic average, and n is the number of items in thegroup of data.

An example of the computations of averagedeviation and standard deviation is given in table 6-1and in the following paragraphs.

Table 6-1.—Computation of Average and Standard Deviation

Januaryyear

Meantemperature

Deviationsfrom mean

Deviationssquared

1978 47 – 4 16

1979 51 + 0 0

1980 53 + 2 4

1981 50 – 1 1

1982 49 – 2 4

1983 55 + 4 16

1984 46 – 5 25

1985 52 + 1 1

1986 57 + 6 36

1987 50 – 1 1

TotalsMean

51051

262.6

1043.2

Suppose, on the basis of 10 years of data(1978-1987), you want to compute the averagedeviation of mean temperature and the standarddeviation for the month of January. First, arrange thedata in tabular form (as in table 6-1). Given the year inthe first column, the mean monthly temperature in thesecond column, the deviations from an arithmeticaverage of the mean temperature in the third column,and the deviations from the mean squared in the fourthcolumn.

To compute the average deviation:

1. Add all the temperatures in column 2 anddivide by the number of years (10 in this case) to get thearithmetic average of temperature.

2. In column 3, compute the deviation from themean or average determined in step 1. (The meantemperature for the 10-year period was 51°F.)

3. Total column 3, disregarding the negative andpositive signs. (Total is 26.)

4. Apply the formula for average deviation:

Σd

n

26

102.6 F= = °

The average deviation of temperature during themonth of January for the period of record, 10 years, is2.6°F.

To compute the standard deviation:

1. Square the deviations from the mean (column3).

2. Total these squared deviations. In this case, thetotal is 104.

3. Apply the formula for standard deviation:

Standard deviation =Σd

n

2

=104

10

10 4. = 3.225 or 3.2°F

The standard deviation of temperature for themonth and period in question is 3.2°F (rounded off tothe nearest one-tenth of a degree).

From the standard deviation just determined, it isapparent that there is a small range of mean temperatureduring January. If we had a frequency distribution oftemperature available for this station for each day of themonth, we could readily determine the percentage ofreadings which would fall in the 6.4-degree spread (3.2either side of the mean). From these data we could thenformulate a probability forecast or the number of days

6-5

within this range on which we could expect the normalor mean temperature to occur. This study could bebroken down further into hours of the day, etc., asrequired.

REVIEW QUESTIONS

Q6-6. If one adds all the daily high temperatures forthe week and divides by 7, whatclimatological parameter would bedetermined by this calculation?

Q6-7. A temperature of 124 degrees Fahrenheit wasthe highest temperature ever recorded at aparticular station. What type ofclimatological parameter was determined?

Q6-8. What is a degree-day?

CLASSIFICATION OF CLIMATE

LEARNING OBJECTIVE: Recognizeclimatic zones and climatic types as they relateto the classification of climate.

The climate of a given region or locality isdetermined by a combination of several meteorologicalelements and not by just one element. For example, tworegions may have similar temperature climates but verydifferent precipitation climates. Their climaticdifference, therefore, becomes apparent only if morethan one climatic factor is considered.

Since the climate of a region is composed of all ofthe various climatic elements, such as dew, ice, rain,temperature, wind force, and wind direction, it isobvious that no two locations can have exactly the sameclimate. However, it is possible to group similar areasinto what is known as a climatic zone.

CLIMATIC ZONES

The basic grouping of areas into climatic zonesconsists of classifying climates into five broad beltsbased on astronomical or mathematical factors.Actually they are zones of sunshine or solar climate andinclude the torrid or tropical zone, the two temperatezones, and the two polar zones. The tropical zone islimited on the north by the Tropic of Cancer and on thesouth by the Tropic of Capricorn, which are located at23 1/2° north and south latitude, respectively. TheTemperate Zone of the Northern Hemisphere is limited

on the south by the Tropic of Cancer and on the north bythe Arctic Circle located at 66 1/2° north latitude. TheTemperate Zone of the Southern Hemisphere isbounded on the north by the Tropic of Capricorn and onthe south by the Antarctic Circle located at 66 1/2°south latitude. The two polar zones are the areas in thePolar Regions which have the Arctic and AntarcticCircles as their boundaries.

Technically, climatic zones are limited byisotherms rather than by parallels of latitude (fig. 6-1).A glance at any chart depicting the isotherms over thesurface of the earth shows that the isotherms do notcoincide with latitude lines. In fact, at some places theisotherms parallel the longitude lines more closely thanthey parallel the latitude lines. The astronomical orlight zones therefore differ from the zones of heat.

CLIMATIC TYPES

Any classification of climate depends to a largeextent on the purpose of the classification. For instance,a classification for the purpose of establishing airstations where favorable flying conditions areimportant would differ considerably from one forestablishing the limits of areas that are favorable for thegrowing of crops. There are three classifications thatmerit particular attention. They are the classificationsof C. W. Thornthwaite, W. Köppen, and G. T.Trewartha.

Thornthwaite’s classification of climates places agreat deal of emphasis on the effectiveness ofprecipitation. Effectiveness of precipitation refers tothe relationship between precipitation and evaporationat a certain locality. Thornthwaite classified climatesinto eight main climatic groups; five groups giveprimary emphasis to precipitation and the other threegroups are based on temperature.

Köppen’s classification includes five main climatictypes. They are tropical rain, dry, warm temperaterainy, cool snow forest (boreal), and polar climates.These main types are further divided into climaticprovinces. The Köppen classification is based mainlyon temperature, precipitation amount, and season ofmaximum precipitation. Numerical values for theseelements constitute the boundaries of the above types,which were selected primarily according to their effecton plant growth. Figure 6-2, shows Köppen’s climatictypes.

6-6

Köppen’s climatic types are still considered validtoday. His climatic zones, like others, are by no meansstatic. Climatic zones shift with long-range weatherpatterns. The most noticeable shifts in these climaticzones have been observed over the northern portions ofNorth America and Asia and over Africa. Russia andCanada, for example, have been able to conductfarming at higher latitudes over the past 200 years dueto milder temperatures. Recent studies, however,indicate a general return of cooler temperatures at highlatitudes, and now the growing region is graduallymoving southward again where temperatures are moremoderate. In Africa, desert regions have made notableshifts southward due to decreasing precipitation.

Trewartha is the most recent classifier of climate.Initially, his climatic classifications were based onKöppen’s; however, over the years, he has madesignificant changes and is now recognized fordeveloping his own six climatic groups. These sixgroups are tropical, dry, subtropical, temperate, boreal,and polar. Five of these groups are based ontemperature and one is based on precipitation (see

Table 6-2). Trewartha’s climatic groups, like Köppen’s,are also further broken down into climatic types andsubtypes.

REVIEW QUESTIONS

Q6-9. List the five climatic belts and theirboundaries.

Q6-10. Name the three classifications of climatictypes.

Q6-11. What are the five climatic types according toKöppen?

CLIMATIC CONTROLS

LEARNING OBJECTIVE: Identify thecontrolling factors that affect climate.

The variation of climatic elements from place toplace and from season to season is due to several factorscalled climatic controls. The same basic factors thatcause weather in the atmosphere also determine the

6-7

NORTH POLAR ZONE

NORTH TEMPERATE ZONE

SOUTH TEMPERATE ZONE

HOT BELT

SOUTH POLAR ZONE

60

40

20

0

20

40

180 160 140 120 100 80 60 40 20 0 20 40 60 80 100 120

50O

F ISOTHERM(WARMEST MONTH)

OMEAN ANNUALISOTHERM OF 68 F

MEAN ANNUALISOTHERM OF 68 F

O

50 F ISOTHERM(WARMEST MONTH)

O

AGf06001

Figure 6-1.—Temperature zones.

6-8

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CLIMATIC TYPES OF THE EARTH (after Koppen)..

LEGEND TO CLIMATIC TYPES

A. TROPICALRAINY CLIMATES

AfAmAw

B. DRY CLIMATES

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C. WARMTEMPERATERAINYCLIMATES

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TROPICAL RAIN FOREST CLIMATETROPICAL MONSOON CLIMATETROPICAL SAVANNA CLIMATE

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Dfa, DwaDfb, DwbDfc, DwcDfd, Dwdwf

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E. POLARCLIMATES

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AG5f 0602

Figure 6-2.—Köppen’s climatic types.

Basis for Classification Climate Group Poleward Boundary

Temperature A. Tropical Frost line over continents and 65°F (18°C) over oceans (coolestmonths)

Precipitation B. Dry Bounded by the outer limits where potential evaporation is equal toprecipitation

Temperature C. Subtropical 50°F (10°C) or above for 8 months of the year

Temperature D. Temperature 50°F (10°C) or above for 4 months of the year

Temperature E. Boreal 50°F (10°C) or above for 1 month (warmest month)

Temperature F. Polar Below 50°F (10°C) entire year

Table 6-2.—Trewartha's Climatic Groups and their Poleward Boundaries

climate of an area. These controls, acting in differentcombinations and with varying intensities, act upontemperature, precipitation, humidity, air pressure, andwind to produce many types of weather and thereforeclimate.

Four climatic controls largely determine theclimate of every ocean and continental region. Thesecontrols are latitude, land and water distribution,topography, and ocean currents. Another factor, whichis now significant in determining a region's climate, isman. Man’s influence on climate through pollution,deforestation, and irrigation, is now considered aclimatic factor.

LATITUDE

Perhaps no other climatic control has such amarked effect on climatic elements as does the latitude,or the position of Earth relative to the Sun. The angle at

which rays of sunlight reach Earth and the number ofSun hours each day depends upon the distance of theSun from the equator. (See fig. 6-3.) Therefore, thelatitude directly influences the extent to which an airmass is heated. Latitude influences the sources anddirection of air masses and the weather they bring withthem into a region.

Comparing an equatorial area to a polar area canshow the importance of latitude as a climatic control. Inthe former, the Sun is close to being directly overheadduring the day throughout the year. Therefore, there islittle difference between mean temperatures for thecoldest and warmest months. In the polar area,however, the Sun never rises far above the horizon; thatis, the angle of the Sun to Earth’s surface is alwaysacute. The radiant energy received per unit area istherefore slight, and the warming effects of the Sun arerelatively weak.

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6 MONTHS NIGHTAT N. POLE TANGENT

SUN RAY SUN RAY

SUN RAY SUN RAY

SUN RAY

SUN RAY

SUN RAY

SUN RAY

SUN RAY

TANGENT

TANGENT TANGENT

TANGENT

TANGENT

PERPENDICULAR PERPENDICULAR

PERPENDICULAR

TROPICOF CANCER

EQUATOREQUATOR TROPIC

OF CANCER

TROPICOF CAPRICORN

TROPICOF CAPRICORN

ARCTICCIRCLE

ARCTICCIRCLE

ANTARCTICCIRCLE

ANTARCTICCIRCLE

12H

O H

10.3 H

10.3 H

13.7 H

24 H12H

13.7 H

24 H

O H

O H

NP

NP

SP

SP

12 H

S P

N P

ARCTIC CIRCLE

TROPIC OF CANCER

EQUATOR

TROPIC OF CAPRICORN

ANTARCTIC CIRCLE

12 H

12 H

12 H

12 H

12 H

6 MONTHSDAY AT N. POLE

6 MONTHSNIGHT AT S. POLE6 MONTHS

DAY AT S. POLE

22 DECEMBER - WINTER SOLSTICE 21 - JUNE - SUMMER SOLSTICE

22 SEPTEMBER - AUTUMN EQUINOX21 - SPRING EQUINOX

POSITION OF PERPENDICULAR ANDTANGENT SUN RAYS DETERMINESTROPICS OF CANCER AND CAPRICORNAND ARCTIC AND ANTARCTIC CIRCLES

POSITION OF DAYLIGHT CIRCLE DETER-MINES LENGTH OF DAY AND NIGHT.

AG5f 0603

Figure 6-3.—Latitude differences in amount of insolation.

In chapter 3, the average world surfacetemperatures are represented on two world charts forJanuary and July in figures 3-2A and 3-2B. These aremean charts and are not meant to be an accurateportrayal of the temperatures on any one particular day.Note that in general the temperatures decrease from lowto high latitudes.

LAND AND WATER DISTRIBUTION

Land heats and cools about four times faster thanwater. Therefore, the location of continents and oceansgreatly influences Earth’s pattern of air temperature aswell as the sources and direction of movement of airmasses.

Influence on Air Temperature

Coastal areas assume the temperaturecharacteristics of the land or water that is on theirwindward side. In latitudes of prevailing westerlywinds, for example, west coasts of continents haveoceanic temperatures and east coasts have continentaltemperatures. These temperatures are determined bythe wind flow.

Since the upper layer of the ocean is nearly alwaysin a state of mixing, heat losses or heat gains occurringat the surface are distributed throughout a large volumeof water. This mixing process sharply reduces airtemperature contrasts between day and night andbetween winter and summer over oceanic areas.

Over land, there is almost no redistribution of heatby turbulence; also, the effect of conduction isnegligible. Thus strong seasonal and diurnal contrastsexist in the interiors of continents. During the winter, alarge part of the incident solar radiation is reflectedback toward space by the snow cover that extends overlarge portions of the northern continents. For thisreason, the northern continents serve as source regionsfor dry polar air.

The large temperature difference between the landand water surfaces, which reverses between the twoseasons, determines the seasonal weather patterns to agreat extent.

In chapter 3, figures 3-2A and 3-2B, the isothermsover the Northern Hemisphere are more closely spacedand parallel in winter than in summer. In the SouthernHemisphere, the temperature gradient does not have asgreat a seasonal change as it does in the NorthernHemisphere. These conditions are due to the unequaldistribution of land and water on the two hemispheres.

Since the Southern Hemisphere has less land and morewater surface than the Northern Hemisphere, thechange due to the greater water surface is less withconsequently more nearly uniform isotherms. Also, thecontinents of the Southern Hemisphere taper toward thepoles and do not extend as far poleward as do those inthe Northern Hemisphere.

The nature of the surface affects local heatdistribution. Color, texture, and vegetation influencethe rate of heating and cooling. Generally, a dry surfacewill heat and cool faster than a moist surface. Forinstance, plowed fields, sandy beaches, and pavedroads become hotter than surrounding meadows andwooded areas during the day. During the night,however, the situation is reversed.

The distribution of water vapor and clouds isanother important factor influencing air temperature.Although areas with a high percentage of cloud coverhave a high degree of reflectivity, the energy, which isnot reflected, is easily trapped in the lower layers due tothe greenhouse effect. Thus, areas of high moisturecontent have relatively high temperature.

Influence on Air Circulation

The higher mean temperature of the NorthernHemisphere is an effect not only of its higherpercentage of land, but also of the fact that its oceansare also warmer than those in the Southern Hemisphereare. This is partly due to the movement of warmequatorial waters from the Southern Hemisphere intothe Northern Hemisphere caused by the southeasttrades crossing the equator. Another factor conducive tohigher mean temperatures in the Northern Hemisphereis the partial protection of its oceans from cold polarwaters and arctic ice by land barriers. There is no suchbarrier between the Antarctic region and the southernoceans.

TOPOGRAPHY

Climates over land may vary radically within veryshort distances because of the elevation and variationsin landforms. Therefore, topography plays anextremely important role in determining the climate ofa region.

The height of an area above sea level exerts aconsiderable influence on its climate. For instance, theclimate at the equator in the high Andes of SouthAmerica is quite different from that found a few feetabove sea level at the same latitude. All climatic valuesare affected by surface elevation.

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An important influence on climate is mountainousterrain, especially the long, high chains of mountainsthat act as climatic divides. These obstacles deflect thetracks of cyclones and block the passage of air massesat the lower levels. If the pressure gradients are strongenough to force the air masses over the mountains, theforced ascent and descent modifies the air masses to agreat extent, thus modifying the climate on both thewindward and leeward sides of the range.

The orientation of the mountain range may blockcertain air masses and prevent them from reaching thelee side of the mountains. For example, the Himalayasand the Alps, which have east-west orientations,prevent polar air masses from advancing southward.Therefore, the climates of India and Italy are warmer inwinter than are other locations of the same latitude. Thecoastal ranges in North America, running in anorth-south line, prevent the passage of unmodifiedmaritime air masses to the lee side.

The most noted effect of mountains is thedistribution of precipitation. The precipitation values,level for level, are much higher on the windward sidethan on the leeward side.

In regions where the prevailing circulation flowsagainst a mountain barrier, the amount of precipitationincreases more or less uniformly with elevation on thewindward side of the range. This steady increasenormally occurs up to elevations of about 10,000 feet.However, in the trade wind zone (such as at theHawaiian Islands), precipitation increases only to about3,000 feet and then decreases gradually. Even with thisdecrease in amount, more rain is received at 6,000 feetthan at sea level.

Another important topographical feature is thepresence of lakes. The lake effect can be notable forlarge unfrozen bodies of water. The lee sides of lakesshow considerable diurnal and annual modification inthe form of more moderate temperatures; increasedmoisture, clouds, and precipitation; and increasedwinds (due to less friction) and land and sea breezeeffects.

OCEAN CURRENTS

Ocean currents play a significant role in controllingthe climate of certain regions. Ocean currents transportheat moving cold polar water equatorward into warmerwaters and moving warm equatorial water polewardinto cooler waters.

Currents are driven by the major wind systems;therefore, cold southward-moving currents flow alongthe west coasts of continents, and warm northwardmoving currents flow along the east coasts ofcontinents. This is true in both hemispheres. Basically,this results in cooler climates along the west coasts andwarmer climates along the east coasts.

A brief explanation of the effects of ocean currentsis presented here.

Effects on the West Coasts

The northern portions of the west coasts ofcontinents generally have cool summers and warmwinters. The summers are cool because of the presenceof cold northern waters along their shores. However,the winters are generally mild because of the transportof warm ocean waters to these latitudes. For example,the south and southeast coasts of Alaska and the westcoasts of Canada, Washington, and Oregon haverelatively warm currents flowing along their shores.These currents are the Aleutian and North Pacificcurrents, which are branches of the warmnorthward-flowing Kuroshio Current. The currentsflow along the West Side of the Pacific high and bringwarm water into southern Alaska and the PacificNorthwest.

As these currents merge and flow southward alongthe British Columbia coast, they move into warmerwaters and become the cold California Current.

The southern portions of the west coasts ofcontinents generally have cooler climates than do theeast coasts of the same latitude. For example, duringsummer, the cold California Current flows southwardalong the shores of California. Due to the Pacific high,the winds normally flow either across the cold currenttoward shore (onshore) or parallel to the coastline. Thisresults in cool air being advected inland allowing citiessuch as San Francisco and Seattle to enjoy relativelycool summers. Unfortunately, when the warm, moist airfrom the Pacific high does move over the underlyingcold current, extensive fog and stratus develop whichalso move inland. This situation is typical along thesouthern portions of the west coasts in bothhemispheres.

Another factor affecting west coasts is upwelling.Upwelling is the process by which cold subsurfacewaters are brought to the surface by wind. It occurs inareas where the wind causes the surface water to betransported away from the coast. The colder subsurfacewater then replaces the surface water. In the Northern

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Hemisphere, upwelling is common where the windblows parallel to the coast and the surface water istransported away from the coast. In the process ofupwelling, the exchange of water takes place only in theupper layers.

Generally, the following statements are trueregarding the effects of ocean currents along the westcoasts of continents:

• The west coasts of continents in middle andhigher latitudes are bordered by warm waters, whichcause a distinct maritime climate characterized by coolsummers and relatively mild winters with small annualrange of temperatures (upper west coasts of the UnitedStates and Europe).

• The west coasts of continents in tropical andsubtropical latitudes (except close to the equator) arebordered by cool waters and their average temperaturesare relatively low with small diurnal and annual ranges.There are fogs, but generally the areas (southernCalifornia, Morocco, etc.) are arid (dry).

Effects on the East Coasts

The effects of currents along the eastern coasts ofcontinents are less dramatic than those of the westcoasts because of the west-to-east flow of weather. Theeffects, however, are just as significant.

In the tropical and subtropical regions, warm oceancurrents introduce warm, rainy climates, especially onthe windward sides of mountainous landmasses. As thewarm currents progress northward into middlelatitudes, warm, moist air produces a hot, humidclimate with frequent rain showers during the summer.Winters are relatively moderate (but still cold) along thecoast due to the transport of warm water. The higherlatitudes along eastern shores normally have coldwaters flowing southward from the polar region; warmocean currents rarely extend very far north. The regionswhere the two currents meet have cool summers andcold winters with extensive fogs. This is especially truealong the Grand Banks of Newfoundland and theKamchatka Peninsula of eastern Asia.

The following general statements are trueregarding the effects of ocean currents along the easterncoasts of continents:

• The east coasts in the tropics and subtropicallatitudes are paralleled by warm currents and have

resultant warm and rainy climates. These areas lie in thewestern margins of the subtropical anticyclone regions(Florida, Philippines, and Southeast Asia).

• The east coasts in the lower middle latitudes(leeward sides of landmasses) have adjacent warmwaters with a modified continental-type climate. Thewinters are fairly cold, and the summers are warm andhumid.

• The east coasts in the higher middle latitudestypically experience cool summers with cool oceancurrents paralleling the coasts.

Other Effects

Ocean currents also affect the location of primaryfrontal zones and the tracks of cyclonic storms. Off theeastern coast of the United States in the winter, two ofthe major frontal zones are located in areas where thetemperature gradient is strong and where a largeamount of warm water is being transported into themiddle latitudes. The fact that these frontal zones arelocated near large amounts of energy suggests thatcyclones developing in these regions along the primaryfront may be of thermodynamic origin. The mainhurricane tracks in the Atlantic and Pacific also appearto follow warm waters. Extratropical cyclones also tendto occur in warm waters in fall and early winter.

CLIMATIC FACTORS

Human activity and vegetation can have markedeffects on the climates of local areas. Eventually man’sactivities could affect larger areas and ultimately wholecontinents.

It has been known for years now that urban areasand industrial complexes have an influence on climate.Atmospheric pollution is increased, for example, andthe radiation balance is thereby altered. This changeaffects the daily maximum and minimum temperaturesin cities, where they tend to be generally higher than innearby suburbs. A higher concentration of hygroscopiccondensation nuclei in cities results in an increasednumber of fogs. Also, with the greater heat sourcefound in cities, increased convection gives rise togreater amounts of cloudiness and precipitation. Anapparent benefit of this increased heat is a slightdecrease in severe weather occurring in large cities(Chicago, for example) as compared to adjacent areas.

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Areas of heavy vegetation generally have distinctclimates, which may differ considerably from climatesof nearly open areas. Falling precipitation caught intrees before reaching the ground may be evaporated,but precipitation, which reaches the ground, does notevaporate or run off readily. Heavily forested areas canabsorb and store considerable quantities of water. Snowin forests can be protected from direct insolation by thetrees and may stay on the ground for much longerperiods than snow on open, exposed surfaces. Inforests, temperature maximums and minimums arehigher than over open land at the same latitude. Relativehumidity is also higher and wind speeds areconsiderably lower.

REVIEW QUESTIONS

Q6-12. Which climatic control has the biggest effecton climatic elements?

Q6-13. A weather station on the western coast of theUnited States will receive the characteristicsof what type air as compared to a weatherstation on the eastern coast?

Q6-14. Generally, how do ocean currents effectclimate?

CLIMATOLOGICAL DATA

LEARNING OBJECTIVE: Describe the useof climatological data in meteorology and whatreferences and services are available.

Climatological records are based on themeteorological observations that are taken at aparticular locality. This information may be presentedin a number of ways.

Temperature records generally include thefollowing temperature values: daily maximums andminimums by months; the extremes; the averagetemperature by year and month; the mean monthly andannual temperature; the mean monthly maximum andminimum temperature; and (sometimes) the monthlyand seasonal degree-days. Of great climaticsignificance is the range between the mean temperatureof the warmest month and the coldest month. Othertemperature data are sometimes given. These mayinclude the number of days with the following

temperatures: maximum of 90°F and above; maximumof 32°F and below; minimum of 32°F and below; andminimum of 0°F and below.

Precipitation records include the mean annual andmonthly totals. The range between the highest and thelowest annual rainfall for a locality is the best indicationof the dependability of the precipitation. The recordsoften show the absolute maximum rainfall and snowfallfor a 24-hour period by months, as well as themaximum and minimum precipitation for each month.

Climatic records usually show data on winds. Suchinformation indicates the mean hourly speed and theprevailing direction by month. Also shown are thespeed and direction of the strongest wind for the 12months and the year in which it occurred.

Data on cloudiness, humidity, thunderstorms, andheavy fog are often included. Other helpful data wouldbe the frequency and distribution of cyclones andanticyclones; passage of fronts; proportion of rainfalland snowfall received from cyclonic storms and local,air mass thunderstorms; and climatological data onupper air conditions.

METHODS OF PRESENTATION

Climatological information is presented in manydifferent ways. Tables are frequently used. Maps areparticularly useful in presenting climatic information incases where geography is an important factor. Winddata can be given by means of a device called a windrose, which presents information on the prevailing winddirections. (See fig. 6-4.)

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4

15

30

15

10

6

10

10

N

(NOTE : NUMBERS REPRESENTTHE % OF WINDS FROMTHAT DIRECTION DURING ASPECIFIC TIME)

AG5f 0604

Figure 6-4.—A wind rose.

Graphs are usually divided into bar and line graphs,or the graph may be a combination of the two. Figure6-5 is an example of a bar graph and a line graphshowing the same information. Figure 6-6 shows acombination of a bar and line graph used to depict cloudcover. This type of depiction is used in the most recentU.S. Navy Marine Climatic Atlas of the World.

AVAILABILITY OF DATA

Every Naval Meteorology OceanographyCommand activity should have climatological recordsavailable for their area and for such other areas as maybe necessary to provide climatological support at thelocal command level. Various climatological recordsare available from Fleet Numerical Meteorology andOceanography Detachment (FNMOD), Asheville, NC28801-2696, or by contacting their website. Theserecords include the Summary of MeteorologicalObservations, Surface (SMOS); Local ClimatologicalData (LCD); Summary of Synoptic MeteorologicalObservations (SSMO); and Summary ofMeteorological Observations, Radiosonde (SMOR).

Frequency SMOS

Frequency SMOS summaries are prepared for allnaval observing stations from navy monthlymeteorological records (MMRs). Each SMOS is for aspecific station. Frequency distributions for variousparameters are presented by time of day, month, andyear. SMOS are revised every 5 years.

Local Climatological Data Summary

The LCD summary is prepared only for selectedcivilian stations in the continental U.S.A. (CONUS). It

consists of means and extremes (temperature,precipitation, wind, etc.) by month, mean temperatureand total precipitation by month for specific years ofrecord, and monthly and seasonal degree-days. TheLCD is revised annually.

Cross-Wind Summary

The Cross Wind Summary presents the percentageof occurrence of cross winds for a given location. It isproduced only on request.

Summary of Synoptic MeteorologicalObservations (SSMO)

The SSMO presents useful monthly and annualtabulations of surface climatological data and variouscombinations of the included parameters. SSMOs werelast updated in the mid-seventies and are supplementedby the Near Coastal Zone Studies.

Near Coastal Zone Studies

Near Coastal Zone Studies are currently beingdeveloped by FNMOD, Asheville, to supplement theSMOS by providing detailed climatological data forareas of higher interest. Near Coastal Zone Studiespresent data in both graphic and tabular formats.

Summary of Meteorological Observations,Radiosonde (SMOR)

The SMOR is used to prepare monthly winds aloftsummaries, which generally include various constantheight and constant pressure levels. The summariescontain winds aloft data, giving speeds and directionsover the period covered.

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PRECIPITATION

50

40

30

20

10

0

50

40

30

20

10

01979 1980 1981 1982 1983 1984 1985 1979 1980 1981 1982 1983 1984 1985

PRECIPITATION

(A) (B)AG5f 0605

Figure 6-5.—A comparison of the bar and line graph method of showing the variable annual precipitationin a time series. (A) Bar graph; (B) Line graphs.

Worldwide Airfield Summary

The Worldwide Airfield Summary providesclimatological data for airfields and geographical areasthroughout the world. There are 10 volumes, somepublished in two or more parts.

CLIMATOLOGICAL REFERENCES

There are many references, which can be used inclimatological work, so many in fact that they would betoo numerous to list here. They are tabularized in thefollowing publications:

• Guide to Standard Weather Summaries(NAVAIR 50-1C-534) contains an index of all thestandard machine-tabulated summaries availablethrough FNMOD, Asheville.

In addition, many navy climatic references arelisted in the Navy Stock List of Forms and Publications,NAVSUP publication 2002, section 2B. Navyclimatology publications are found under theNA-SO-1C-series.

The following publications can also be used toprepare climatological briefings and packets:

• U.S. Navy Marine Climatic Atlas of the World,volumes 1 through 7 and 9 (NAVAIR 50-1C-528through 533, 550, 554, s and 565). Thesepublications contain climatic data for all theprincipal ocean areas of the world. They haveboth land and ocean sections. The surface

section contains data presented by graphs,tables, and isopleths on such elements as surfacewinds, visibility, precipitation, storm tracks, etc.The oceanographic section includes charts oftidal data, currents, and ice.

• U.S. Navy Hindcast Spectral Ocean Wave ModelAtlases, volume 1, North Atlantic (NAVAIR50-1C-538), volume 2, Pacific (NAVAIR50-1C-539). These atlases represent ocean wavedata by tables, bar graphs, and isopleths. Data isbased on numerically derived historical data inthe form of wind and wave climatology. Thesepublications are designed to provide a moreaccurate representation of overall ocean waveclimatic data for some applications. They aredesigned to supplement but not supersede theconventional Marine climatic atlases.

Local Area Forecaster’s Handbooks

The Local Area Forecaster’s Handbooks, asrequired by NAVMETOCCOM Instruction 3140.2( ),contain valuable information on local and area weatheras follows: A description of the local topography,terrain and general synoptic characteristics of weatheroccurrences in the area. Mean storm tracks for theregion, a limited amount of climatological data, andlocal forecasting rules and techniques are alsoavailable. A handbook can serve as a compositesummary of expected weather events and the effects ofcertain parameters on local weather.

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0 1 2 3 4 5 6 7 8100

90

80

70

60

50%

40

30

20

10

0NEN E SE S SW W NW C

0

*

1234 }

}Total Cloud Amount

Low Cloud Amount

CLOUD AMOUNT IN EIGHTHS

Cumulative percent frequency of indicated cloud amount equal to orless than the amount intersected by the curve.

Number of observations.

Obscurations

(77% of all total cloud amounts were 7/8.)<

<(46% of all low cloud amounts were 2/8.)

Low cloud amount: Percent frequency of observationsfrom each direction and calm that were accompaniedby low cloud amounts 5/8 and 7/8. Low cloudsare clouds with bases <8000 feet.

< <<<}

}}5/8

5/86/87/8

&

(28% of all SE winds were accompanied by low cloud amounts5/8 and 14% by low cloud amounts 7/8.)< <

An asterisk indicates that the percentage is based on 10 - 30 obser-vations of wind direction, total and low cloud amount. 0 replacesbar graph when no low cloud amounts 5/8 were observed witha wind direction or calm. 0 or bar is omitted when number ofobservations of total and low cloud amount from a wind directionor calm is less than 10.

<

AG5f 0606

Figure 6-6.—Combination bar and line graph (with legend and instructions for use).

Naval Intelligence Survey (NIS) Publications

The Naval Intelligence Survey (NIS) publicationshave been discontinued, and distribution is limited.However, when available, these classified publicationsare a valuable source of information about generalclimatic influences and topographic/oceanic effects onregions from which unclassified data may no longer beavailable.

Miscellaneous Publications

The following publications contain generally thesame type of climatological information or specificdata. They have proven to be extremely useful.

1. Climatic Summaries for Major Seventh FleetPorts and Waters, NAVAIR 50-1C-62.

2. Climatic Summaries of Indian Ocean Ports andWaters, NAVAIR 50-IC-63.

3. A Climatic Resume of the Mediterranean Sea,NAVAIR 50-1C-64.

4. Upper Wind Statistics of the NorthernHemisphere, volumes 1, 2, and 3, NAVAIR 50-1C-535.

5. Marine Climatic Guide to Tropical Storms atSea, NAVAIR 50-IC-61.

6. Sea Ice Climatic Atlases, volume 1, Antarctic,NAVAIR 50-1C-540. Volume 2, Arctic East, NAVAIR50-IC-541. Volume 3, Arctic West, NAVAIR50-1C-542.

CLIMATOLOGICAL SERVICES

Requests for climatic support should be made to theMeteorology Oceanography Facility or Center in yourchain of command. Requests that cannot be fulfilledare forwarded to:

Fleet Numerical Meteorology andOceanography FacilityAsheville, NC 28801-5014

Additional Climatic Sources

In addition to navy climatic publications, there areother sources for air/ocean climatology data, which areavailable to the Aerographer’s Mate for preparingclimatic studies. They are as follows:

• The Warfighting Support Center (WSC), StennisSpace Center Mississippi, provides oceanographic

support. Available data includes tides, currents, andwater structure, etc.

• The Air Weather Service EnvironmentalTechnical Application Center (ETAC) providesclimatic information for Air Force operations.However, data produced by ETAC can be used for navalapplications. A listing of climatology studies availablefrom the Air Weather Service can be found in Index ofAir Weather Service Technical Publications(AWS/TI-84/00 1). Requests for Air Weather Servicepublications must be made to Commander, NavalMeteorology Oceanography Command, Stennis SpaceCenter, Mississippi.

INTERPRETATION

Climatological records must be interpretedcorrectly to gain the needed information. Properinterpretation requires that all of the meteorologicalelements be studied so they present a compositepicture. One meteorological element alone may meanvery little. For instance, it is possible to conclude thatCairo, Egypt, and Galveston, Texas, has about the samekind of weather based solely on the temperature, sincethe yearly and monthly means and annual range areapproximately the same. However, Galveston has about40 times as much precipitation. Thus, their weatherconditions over the year differ greatly.

To interpret just one meteorological elementrequires a study of several factors. For example, thetemperature of a particular locality must be studiedfrom the standpoint not only of the mean but also of theextremes and the diurnal and annual ranges. Theeffectiveness of precipitation also depends on severalfactors, such as amount, distribution, and evaporation.The mean precipitation for a particular month for alocality may be several inches, but the interpreter mayfind from a study of the locality’s records that in someyears the precipitation for that month is less than aninch, possibly not even a trace.

APPLICATION TO WEATHER PREDICTION

Climatology is introduced where operationalplanning is required for a length of time beyond therange covered by weather-forecasting techniques. Astudy of the climate of an area or region may wellforetell the general weather pattern to be expected.

Both the experienced and the inexperiencedforecaster and assistant forecaster can make a moredirect application of climatology. Those personnel

6-16

having personal experience at a particular station canuse climatology as a refresher for the overall weatherpatterns that can be expected for the ensuing season.This knowledge can help them to be more perceptive intheir everyday analyses, to be alert for changingpatterns with the seasons, and to produce a higherquality forecast.

The personnel who have had no experience at aparticular station must rely on climatology as asubstitute for their experience. Forecasters andassistant forecasters cannot be expected to becomefamiliar overnight with the weather peculiarities oftheir new area of responsibility. The stationcertification period can be greatly reduced if the newpeople are furnished with “packaged experience” in aform that can place them more nearly on a par withthose forecasters already experienced at that station.The Local Area Forecaster’s Handbooks are goodexamples of this type of packaged information.

The Naval Meteorology Oceanography Commandmakes many uses of climatological data. In using thedata, however, it must be clear that climatology has itslimitations in the field of meteorology. It may be putthis way. Climatology is an essential supplement tometeorology, but it must never be considered asubstitute for the meteorological situation thatconstitutes current weather conditions.

REVIEW QUESTIONS

Q6-15. What is the correct method to obtainclimatology information?

Q6-16. What publication is also useful for obtainingclimatology information for a particularweather station?

WORLD WEATHER

LEARNING OBJECTIVE: Identify thevarious types of weather and climate of theoceans and continents.

Aerographer’s Mates are stationed, and may travel,around the world. Ships and aircraft are constantly inglobal transit. Therefore, the Aerographer’s Mate musthave a general knowledge of types of weatherencountered during various seasons in regions allaround the world. This knowledge also increasesinsight into atmospheric circulation, weatherdevelopment and movement, weather effects on theenvironment, and credibility as a knowledgeableanalyst, interpreter, and briefer.

NOTE: You will find that a world atlas can beextremely useful and informative if used in conjunctionwith the information that follows.

OCEANIC WEATHER

Naval vessels of the United States operate invirtually all the oceanic areas of the world; therefore,the Aerographer’s Mates must be acquainted withoceanic weather. Some general considerations of theweather encountered over ocean areas are discussed inthis lesson.

Because land and water heat and cool at differentrates, the location of continents and oceans greatlyaffects the Earth’s pattern of air temperature andtherefore influences the weather. The upper layers ofthe ocean are almost always in a state of motion. Heatloss or gain occurs at the sea surface and is distributedthroughout large volumes of water. This mixing processsharply reduces the temperature contrasts between dayand night and between winter and summer.

Oceanic Weather Control

It has long been recognized that the ocean plays animportant part in climate and weather, particularly inthe realms of temperature, humidity, and precipitation.This is only natural, since three-fourths of Earth’ssurface is covered by water.

The two climatic extremes that relate to water andland distribution over Earth are maritime andcontinental. A wide range in annual and diurnaltemperatures, little cloudiness, and little precipitationgenerally evidences Continental climate. Continentalclimate is a product of a minimal influence from theoceans. Maritime climate prevails over the oceans andis characterized by a small temperature range, bothannual and diurnal, and considerable precipitation andcloudiness.

Water vapor is considered one of the mostimportant variables in meteorology. The state of theweather is largely expressed in terms of the amount ofwater vapor present and what is happening to the watervapor. Two principal elements of climate, precipitationand humidity are dependent upon water vapor. Sincethe oceans are the main source of water vapor, it followsthat the oceans largely control weather.

Effects of Air-Sea Interchange

The atmosphere and the oceans have tremendouseffects on each other. These effects are principally in

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the realms of temperature and water vapor. Theprocesses of radiation, the exchange of sensible heat,and the evaporation and condensation of water vapor onthe sea surface maintain the heat balance of the oceans.

The amount of radiant energy absorbed by the seadepends upon the amount of energy reaching thesurface and the amount of reflection by the surface.When the Sun is directly overhead, the amount of itsenergy reflected amounts to only about 3 percent. Evenwhen the Sun is 30° above the horizon, the amount ofreflection is just 6 percent. However, there is areflection of about 25 percent of the energy when theSun is 10° above the horizon. (See fig. 6-7.) Reflectionloss is especially great in the presence of waves whenthe Sun is low.

Much of the insolation is absorbed in the first meterof seawater. This is true of the clearest water as well asof quite turbid (opaque) water. In water that isextremely turbid, the absorption is in the veryuppermost layers. Foam and air bubbles are two majorcauses of a proportionately greater amount ofabsorption in the uppermost meter of the sea. However,due to vertical mixing, the heat absorbed in the upperlayer is carried to great depths of the ocean, which actsas a great heat storage reservoir.

There is an exchange of energy between the oceansand the atmosphere. The surface of the oceans emitslong-wave heat radiation. The sea surface at the sametime receives long-wave radiation from the atmosphere.Although some of this incoming radiation from the

atmosphere is reflected from the surface of the oceans,most of it is absorbed in a very thin layer of the watersurface. The difference between the incominglong-wave atmospheric radiation and the outgoinglong-wave radiation from the sea surface is known asthe effective back radiation. The effective backradiation depends primarily on the temperature of thesea surface and on the water vapor content of theatmosphere. The time of day and the season have littleeffect on effective back radiation, since the diurnal andannual variation of the sea-surface temperature and ofthe relative humidity of the air above the oceans isslight.

For conduction to take place between the oceansand the atmosphere there must be a temperaturedifference between the ocean surface and the airimmediately overlying it. On the average, thetemperature of the surface of the oceans is higher thanthat of the overlaying air. It might be expected that all ofthe ocean’s surplus of heat is either radiated orconducted to the atmosphere. This is not the case. Onlya small percentage of the ocean’s surplus heat isactually conducted to the atmosphere. About 90 percentof the surplus are used for evaporation of ocean water.

Due to the processes of radiation and mixing, theoceans act as a thermostat relative to the atmosphere.The energy stored at one place during one season maybe given off at another locality and during a laterseason. Hence, there seems to be a constant effort bythe atmosphere and the oceans to keep theirtemperatures in balance by an interchange of heat.

STABILITY.—The deciding factor of mostweather phenomena is the stability of the atmosphere.Air masses may become more stable or less stable asthey move over ocean surfaces. The temperaturecontrast between the ocean surface and the lowestlayers of the overlying air determines whether theocean will promote stability or instability.

When the air moving over the ocean has a highertemperature than that of the ocean surface, the lowerlayers of the air become stable in time. On the otherhand, when the air mass is colder than the ocean surfaceover which it is moving, instability results. As thecolder air is warmed by the ocean, convective activityeventually develops. If the warming is sufficientlyintense, thunderstorms develop.

MOISTURE CONTENT.—The interchange ofmoisture between the atmosphere and the oceans is oneof the most important features of the wholemeteorological picture. Without this interchange,

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80O

60O

50O

70O

40O

30O

20O

10O

90O

0O

40% 35% 30% 20% 10%15%25% 5% 0

PERCENTAGE OF REFLECTION

ALT

ITU

DE

OF

SU

NIN

DE

GR

EE

S

AG5f 0607

Figure 6-7.—Percentage of reflected radiation.

weather, as we know it, could not exist; there would beno clouds and no precipitation. The oceans are by farthe greatest source of moisture for the atmosphere.Other moisture sources are negligible in comparison.

Whether the atmosphere gives up some of itsmoisture to the ocean or vice versa depends greatlyupon vapor pressure. Vapor pressure is the pressureexerted by the molecules of water vapor in theatmosphere or over the surface of liquid water. Whenthe vapor pressure of a liquid is equal to that of theatmosphere above the liquid, there is little or noapparent interchange of moisture. In other words, atequal vapor pressure, just as many molecules escapefrom the liquid to the atmosphere and vice versa. This isthe case when air becomes saturated. The saturationvapor pressure increases with increasing temperature.

If the temperature of the surface water is warmerthan that of the air, the vapor pressure of the water at itssurface is greater than that of the air. When thiscondition exists, there can be abundant evaporationfrom the ocean surface. This evaporation is aided by theturbulence of the air brought on by the unstablecondition of the lower layers. It follows, then, that thegreatest evaporation takes place when cold air flowsover warm ocean waters.

Let us consider the opposite condition—warm airflowing over a relatively cold body of water. When thishappens, there is stable stratification in the lower layersof the atmosphere. The vapor pressure of the air soonreaches a state of equilibrium with that of the watersurface. Evaporation stops. However, if the warm air isquite moist, it is possible for the moisture in the air tocondense on the water surface. Contact of the warm airwith the cold water may result in the formation of fogby lowering the air temperature to the dew point.

The direct interchange of moisture from theatmosphere to the oceans occurs through precipitationand, to a lesser extent, condensation. The directinterchange, however, is not as importantmeteorologically as the indirect interchange. Theindirect interchange is a sequence of events beginningwith the evaporation of water from the ocean surfacesand ending with the subsequent condensation andprecipitation over land areas.

Generally, precipitation occurs more frequentlyover land than over the oceans. Though the oceans are asource of abundant moisture, they normally lack therequired precipitation mechanisms, such as verticalmixing, strong temperature contrasts, and orographiclifting.

Equatorial and Tropical Weather

In the Temperate Zone, where westerly windspredominate, pressure patterns move in an easterlydirection. In the tropics, however, weather usuallymoves in the opposite direction. Normally, a moistlayer, 5,000 to 8,000 feet deep exists in this region.During unfavorable weather, this layer deepens to morethan 12,000 feet. Convergence occurs in opposing tradewind streams, northward flowing air, and areas ofcyclonic curvature. The presence of a deep, moist layerand convergent winds account for the weather inequatorial and tropical regions.

North Atlantic and North Pacific Oceans

In the winter, the most favorable conditions forvigorous frontal activity are concentrated along the eastcoasts of North America and Asia. These conditions areassociated with polar front activity. Cold air massesfrom continental sources meet warm, moist air fromover the oceans. The warm ocean currents along thesecoasts greatly accentuate the frontal activity. The greattemperature difference of the air masses, caused by thecontrasting characteristics and proximity of theirsources and the moisture that feeds into the air from thewarm ocean currents, accounts for the intensity andpersistence of these frontal zones off the east coasts inthe winter. Modification of the air masses as they sweepeastward across the ocean leads to modified frontalactivity on the west coasts. Refer back to chapter 4, tofigures 4-25 and 4-26 for the location of the followingfrontal zones:

1. Polar fronts in the Atlantic. In the Atlantic, inwinter, polar fronts are found situated in variouslocations between the West Indies and the Great Lakesarea. Intensity is at a maximum when the frontscoincide with the coastline. Waves, with cold and warmfronts, form along the polar front and movenortheastward along the front. Like all cyclonic waves,they develop low-pressure centers along the frontaltrough. They may grow into severe disturbances and gothrough the usual stages of development: formation,growth, occlusion, and dissipation.

These cyclonic waves occur in families. Eachfamily of waves is associated with a southward surge,or outbreak, of cold polar air. The polar frontcommonly extends approximately through the GreatLakes area. As the polar air advances, it pushes the frontsouthward. The outbreak occurs, and polar air, joiningthe trade winds, spills equatorward.

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There is no regular time interval for these largeoutbreaks of polar air, but the average period is about5 1/2 days between them. Under average conditions,there are from three to six cyclonic waves on the polarfront between each outbreak of polar air. The first ofthese usually travels along the front that lies farthest tothe north. As the polar air accumulates north of thefront, the front is pushed southward, and the last wavetherefore follows a path that starts farther south than thepath followed by the first wave. These families of polarfront cyclones appear most frequently over the NorthAtlantic and North Pacific in the winter.

During the summer months, the polar fronts of theAtlantic recede to a location near the Great Lakesregion, with the average summer storm track extendingfrom the St. Lawrence Valley, across Newfoundland,and on toward Iceland. Polar outbreaks, with theiraccompanying family groupings of cyclones, are veryirregular in summer and often do not exist at all. Frontalactivity is more vigorous in the winter than in thesummer because the polar and tropical air masses havegreater temperature contrasts in the winter, and polarhighs reach maximum development in the winter. Bothof these factors increase the speed of winds flowing intofronts. Over oceans of middle latitudes, a third factorhelps to make winter fronts more vigorous thansummer fronts. In the winter, continental air becomesvery unstable when it moves over the comparativelywarm ocean surface; in the summer, it remainsrelatively stable over the comparatively cool ocean.Summer frontal activity (in middle latitudes) istherefore weak over oceans as well as over land. Thehigh moisture content of maritime air causes muchcloudiness, but this moisture adds little energy tofrontal activity in the relatively stable summer air.

2. The polar fronts in the Pacific. These fronts aresimilar to those of the Atlantic, except that in the winterthere are usually two fronts at once. When one highdominates the subtropical Pacific in the winter season,the pacific polar front forms near the Asiatic coast. Thisfront gets its energy from the temperature contrastbetween cold northerly monsoon winds and the tropicalmaritime air masses, and from the warm, moistKuroshio Current. In moving along this polar front ofthe Asiatic North Pacific in winter, storms occludebefore reaching the Aleutian Islands or the Gulf ofAlaska. Because of its steady cyclonic circulation, theAleutian low becomes a focal center, or a gatheringpoint, for cyclones. The occluded fronts move aroundits southern side like wheel spokes. This frontalmovement is limited to the southern side of theAleutian low because mountains and the North

American winter high-pressure center prevent frontsfrom passing northward through Alaska withoutconsiderable modification.

In the winter the cyclones reach the Aleutians andthe Gulf of Alaska. Here, Arctic air from the northmeets the relatively warmer maritime air from thesouth. The Pacific arctic front of winter is found in thisregion. Although many occluded storms dissipate in theGulf of Alaska, others strongly regenerate with wavesdeveloping on what were once occluded fronts.

When the Pacific subtropical high divides into twocells or segments (as it does 50 percent of the time in thewinter and 25 percent of the time in the summer), afront forms in the vicinity of Hawaii. Along this front,storms develop and move northeastward. These stormscalled Kona storms, have strong southwest winds andbring heavy rains to the islands. Those storms thatsucceed in moving beyond the realm of the northeasttrade winds, which stunt them, may develop quitevigorously and advance to the North American coast,generally occluding against the mountains. When thissecond polar front exists, two systems of cyclonicdisturbances move across the Pacific. Because of theirgreater sources of energy, however, storms thatoriginate over the Kuroshio Current and move towardthe Aleutians are almost always more severe. In theAtlantic, a second polar front, similar in nature andsource to the second polar front of the Pacific,sometimes—though rarely—develops.

During the summer months, the Pacific polar frontlies to the north of Kamchatka and the Aleutians andshows no rhythmic polar outbreaks.

Air-Mass Weather

Flying weather is usually best in tropical maritimeair, at its source, within the subtropical highs. Scatteredcumulus and patches of stratocumulus clouds maydevelop, but the sky is almost never overcast. Scantprecipitation falls in scattered showers and variable,mild winds prevail.

The excellent flying weather in these mT sourceregions commonly extends through the moving airmasses some distance from the sources. Cloudiness inthe mT air increases with an increase in distance fromthe source.

On flights from Hawaii or from the Azoresnorthward, through northward-moving mT air,stratiform clouds increase. On flights from Hawaii orthe Azores southward, through southward-moving mT

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air (or the northeast trades), cumuliform cloudsincrease. Here we are considering only NorthernHemisphere situations; however, a comparable patternexists in the Southern Hemisphere.

A typical breakdown of the weather conditions youmay encounter in air masses around the subtropicalhighs (fig. 6-8) is as follows:

1. North of a subtropical high. Any mT air thatmoves northward becomes cooled over the cool oceansurface. A stratus overcast may form, and drizzle mayfall. Farther north, low ceilings (usually below 1,000feet) may reach the surface, producing fog. The mT airsurges farthest north in summer because subtropicalhighs are best developed and polar fronts lie farthestnorth. This mT air brings most of the summer fogginessto northern seas and coasts. It brings the greatestfogginess in the Atlantic where it blows from the warmGulf Stream over the cold Labrador Current (nearNewfoundland), and in the Pacific where it blows fromthe warm Kuroshio Current over the cold Oyashiocurrent (near the Kamchatka peninsula).

2. East of a subtropical high. Along the Californiacoast, and along the Atlantic coast of North Africa, themT air blows from the west and the northwest. This airtends to remain stable for the following reasons:

a. It is coming from the northern, coolerportion of the source region.

b. Its surface layers remain cool because itmoves over cold ocean currents.

c. Its upper portions warm adiabaticallybecause of subsidence.

Throughout the year, airways are smooth. The skiesare clear to partly cloudy. Clouds are generally patchesof stratocumulus, and rain is rare. The chief flight

hazard in this air is coastal fog, which often hides theCalifornia or European coastal land. Stratus andstratocumulus clouds may cause the sky to be overcast,develop low ceilings, and produce drizzle that reducesvisibility.

3. South of a subtropical high. Where the mT airmoves southward or southwestward (as trade winds),its lower layers are warmed by the tropical oceansurface. This produces scattered cumulus. Near theequator, after absorbing much moisture and beingheated, this air may develop cumulonimbus.

4. West of a subtropical high. This mT air blowsfrom the east and the southeast. Since it flows overwarm water all of the way, the air neither cools norwarms. Over the ocean near the Philippines (and nearFlorida and the West Indies), this trade wind bringsgood flying weather—clear or scattered cumulusclouds. When it is moving over land, this warm, moistair becomes unstable and turbulent and is a source ofthunderstorms. When it moves over cold land (forexample, southeastern United States in the winter), itbecomes stable and produces stratus clouds or fog.Over cold ocean surfaces, such as the Sea of Japan andthe Kamchatka and Labrador currents, it develops thepersistent low stratus and fogs characteristic of theseareas.

ARCTIC AND ANTARCTIC WEATHER

Geographically, the arctic zone is north of theArctic Circle (66.5°N) and the Antarctic zone is southof Antarctic Circle (66.5°S) The Arctic is extremelyimportant to the military defense of Canada and theUnited States and is the subject of ever-increasingmilitary operations. Therefore, Aerographer’s Matesmust familiarize themselves with the prevailingweather and peculiarities of these regions.

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SUBTROPICAL

ANTICYCLONEH

AIR STABLE

TRADES (DRY LITTLERAINFALL)

AIR UNSTABLE(WET)

AIR STABLE (POSSIBLE FOG & STRATUS)

SUBSIDENCE INVERSION(STRONG) AIR STABLE

(RELATIVELY DRYCLIMATE)

SUBSIDENCEWEAKAIR NEUTRAL

ORUNSTABLE

(WET)(PRECIPITATIONABUNDANT)

AG5f 0608

Figure 6-8.—Weather, winds, and stability conditions around the subtropical high.

Arctic Weather

The Arctic is the aerial crossroads of the world.This is not only due to the shorter arctic routes betweensome of the major cities of the world, but also becauseflying weather over the Arctic is generally better thanthat encountered over the familiar ocean routes. Tounderstand some of the important weather andproblems of the Arctic, you must understand the broadunderlying causes of the arctic climate.

SEASONAL TEMPERATURE VARIA-TIONS.—From our previous discussion of climaticcontrols, we have seen that the most important factorthat determines the climate of an area is the amount ofenergy it receives from the Sun. During the wintermuch of the Arctic receives little or no direct heat fromthe Sun. The cold winter temperatures common in theArctic result from a lack of the Sun’s energy.

The Sun is not the only factor responsible for thearctic climate. Two other factors, the land-sea-icedistribution and mountain barriers, contribute to thetremendous variation in climate at different points ofsimilar latitude.

1. Land-sea-ice features. In the NorthernHemisphere, the water features include the Arctic,North Atlantic, and North Pacific oceans. These bodiesof water act as temperature moderators since they donot have large temperature variations. A majorexception occurs when large areas are covered by ice inwinter. The land features are the northern continents ofEurasia, North America, the island of Greenland, andthe Canadian Archipelago. As opposed to the waterareas, the land areas tend to show the direct results ofthe extremes of seasonal heating and cooling by theirseasonal temperature variations.

2. Mountains. The arctic mountain ranges ofSiberia and North America are factors, whichcontribute to the climate and air mass characteristics ofthe regions. These mountain barriers, as inmid-latitudes, restrict the movement of air from west toeast. During periods of weak circulation, the air isblocked by the ranges and remains more or lessstagnant over the area. It is during these periods that theair acquires the temperature and moisturecharacteristics of the underlying surface. Thus, theseareas are air-mass source regions, and they areparticularly effective as source regions during thewinter when the surface is covered with snow and ice.

The Greenland ice cap is essentially a mountainrange more than 10,000 feet above mean sea level. It

restricts the movement of weather systems, oftencausing low-pressure centers to move northward alongthe West Coast of Greenland. Some of the largest ratesof falling pressure in the world (other than hurricanesand tornadoes) are recorded here. The deep, low centersthat move along the west coast of Greenland areprimarily responsible for the high winds that arerecorded occasionally in that area.

At times, winter temperatures in the Arctic areunusually high. This situation is brought about by deep,low centers moving into the Arctic, coupled withcompression of air (the Foehn effect) as it often blowsdown off the sloping edges of the ice caps, primarily theGreenland ice cap.

ARCTIC AIR MASSES.—The moisture contentof air masses that originate over land is low at allaltitudes in the winter. The distinction between airmasses almost disappears during the summer becauseof the nearly uniform surface conditions over the arcticand subpolar regions. The frozen surface thaws underthe influence of lengthened or continual daylight, thesnow melts from the glaciers and pack ice, the ice meltsin the lake areas in the Arctic, and the water areas of thepolar basin increase markedly. Thus, the polar areabecomes mild, humid, and semimaritime in character.Temperatures are usually between freezing and 50°F.Occasionally, strong disturbances from the southincrease the temperature for short periods. Dailyextremes, horizontal differences, and day-to-dayvariability are slight.

During the winter months, air masses are formedover areas that are completely covered by ice and snow.The air masses are characterized by very cold surfaceair and a large temperature inversion in the lowest fewthousand feet. Since the amount of moisture the air canhold depends on the air temperature, the cold arctic airis very dry (low absolute humidity). The air mass thatoriginates over oceans does not have a surfacetemperature inversion in the winter, the surface airtemperature is warmer, and there is a correspondingincrease in the moisture content of the air. It is duringmovement inland of moist air from the warmer watersthat most of the rather infrequent arctic cloudiness andprecipitation occurs during this season.

During the summer months, the large expanse ofopen water and warmer temperatures in the Arcticresult in increased moisture. Consequently, the largestamount of cloudiness and precipitation occurs duringthese summer months.

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ARCTIC FRONTS.—The weather associatedwith fronts in the Arctic has much the same cloudstructure as with polar fronts, except that the middleand high cloud types are generally much lower, and theprecipitation is usually in the form of snow.

Periods of maximum surface wind usually occurduring and just after a frontal passage. This strong windflow often creates hazards, such as blowing snow andturbulence, which make operational flying difficult.

The best flying weather in the Arctic over landusually occurs in midsummer and midwinter; the worst(low ceilings and visibility) is during the transitionalperiods between the two seasons. Winter ischaracterized by frequent storms and well-definedfrontal passages, but because of the dryness of the air,cloudiness and precipitation are at a minimum. In thesummer, there are fewer storm passages and fronts areweaker; however, the increased moisture in the airresults in more widespread clouds and precipitation.Over the sea areas the summer weather is very foggy,but winds are of lower speeds than in the winter.

During the transitional periods of spring and fall,operational flying conditions are usually the worst.Frontal systems are usually well defined, active, andturbulent. Icing may extend to high levels.

TEMPERATURES IN THE ARCTIC.—Temperatures in the Arctic, as one might expect, arevery cold most of the year. But contrary to commonbelief, the interior areas of Siberia, northern Canada,and Alaska have pleasantly warm summers with manyhours of sunshine each day. There are large differencesin temperature between the interior and coastal areas.

In the interior during the summer days,temperatures climb to the mid 60s or low 70s andfrequently rise to the high 70s or low 80s, occasionallyeven into the 90s. Fort Yukon, Alaska, which is justnorth of the Arctic Circle, has recorded an extreme hightemperature of 100°F, while Verkhoyansk in northcentral Siberia has recorded 94°F.

During the winter, the interior areas of Siberia,northern Canada, and Alaska act as a source region forthe cold arctic air that frequently moves southward intothe middle latitudes. The coldest temperatures onrecord over the Northern Hemisphere have beenestablished in Siberia.

In the northern areas of the interior regions,temperatures are usually well below zero during thewinter months. In fact, during these long periods ofdarkness and near darkness, the temperature normally

falls to –20°F or –30°F, and in some isolated areas thenormal daily minimum temperature may drop to –40°F.In north central Siberia the normal minimum dailytemperature in the winter is between –45°F and –55°F.

The arctic coastal regions, which include theCanadian Archipelago, are characterized by relativelycool, short summers. During the summer months thetemperatures normally climb to the 40s or low 50s andoccasionally reach the 60s. There is almost no growingseason along the coasts, and the temperatures may fallbelow freezing during all months of the year. At PointBarrow, Alaska, the minimum temperature rises abovefreezing on no more than about 42 days a year.

Over the Arctic Ocean, the temperatures are verysimilar to those experienced along the coast; however,the summer temperatures are somewhat lower. Wintertemperatures along the Arctic coast are very low but notnearly as low as those observed in certain interior areas.Only on rare occasions does the temperature climb toabove freezing during the winter months. The coldestreadings for these coastal areas range between –60° and–70°F.

These figures may seem surprising. At first onemight think that the temperatures near the North Polewould be lower than those over the northern continentalinteriors. Actually the flow of heat from the water underthe ice has a moderating effect upon the air temperaturealong the coast.

CLOUDINESS.—Cloudiness over the Arctic is ata minimum during the winter and spring and at amaximum during the summer and fall, again due to thelow-moisture capacity of cold air. The average numberof cloudy days for the two 6-month periods on climaticcharts shows a general decrease in cloudiness in theentire arctic area during the winter months. The greatestseasonal variation is found in the interior, and the leastis found along the coasts.

During the warm summer afternoons in the interiorregions, scattered cumulus form and occasionallydevelop into thunderstorms. The thunderstorms arenormally widely scattered and seldom form continuouslines. Along the arctic coast and over the Arctic Ocean,thunderstorms occur infrequently. Although tornadoeshave been observed near the Arctic Circle, theiroccurrence is extremely rare. In these areas, summersare quite cloudy, with stratiform clouds predominating.

Seasonal changes in cloudiness take place quiterapidly. Winters are characterized by extensivecloudiness in the coastal regions. These clouds are

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associated with migratory lows and generally disperseinland as the systems lose their moisture.

WINDS.—Wind speeds are generally light in thecontinental arctic interior throughout the year. Thestrongest winds in the interior normally occur duringthe summer and fall. During the winter, the interiorcontinental regions are areas of strong anticyclonicactivity that produce only light surface winds.

Strong winds occur more frequently along thearctic coast than in the continental interiors. Thefrequency with which these high winds occur in coastalareas is greater in the fall and winter than in thesummer. These winds frequently cause blowing snow.

Very strong wind speeds have been observed atmany arctic coastal stations. Strong winds areinfrequent over the ice pack, but the wind blows almostcontinuously because there are no natural barriers (suchas hills and mountains) to retard the wind flow. As aresult, the combination of wind speed and lowtemperatures produces equivalent wind chilltemperatures that are extreme and severely limitoutdoor human activity.

PRECIPITATION.—Precipitation amounts aresmall, varying from 5 to 15 inches annually in thecontinental interior and 3 to 7 inches along the arcticcoastal area and over the ice pack. The climate over theArctic Ocean and adjoining coastal areas is as dry assome of the desert regions of the mid-latitudes. Most ofthe annual precipitation falls as snow on the ArcticOcean and adjacent coastal areas and ice caps. On theother hand, most of the annual precipitation falls as rainover the interior.

RESTRICTION TO VISIBILITY.—Two factorsmake the visibility in the Polar Regions a very complexmatter. Arctic air, being cold and dry, is exceptionallytransparent, and extreme ranges of visibility arepossible. On the other hand, there is a lack of contrastbetween objects, particularly when a layer of new snowcovers all distinguishable objects. Limitations tovisibility in the Arctic are primarily blowing snow, fog,and local smoke. Local smoke is serious only in thevicinity of larger towns and often occurssimultaneously with shallow radiation fogs of winter.

1. Blowing snow. Blowing snow constitutes amore serious hazard to flying operations in the Arcticthan in mid-latitudes because the snow is dry and fineand is easily picked up by moderate winds. Winds inexcess of 8 knots may raise the snow several feet off theground, and the blowing snow may obscure surfaceobjects such as runway markers.

2. Fog. Of all the elements that restrict flying inthe Arctic regions, fog is perhaps most important. Thetwo types of fog most common to the Polar Regions areadvection fog and radiation fog.

Fog is found most frequently along the coastalareas and usually lies in a belt parallel to the shore. Inthe winter, the sea is warmer than the land, andrelatively warm, moist air is advected over the cool landcausing fog. This fog may be quite persistent. In thesummer, warm, moist air is advected over sea ice,which is now melting, creating the same situation,which is found over land in winter.

3. Ice fog. A fog condition peculiar to Arcticclimates is ice fog. Ice fog is composed of minute icecrystals rather than water droplets of ordinary fog and ismost likely to occur when the temperature is about–45°C (–50°F) or colder but can occur whentemperatures are as warm as -30°C (–20°F).

4. Sea smoke or steam fog. The cold temperaturesin the Arctic can have effects, which seem peculiar topeople unfamiliar with the area. During the wintermonths, the inability of the air to hold moisture resultsin an unusual phenomenon called sea smoke. Openbodies of comparatively warm water existingsimultaneously with low air temperature cause this.Actually, this phenomenon is similar to that of steamforming over hot water.

In the case of sea smoke, the temperatures of boththe air and the water are quite low, but the airtemperature is still by far the lower of the two, causingsteam to rise from the open water to form a fog layer.This fog occurs over open water, particularly over leads(navigable passages) in the ice pack and is composedentirely of water droplets.

5. Arctic haze. This is a condition of reducedhorizontal and slant visibility (but good verticalvisibility) encountered by aircraft in flight over arcticregions. Color effects suggest this phenomenon to becaused by very small ice particles. Near the ground, it iscalled arctic mist or frost smoke; when the sun shineson the ice particles, they are called diamond dust.

ARCTIC WEATHER PECULIARITIES.—Thestrong temperature inversions present over the Arcticduring much of the winter causes several interestingphenomena. Sound tends to carry great distances underthese inversions. On some days, when the inversion isvery strong, human voices can be heard over extremelylong distances as compared to the normal range of thevoice. Light rays are bent as they pass through theinversion at low angles. This may cause the appearance

6-24

above the horizon of objects that are normally belowthe horizon. This effect, known as looming, is a form ofmirage. Mirages of the type that distort the apparentshape of the Sun, Moon, or other objects near thehorizon are common under inversion conditions.

One of the most interesting phenomena in theArctic is aurora borealis (northern lights). These lightsare by no means confined to the Arctic but are brightestat the arctic locations. Their intensity varies from a faintglow on certain nights to a glow, which illuminates thesurface of the Earth with light almost equal to that of thelight from a full moon. The reactions resulting in theauroral glow have been observed to reach a maximumat an altitude of approximately 300,000 feet.

The amount of light reflected from a snow-coveredsurface is much greater than the amount reflected fromthe darker surfaces of the middle latitudes. As a result,useful illumination from equal sources is greater in theArctic than in lower latitudes. When the sun is shining,sufficient light is often reflected from the snow surfaceto nearly obliterate shadows. This causes a lack ofcontrast, which, in turn, results in an inability todistinguish outlines of terrain or objects even at shortdistances. The landscape may merge into a featurelessgrayish-white field. Dark mountains in the distancemay be easily recognized, but a crevasse immediatelyahead may be obscured by the lack of contrast. Thesituation is even worse when the unbroken snow coveris combined with a uniformly overcast sky and the lightfrom the sky is about equal to that reflected from thesnow cover. In this situation, all sense of depth andorientation is lost in what appears to be a uniformlywhite glow; the term for this optical phenomenon iswhiteout.

Pilots have reported that the light from a half-moonover a snow-covered field is sufficient for landingaircraft at night. It is possible to read a newspaper onoccasions by the illumination from a full moon in theArctic. Even the illumination from the stars createsvisibility far beyond what one would expect elsewhere.It is only during periods of heavy cloud cover that thenight darkness begins to approach the degree ofdarkness in lower latitudes. In lower latitudes, south of65° north latitude, there are long periods of moonlight,since the Moon may stay above the horizon for severaldays at a time.

Antarctic Weather

Many of the same peculiarities prevalent over thearctic regions are also present in the Antarctic. For

instance, the aurora borealis has its counterpart in theSouthern Hemisphere, called aurora australis. Thesame restrictions to visibility exist over the Antarcticregions as over the Arctic. Some other characteristics ofthe Antarctic regions are as follows:

Precipitation occurs in all seasons, with themaximum occurring in summer. The amount ofprecipitation decreases poleward from the coast.Temperatures are extremely low. The lowesttemperature in the world, –127°F, was recorded atVostok, Antarctica. In the winter, temperaturesdecrease from the coast to the pole, but there is somedoubt that this is true in the summer. The annualvariation of temperature as indicated by Macmurdostation shows the maximum in January and theminimum in early September. A peculiar, and to dateunexplained, feature of Antarctic temperaturevariations during the Antarctic night is the occurrenceof maximum temperatures on cloudless days in theearly hours after midnight. On cloudy days, however,the day is warmer than the night.

UNITED STATES WEATHER

The weather in the United States, with minorexceptions, is typical of all weather types within thetemperate regions of the North American, European,and Asiatic continents. The general air circulation inthe United States, as in the entire Temperate Zone of theNorthern Hemisphere, is from west to east. All closedsurface weather systems (highs and lows) tend to movewith this west-to-east circulation. However, since this isonly the average circulation and weather systems movewith the general flow, the fronts associated with themigratory lows also tend to move southward if they arecold fronts and northward if they are warm fronts.Surface low-pressure centers, with their associatedweather and frontal systems, are referred to ascyclones. Knowledge of the mean circulation in thetemperate region makes it possible to observe and plotaverage storm tracks and to forecast future movementwith a reasonable degree of accuracy.

Certain geographical and climatic conditions tendto make specific areas in the United States favorable forthe development of low-pressure systems such as westTexas, Cape Hatteras, central Idaho, and the northernportions of the Gulf of Mexico. Once a low has formed,it generally follows the same mean track as the last lowthat formed in that area. The averages, or mean paths,are referred to as storm tracks.

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These storms (lows) are outbreaks on the polarfront or the generation or regeneration of a storm alongthe trailing edge of an old front. The low pressure alongthese fronts intensifies in certain areas as the frontsurges southward ahead of a moving mass of cold polarair. Much of the weather, especially the winter weather,in the Temperate Zone is a direct result of these storms.

Air-mass weather also affects temperate climates.Air-mass weather is the name given to all weather otherthan the frontal weather in the temperate region.Air-mass weather is the net effect of local surfacecirculation, terrain, and the modifying effect ofsignificant water bodies.

There are many subdivisions of weather regions inthe United States. For the purpose of this discussion, wehave divided the continental United States into sevenregions as indicated in figure 6-9.

Northwest Pacific Coast Area

The northwest pacific coast area has moreprecipitation than any other region in North America.Its weather is primarily the result of frontal phenomena,consisting mainly of occlusions, which move in overthe coast from the area of the Aleutian low andorographic lifting of moist, stable maritime air.Predominant cloud forms are stratus and fog, which arecommon in all seasons. Rainfall is most frequent in thewinter and least frequent in the summer.

Southwest Pacific Coast Area

The southwest pacific coast area experiences aMediterranean-type climate and is distinctivelydifferent from any other North American climate. Thisclimate occurs exclusively in the Mediterranean andsouthern California in the Northern Hemisphere. In theSouthern Hemisphere, it occurs over small areas ofChile, South Africa, and southern Australia.

This climate is characterized by warm to hotsummers, tempered by sea breezes, and by mild wintersduring which the temperatures seldom go belowfreezing. Little or no rainfall occurs in the summer andonly light to moderate rain in the winter.

Cold fronts rarely penetrate the southwest pacificcoast region. The weather over this region is due to thecirculation of moist pacific air from the west beingforced up the slope of the coastal range. In the summer,air is stable, and stratus and fog result. In the winter,unstable air, which is forced over the mountain rangescauses showers or snow, showers in the mountains.

Intermountain West Central Area

The intermountain west central area includes theGreat Plains region. This region is located east of theCascade and coastal ranges, west of the MississippiValley, and north of the southwest desert area. Theclimate is generally cold and dry in the winter, andwarm and dry in the summer. Most of the region is

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WEST CENTRALINTERMOUNTAIN

CENTRAL PLAINSNORTHATLANTICCOASTAL

SOUTHEASTAND GULF STATES

S. WEST DESERTAND MOUNTAIN

S. WESTPACIFIC

COAST

N. WESTPACIFIC

COAST

AG5f 0609

Figure 6-9.—United States weather regions.

semiarid. The western mountain range, which acts as aclimatic barrier, has an extreme drying effect on the airin the westerly circulation.

Maximum rainfall occurs in the spring and is duemainly to the predominance of cyclonic storm passagesduring this season. In midwinter a cold high isgenerally centered in this region which prevents thepossibility of storm passages. Annual precipitation isnormally light.

Southwest Desert and Mountain Area

The southwest desert and mountain area includesLower California and some of southeast California aswell as the southern portions of Arizona, New Mexico,and Texas. It is an area almost completely surroundedby high mountains and is either very arid or actualdesert. Annual rainfall seldom exceeds 5 inches. Themore northerly sections have cold winters, and all partshave extremely hot summers. The chief flying hazardresults from a predominance of summer and springthunderstorms caused mainly by maritime tropical airbeing forced aloft at the mountains. For this reasonnearly all significant peaks and ranges havethundershowers building over them in the spring andsummer. The thunderstorms are generally scattered andare almost always severe; however, pilots can usuallyavoid them by circumnavigating them.

Central Plains Area

The Central Plains area includes the continentalclimate regions of the Great Plains, Mississippi Valley,and Appalachian Plateau between the RockyMountains to the west, the Appalachians to the east,and the Gulf States to the south. The western section isgenerally drier than the eastern section. Wintertimeoutbreaks and associated wave phenomena along polarfronts cause the main weather hazards. Convectiveair-mass thunderstorms, which are prevalent over thisarea in summer, also pose a threat to flying.

Frontal passages, both cold and warm, andassociated weather is common in this area.Thunderstorms are usually of convective origin and aremost violent if they have developed in maritime tropicalair. This occurs often in the spring, and tornado activitybecomes a climatic feature due to its frequency.

Southeast and Gulf States Area

The southeast and Gulf States area includes all thestates bordering on the Gulf of Mexico as well as South

Carolina and Georgia. Stagnating southbound coldfronts, rapidly moving squall lines, air-massthunderstorms, and stratus clouds occur in variouscombinations to make this area an especially complexone for the forecaster.

Frontal passages can be expected only in the latefall, winter, and early spring. A circulationphenomenon known as gulf stratus affects this area. Inthe winter, when the circulation near the surface issoutherly, the warm, moist gulf air is cooled from belowto saturation. When this occurs, fog and the gulf stratusmay form and may persist over the area for severaldays. The southerly circulation in summer causeswarm, moist air to be heated from below, andconvective thunderstorms are common. Since the air isgenerally quite moist and unstable, these storms aregenerally severe.

North Atlantic Coastal Area

The North Atlantic coastal area is an area of stormtrack convergence, and cyclonic storm activity isfrequent in winter. Moreover, the heating and additionof moisture to the air intensify these storms over theGreat Lakes. The lake effect is directly accountable forthe large amounts of snowfall often found over this areain the winter. Generally good weather prevails insummer due to the predominant influence of theBermuda high.

EUROPEAN WEATHER

Most of Europe has a relatively mild climate, whichis largely due to its oceanic exposure to the north, west,and south. The east-west orientation of the mountainsin Europe normally prevents extremely cold arctic airfrom penetrating southward to the Mediterranean. As aresult, very cold weather is limited to the northernlimits. The southern coast and Mediterranean countriesenjoy moderate temperatures year round becauserelatively warm maritime air masses move inland fromthe Atlantic and because of the moderating influence ofthe Mediterranean Sea. However, this inflow ofmaritime air also brings frequent cloudiness,considerable precipitation, and high humidity.

When continental air masses dominate, Europe issubjected to low-temperature extremes, low humidity,and clear skies much the same as North America. Thisis especially true north of the Alpine Mountains. Southof this region, somewhat normal migratory patterns doexist. The end result is relatively dry summers and wet

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winters characteristic of the western coastal region ofNorth America and Canada.

Temperatures are highest in Europe during thesummer; Athens, Greece averages 80°F; Granada,Spain 77°F; Greenwich, England 63°F; and Paris,France 65°F. Farther north, summer temperaturesaverage as much as 20 to 25 degrees less. During thewinter, the Mediterranean temperatures average in theupper 40°F to low 50°F range while the extremenorthern sections average 10°F or less. The Atlanticcoastal countries with their predominantly maritimeclimates maintain far less temperature extremesbetween summer and winter.

Precipitation in the form of rain and drizzle iscommon along the European Atlantic coast and nearthe Mediterranean Sea. Snow does occur at times inareas east of Spain and north of the Mediterranean Sea.At higher elevations inland, snow is common andfrequently abundant. Central Spain and southernRussia, by contrast, experience semiarid and aridclimates.

ASIATIC WEATHER

Asia’s climate is predominantly continental. Theonly exceptions are the heavily populated coastal areasthat have tropical and maritime climates during thesummer. This primarily continental climate results inlimited precipitation and large temperature ranges bothdaily and seasonally.

Asia is a huge continent with large expanses of landextending far northward. The Himalaya Mountainsstretch across the southern portion in an east-westdirection; mountains also parallel the eastern coast.These geographical features often contain continentalarctic and polar air inland, resulting in the most extremetemperature ranges found in the Northern Hemisphere.Northeastern Siberia’s temperatures often range from–60°F in the winter to above 60°F in the summer.Extremes range as high as 98°F and as low as –90°F.The large interior of Asia also results in extremepressure difference. In the winter, a cold high-pressurearea dominates the continent. In the summer, a warmlow-pressure area dominates the continent. Thisaccounts for the northeast winter monsoons andsouthwest summer monsoons.

In the winter the interior is dry, receiving less than 1inch of precipitation. Coastal areas under maritimeinfluence receive normal amounts (about 8 inches) ofprecipitation. In the summer, precipitation is plentifulexcept well inland. Rain is so abundant in some regions,

such as India, that the yearly rainfall average (425inches or more) is among the highest in the world.

The extreme south and southeast regions of Asiadiffer sharply from its northern neighbors. Thesesouthern regions enjoy the tropical and maritimeclimates that feature only minor seasonal temperaturevariations. Eastern Asia enjoys a climate very similar tothat found along the eastern coast of North Americafrom the Florida Keys to eastern Canada. East andSoutheast Asia, like the eastern and southeasternUnited States, is also subject to an occasional tropicalcyclone (typhoon) in the summer and in the fall.

SOUTH AMERICAN WEATHER

South America has a variety of climates but lacksthe severe weather of North America. Continental polarair does not exist here because the continent taperssharply from north to south. The larger northern area isclose to the equator and does not experience the influxof cold maritime polar air from the south. Tropicalclimates prevail over much of the continent. Yet, due tothe high Andes Mountains along the western coast,there are areas that are extremely dry and others that areextremely wet.

Northeastern Climate

The South American northeast’s climate consistsmainly of high temperature and humidity and copiousrainfall throughout the year. September is the warmestmonth with average temperatures of around 82°F.January is the coolest month with average temperaturesof around 79°F. Nighttime temperatures rarely fallbelow 65°F. Rainfall averages 87 inches annually with12 inches falling in June and just over 2 inches falling inOctober. The higher elevations of northeastern SouthAmerica have greater ranges of temperature, humidity,and precipitation; however, these ranges are notextreme.

Southern Climate

In the southern region, below 200 south latitude,South America has distinct seasons very similar tothose in the southeastern United States. These seasons,however, are reversed. The warmest month is January,which averages 74°F; July, which averages 49°F is thecoolest month. Precipitation occurs fairly evenlythroughout the year and averages 38 inches. There is nodistinct rainy season.

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Below 40° south latitude, the climate isprogressively drier and cooler. However, the extremesouthern tip of South America is characterized by yearround cold and damp climate due to a strong maritimeinfluence.

West Coast Climate

The West Coast, from northern Peru to the middleof Chile, is a desert. North and south of this desertmidsection, the climate is quite humid. The northwestcoast has a typical tropical climate with wet and dryseasons.

Below central Chile, the climate again shows atypical Southern Hemisphere seasonal reversal of thatfound in North America. The weather in this region issimilar to that found along the northwest coast of NorthAmerica. The climate is generally rainy and cool.Summer does not seem to exist as we know it. Yet,winter temperatures average above freezing.

AFRICAN WEATHER

Africa’s climate is unlike that of any othercontinent for several reasons. The most important is thefact that the entire continent is within the tropical zone.The equator bisects the continent; therefore, in the areanorth and south of the equator, the climates are similar,yet they differ because the region north of the equator ismuch larger than the southern region. Since thenorthern area is so broad in the east-west direction,maritime effects inland are minimal. Also, an extensivelow-pressure area develops inland due to extreme landmass heating. A belt of high pressure, however, with itsmaritime influences dominates the southern section,during winter and by low pressure during summer.

Another factor is the cold currents, which existalong its western shores. These currents allow an influxof cool winds and associated weather to the West Coast.The final factor involves the lack of high mountainranges common to other continents. Since there are noprominent mountain ranges, the various climate typesin Africa blend together, showing no sharp distinctions.

The most important climatic element in Africa isprecipitation. Precipitation is greatest near the equator(60 to 80 inches to over 120 inches in places). Itdecreases sharply to the north (less than 10 inches), anddecreases gradually south of the equator (average of 20to 40 inches). Because Africa is in the tropical zone, theprecipitation belt of the intertropical convergence zone(ITCZ) moves with the seasons. This belt ofprecipitation moves northward in the summer and

southward in the winter. Africa does have distinctclimatic regions. Air-mass movement and influencesallow for a division of eight climatic regions.

Northern Region

The northern region includes the great Saharadesert. The desert is a source region for drycontinental-type air masses. While maritime air maytransit the area, the air masses are highly modified andoften exhibit continental properties after movinginland. This desert region is extremely hot during theday throughout the year but is very cool at night due to alack of moisture; hence, strong radiational cooling.

Southwestern Region

The southwest region is an arid to semiarid area,which is known as the Kalahari Desert. Thetemperatures are not as extreme as in the Saharabecause the land area involved is much smaller.

North Central Region

The north central region is a semiarid area locatedalong the edge of the Sahara. While the temperaturesare similar to those of the neighboring desert (50°F inwinter to well above 80°F in summer), this areaoccasionally gets precipitation in the winter. The sourceof this precipitation for the northern area is maritime airfrom the Mediterranean; in the south, it is the spottyrainfall provided by the meandering ITCZ.

Sub-Equatorial Region

The sub-equatorial region extends toward theequator from the semiarid region in the north. Theregion is marked by seasonal rainfall associated withthe position of the ITCZ. The region is wet for about 5months (Nov-Mar), and dry during the rest of the year.Temperatures show little seasonal variation (68°F to86°F) because of the close proximity to the equator.The only exception to this temperature stability occursin the western portion which, during the winter, isoccasionally influenced by cool weather from thenorth.

Equatorial Region

The equatorial region includes the southwest tip ofnorthern Africa and the region between 5° north andsouth latitudes, extending from the western coast toLake Victoria. It is the wettest climate in all Africa.

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These areas have two distinct rainy seasons associatedwith the northward and southward movement of theITCZ. Rainfall averages over 120 inches annually insome areas. Throughout the rest of the year,precipitation remains plentiful because of the influx ofmaritime air from the west. There are no significantmountains in the region to prevent this maritime airfrom migrating inland. Temperatures are moderate yearround.

Southeast Coastal Region

The southeast coastal region has a humidsubtropical climate. This region has rainfall all year (45inches on the average) and temperatures remaingenerally moderate all year, ranging from an averagemaximum of 72°F in winter (July) to 89°F in summer(January).

Southeastern Interior Region

This region has a wet-and-dry type of maritimeclimate; however, it is considered temperate because ofthe lower temperatures common to the higher elevation.

AUSTRALIA AND NEW ZEALAND WEATHER

Australia has a generally mild climate with coolwinters in the south and warm winters in the north.Summers are warm along the coasts and generally hotin the interior. Freezing temperatures are infrequent.Australia’s climatic zones are relatively uncomplicateddue to the lack of high mountain ranges.

The northern third of Australia is located within thetropical zone. The region has a rainy season that runsfrom January to April. Annual precipitation is greatest(nearly 100 inches) in the extreme north and tapers offto the south and inland toward the semiarid interior. Theinterior, along the Tropic of Capricorn, is very hot anddry in the summer with average maximum

temperatures at or above 90°F. In the winter, averagemaximum temperatures in some areas drop to 68°F.

The southern two-thirds of Australia is under theinfluence of the high-pressure belts of the SouthernHemisphere as well as of the migratory lows foundfarther southward. The southwest and southernportions of this region have rainy winters andnear-drought conditions in the summer similar to theMediterranean climate. Temperatures average 80°F inJanuary and 55°F in July. The climate of the southeastcorner is very similar to the southwest region except itexperiences a shorter winter and less annualprecipitation.

New Zealand is located southeast of Australia. It isa very narrow country with a southwest to northeastorientation and is exposed to the prevailing westerlies.Therefore, the climate is moderate and predominantlymaritime with moderate precipitation occurringthroughout the year. The northern part of New Zealandhas a subtropical climate; however, winter frost andoccasional snow can occur at locations farther south inhighland areas. Fog is often widespread and verypersistent over much of the country in advance ofapproaching frontal systems. Precipitation averages 49inches in the northern half of the country and up to 170inches in the southern half. Temperatures range from anannual average of 59°F in the north and 55°F in thecentral region to 50°F in the south.

REVIEW QUESTIONS

Q6-17. What are the two climatic extremes that relateto water and land distribution over the earth?

Q6-18. What region in the United States experiencesMediterranean type climate?

Q6-19. What is the major cause of the winter andsummer monsoons near Asia?

Q6-20. Why does South America lack the severeweather that is common in North America?

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APPENDIX I

GLOSSARY

ABSOLUTE INSTABILITY—The state of a columnof air in the atmosphere when it has asuperadiabatic lapse rate of temperature. An airparcel displaced vertically would be accelerated inthe direction of the displacement.

ABSOLUTE STABILITY—The state of a column ofair in the atmosphere when its lapse rate oftemperature is less than the saturation adiabaticlapse rate. An air parcel will be denser than itsenvironment and tend to sink back to its level oforigin.

ADVECTION—The horizontal transport of anatmospheric property solely by the mass motion(velocity field) of the atmosphere.

ADVECTION FOG—Fog caused by the advection ofmoist air over a cold surface, and the consequentcooling of that air to below its dew point.

AIR MASS—A widespread body of air that isapproximately homogeneous in its horizontalextent, with reference to temperature and moisture.

ANABATIC WIND—An upslope wind; usuallyapplied only when the wind is blowing up a hill ormountain as the result of surface heating.

ANTARCTIC FRONT—The semi permanent, semicontinuous front between the Antarctic air of theAntarctic Continent and the polar air of thesouthern oceans; generally comparable to the arcticfront of the Northern Hemisphere.

ANTICYCLOGENESIS—The strengthening ordevelopment of an anticyclonic circulation in theatmosphere.

ANTICYCLOLYSIS—The weakening of ananticyclonic circulation in the atmosphere.

ANTICYCLONE—A closed circulation in theatmosphere that has a clockwise rotation in theNorthern Hemisphere and a counterclockwiserotation in the Southern Hemisphere. Usedinter-changeably with high.

ANTICYCLONIC—Refers to the rotation pattern ofanticyclones. See ANTICYCLONE.

ARCTIC FRONT—The semi permanent, semicontinuous front between the deep, cold arctic airand the shallower, basically less cold polar air ofnorthern latitudes; generally comparable to theAntarctic front of the Southern Hemisphere.

AUTOCONVECTIVE LAPSE RATE—Thetemperature lapse rate in an atmosphere wheredensity is constant with height.

BACKING—A change in wind direction in acounterclockwise manner in the NorthernHemisphere and a clockwise manner in theSouthern Hemisphere.

BLOCKING HIGH—An anticyclone that re-mainsstationary or moves slowly westward so as toeffectively block the movement of migratorycyclones across its latitudes.

BUYS BALLOT’S LAW—The law describing therelationship of horizontal wind direction topressure: In the Northern Hemisphere, with yourback to the wind, the lowest pressure will be to yourleft; in the Southern Hemisphere, the reverse istrue.

CENTER OF ACTION—Any one of the semipermanent high or low-pressure systems.

CENTRAL PRESSURE—The atmospheric pressureat the center of a high or low; the highest pressure ina high, the lowest in a low.

CHROMOSPHERE—A thin layer of relativelytransparent gases above the photosphere of the Sun.

CLOSED HIGH—A high that is completely encircledby an isobar or contour line.

CLOSED LOW—A low that is completely encircledby an isobar or contour line.

COLD-CORE HIGH—Any high that is generallycharacterized by colder air near its center thanaround its periphery at a given level in theatmosphere.

COLD-CORE LOW—Any low that is generallycharacterized by colder air near its center thanaround its periphery at a given level in theatmosphere.

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CONDENSATION—The physical process by which avapor becomes a liquid or solid.

CONDITIONAL INSTABILITY—The state of acolumn of air in the atmosphere when itstemperature lapse rate is less than the dry adiabaticlapse rate but greater than the saturation adiabaticlapse rate.

CONVECTION—Atmospheric motions that arepredominantly vertical, resulting in the verticaltransport and mixing of atmospheric properties.

CORONA—(1) A set of one or more prismaticallycolored rings of small radii, concentricallysurrounding the disk of the Sun, Moon, or otherluminary when veiled by a thin cloud. A coronamaybe distinguished from the relatively common22° halo by its color sequence, which is from blueinside to red outside, the reverse of that of the 22°halo. Diffraction and reflection of light from waterdroplets produce coronas. (2) The pearly outerenvelope of the Sun.

COUNTERRADIATION—(also called backradiation) The downward flow of atmosphericradiation passing through a given level surface,usually taken as Earth’s surface. It is the principalfactor in the GREENHOUSE EFFECT.

CUT-OFF HIGH—A warm high displaced and lyingpoleward of the basic westerly current.

CUT-OFF LOW—A cold low displaced and lyingequatorward of the basic westerly current.

CYCLOGENESIS—Any development orstrengthening of cyclonic circulation in theatmosphere. The initial appearance of a low ortrough, as well as the intensification of an existingcyclonic flow.

CYCLOLYSIS—Any weakening of cycloniccirculation in the atmosphere.

CYCLONIC—A counterclockwise rotation in theNorthern Hemisphere and a clockwise rotation inthe Southern Hemisphere.

DISPERSION—The process in which radiation isseparated into its component wavelengths. Itresults when an optical process, such as diffraction,refraction, or scattering, varies according towavelength. All of the coloration displayed byatmospheric optical phenomena are the result ofdispersion.

DOLDRUMS—A nautical term for the equatorialtrough, with special reference to the light andvariable nature of the winds.

DOWNWIND—The direction toward which the windis blowing; with the wind.

DRY AIR—In atmospheric thermodynamics andchemistry, air that contains no water vapor.

ELECTROMAGNETIC WAVES—Disturbances inelectric and magnetic fields in space or in materialmedia, resulting in the propagation ofelectromagnetic energy (radiation).

EQUINOX—(1) Either of the two points ofintersection of the Sun’s apparent annual path andthe plane of Earth’s equator. (2) Popularly, the timeat which the Sun passes directly above the equator;the “time of the equinox.” In the NorthernHemisphere, the vernal equinox falls on or about 21March, and the autumnal equinox on or about 22September. These dates are reversed in theSouthern Hemisphere.

EVAPORATION—The physical process by which aliquid or solid is transformed to the gaseous state.

FRONT—The interface or transition zone betweentwo air masses of different density. Sincetemperature distribution is the most importantregulator of atmospheric density, a front almostinvariably separates air masses of differenttemperature.

FRONTAL INVERSION—A temperature inversionin the atmosphere, encountered upon verticalascent through a sloping front.

FRONTAL SURFACE—Refers specifically to thewarmer side of the frontal zone.

FRONTAL SYSTEM—Simply, a system of fronts asthey appear on a synoptic chart. This is used for (a)a continuous front and its characteristics along itsentire extent, including its warm, cold, stationary,and occluded sectors, its variations of intensity, andany frontal cyclones along it; and (b) theorientation and nature of the fronts within thecirculation of a frontal cyclone.

FRONTAL ZONE—The transition zone between twoadjacent air masses of different densities boundedby a frontal surface.

FRONTOGENESIS—The initial formation of a frontor frontal zone.

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FRONTOLYSIS—The dissipation of a front or frontalzone.

GENERAL CIRCULATION—(also called planetarycirculation) In its broadest sense, the completestatistical description of atmospheric motions overEarth.

GEOSTROPHIC FLOW—A form of gra-dient flowwhere the Coriolis force exactly balances thehorizontal pressure force.

GEOSTROPHIC WIND The wind velocity for whichthe Coriolis acceleration exactly balances thehorizontal pressure force. The geostrophic wind isdirected along the contour lines on aconstant-pressure surface (or along the isobars in ageopotential surface) with low pressure to the leftin the Northern Hemisphere and to the right in theSouthern Hemisphere.

GEOSTROPHIC-WIND SCALE—A graphicaldevice used for the determination of the speed ofthe geostrophic wind from the isobar or contourline spacing on a synoptic chart.

GRADIENT—The space rate of decrease of afunction. It is often used to denote the magnitude ofpressure change in the horizontal pressure field.

GRADIENT WIND—Any horizontal wind velocitytangent to the contour line of a constant-pressuresurface (or the isobar of a geopotential surface) atthe point in question. At such points, where thewind is gradient, the Coriolis acceleration andcentripetal acceleration together exactly balancethe horizontal pressure force.

GRAVITY WIND—(also called drainage wind;sometimes called katabatic wind) A wind (orcomponent thereof) directed down the slope of anincline and caused by greater air density near theslope (caused by surface cooling) than at the samelevels some distance horizontally from the slope.

GREENHOUSE EFFECT—The heating effectexerted by the atmosphere upon Earth by virtue ofthe fact that the atmosphere (mainly, its watervapor) absorbs and re-emits infrared radiation. Indetail: The shorter wavelengths of insolation aretransmitted rather freely through the atmosphere tobe absorbed at Earth’s surface. Earth then re-emitsthis as long-wave (infrared) terrestrial radiation, aportion of which is absorbed by the atmosphere andagain emitted as atmospheric radiation. The watervapor (cloud cover) acts in the same way as theglass panes of a greenhouse; the heat gained during

the day is trapped beneath the cloud cover, and thecounter-radiation adds to the warming of Earth.

HALO—Any one of a large class of atmosphericoptical phenomena (luminous meteors) that appearas colored or whitish rings and arcs about the Sunor Moon when seen through an ice crystal cloud orin a sky filled with falling ice crystals. The halosexperiencing prismatic coloration are produced byrefraction of light by the ice crystals, and thoseexhibiting only whitish luminosity are produced byreflection from the crystal faces.

HEAT BALANCE—The equilibrium, which exists onthe average, between the radiation received byEarth and its atmosphere and that emitted by Earthand its atmosphere.

HEATING DEGREE-DAY—A form of degree-dayused as an indication of fuel consumption; inUnited States usage, one heating degree-day isgiven for each degree that the daily meantemperature departs below the base of 65°F.

HEAT TRANSFER—The transfer or ex-change ofheat by radiation, conduction, or convection in afluid and/or between the fluid and its surroundings.The three processes occur simultaneously in theatmosphere, and it is often difficult to assess thecontributions of their various effects.

HIGH—An “area of high pressure,” refer-ring to amaximum of atmospheric pressure in twodimensions (closed isobars) on the synoptic surfacechart, or a maximum of height (closed contours) onthe constant-pressure chart. Highs are associatedwith anticyclonic circulations, and the term is usedinterchangeably with anticyclone.

HORSE LATITUDES—The belts of latitude over theoceans at approximately 30 to 35 degrees north andsouth where winds are predominantly calm or verylight and the weather is hot and dry.

ICELANDIC LOW—The low-pressure centerlocated near Iceland (mainly between Iceland andsouthern Greenland) on mean charts of sea-levelpressure. It is a principal center of action in theatmospheric circulation of the NorthernHemisphere.

INACTIVE FRONT—(or passive front) A frontor portion thereof that produces very littlecloudiness and no precipitation, as opposed to anactive front.

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INFERIOR MIRAGE—A spurious image of anobject formed below the true position of that objectby abnormal refractive conditions along the line ofsight; one of the most common of all types ofmirage, and the opposite of a superior mirage.

INSOLATION—(contracted from incoming solarradiation) In general, solar radiation received atEarth’s surface.

INSTABILITY—A property of the steady state of asystem such that certain disturbances orperturbations introduced into the steady state willincrease in magnitude, the maximum perturbationamplitude always remaining larger than the initialamplitude.

INSTABILITY LINE—Any non-frontal line or bandof convective activity in the atmosphere.

INVERSION—The departure from the usual decreaseor increase with altitude of the value of anatmospheric property. The layer through which thisdeparture occurs is known as the inversion layer,and the lowest altitude at which the departure isfound is known as the base of the inversion. Theterm is almost always used in reference totemperature, but may be applied to moisture andprecipitation.

KATABATIC WIND—Any wind blowing down anincline; the opposite of anabatic wind. If the windis warm, it is called a foehn; if cold, it may be a fallor gravity wind.

KINETIC ENERGY—The energy that a bodypossesses as a consequence of its motion, definedas the product of one-half of its mass and the squareof its speed, 1/2mv squared.

LAND BREEZE—A coastal breeze blowing fromland to sea, caused by the temperature differencewhen the sea surface is warmer than the adjacentland.

LAPSE RATE—The decrease of an atmosphericvariable with height, the variable beingtemperature, unless otherwise specified.

LATERAL MIRAGE—A very rare type of mirage inwhich the apparent position of an object appearsdisplaced to one side of its true position.

LIGHT—Visible radiation (about 0.4 to 0.7 micron inwavelength) considered in terms of its luminousefficiency.

LOOMING—A mirage effect produced bygreater-than-normal refraction in the loweratmosphere, thus permitting objects to be seen thatare usually below the horizon.

LOW—An “area of low pressure,” refer-ring to aminimum of atmospheric pressure in twodimensions (closed isobars) on a constant-heightchart or a minimum of height (closed contours) ona constant-pressure chart. Lows are associated withcyclonic circulations, and the term is usedinterchangeably with cyclone.

MACROCLIMATE—The general large-scale climateof a large area or country, as distinguished from themesoclimate and microclimate.

MAGNETIC NORTH—At any point on Earth’ssurface, the horizontal direction of the Earth’smagnetic lines of force (direction of a magneticmeridian) toward the north magnetic pole, i.e., adirection indicated by the needle of a magneticcompass. Because of the wide use of the magneticcompass, magnetic north, rather than TRUENORTH, is the common 0° (or 360°) reference inmuch of navigational practice, including thedesignation of airport runway alignment.

MARITIME AIR—A type of air whosecharacteristics are developed over an extensivewater surface and which, therefore, has the basicmaritime quality of high moisture content in at leastit's lower levels.

MEAN SEA LEVEL—The average height of the seasurface, based upon hourly observation of tideheight on the open coast or in adjacent waterswhich have free access to the sea. In the UnitedStates, mean sea level is defined as the averageheight of the surface of the sea for all stages of thetide over a 19-year period.

MESOCLIMATE—The climate of small areas ofEarth’s surface that may not be representative of thegeneral climate of the district. The placesconsidered in mesoclimatology include smallvalleys, “frost hollows,” forest clearings, and openspaces in towns, all of which may have extremes oftemperature differing by many degrees from thoseof adjacent areas. The mesoclimate is intermediatein scale between the microclimate andmicroclimate.

MESOPAUSE—The top of the mesosphere. Thiscorresponds to the level of minimum temperature at70 to 80 km.

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MESOSPHERE—The atmospheric shell betweenabout 20 km and about 70 or 80 km, ex-tendingfrom the top of the stratosphere to the uppertemperature minimum (the menopause). A broadtemperature maximum at about 50 kmcharacterizes it, except possibly over the winterpolar regions.

METEOROLOGY—The study dealing with thephenomena of the atmosphere. This includes notonly the physics, chemistry, and dynamics of theatmosphere, but is extended to include many of thedirect effects of the atmosphere upon Earth’ssurface, the oceans, and life in general.

MICROCLIMATE—The fine climate structure of theair space that extends from the very surface ofEarth to a height where the effects of the immediatecharacter of the underlying surface no longer canbe distinguished from the general local climate(mesoclimate or microclimate).

MIGRATORY—Moving; commonly applied topressure systems embedded in the wastrels and,therefore, moving in a general west-to-eastdirection.

MILLBRAE—(abbreviated MB) A pressure unit of1,000 dynes per centimeter, convenient forreporting atmospheric pressures.

MIRAGE—A refraction phenomenon wherein animage of some object is made to appear displacedfrom its true position.

MOISTURE—A general term usually refer-ring to thewater vapor content of the atmosphere, or to thetotal water substance (gas, liquid, and solid)present in a given volume of air.

MONSOON—A name for seasonal wind. It was firstapplied to the winds over the Arabian Sea, whichblow for 6 months from the northeast and 6 monthsfrom the southwest, but it has been extended tosimilar winds in other parts of the world.

MONSOON CLIMATE—The type of climate that isfound in regions subject to monsoons. It is bestdeveloped on the fringes of the tropics.

NEUTRAL STABILITY—The state of anunsaturated or saturated column. of air in theatmosphere when its environmental lapse rate oftemperature is equal to the dry-adiabatic lapse rateor the saturation-adiabatic lapse rate, respectively.Under such conditions a parcel of air displacedvertically will experience no buoyant acceleration.

OCCLUDED FRONT—(commonly calledocclusion; also called frontal occlusion) Acomposite of two fronts, formed as a cold frontovertakes a warm front or quasi-stationary front.This is a common process in the late stages of wavecyclone development, but it is not limited tooccurrence within a wave cyclone.

OCCLUSION—Same as OCCLUDED FRONT.

OROGRAPHIC LIFTING—The lifting of an aircurrent caused by its passage up and overmountains.

OVERRUNNING—A condition existing when an airmass is in motion aloft above another air mass ofgreater density at the surface. This term is usuallyapplied in the case of warm air ascending thesurface of a warm or quasi-stationary front.

PARTIAL PRESSURE—The pressure of a singlecomponent of a gaseous mixture, according toDalton’s Law.

PERTURBATION—Any departure introduced intoan assumed steady state of a system. In synopticmeteorology, the term most often refers to anydeparture from zonal flow within the major zonalcurrents of the atmosphere. It is especially appliedto the wave-like disturbances within the tropicaleasterlies.

PHOTOSPHERE—The intensely bright portion ofthe Sun visible to the unaided eye. It is a shell a fewhundred miles in thickness marking the boundarybetween the dense interior gases of the Sun and themore diffuse cooler gases in the outer portions ofthe Sun.

PLANETARY BOUNDARY LAYER—(also calledfriction layer or atmospheric boundary layer) Thatlayer of the atmosphere from Earth’s surface to thegeostrophic wind level, including therefore, thesurface boundary layer and the Eckman layer.

PLANETARY CIRCULATION—The sys-tem oflarge-scale disturbances in the troposphere whenviewed on a hemispheric or worldwide scale. Sameas GENERAL CIRCULATION.

POLAR AIR—A type of air whose characteristics aredeveloped over high latitudes, especially within thesubpolar highs. Continental polar air (cP) has lowsurface temperature, low moisture content, and,especially in its source regions, great stability in thelower layers. It is shallow in comparison witharctic air.

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POLAR EASTERLIES—The rather shallow anddiffuse body of easterly winds located poleward ofthe subpolar low-pressure belt. In the mean in theNorthern Hemisphere, these easterlies exist to anappreciable extent only north of the Aleutian lowand Icelandic low.

POLAR FRONT—According to the polar-fronttheory, the semi permanent, semi continuous frontseparating air masses of tropical and polar origin.This is the major front in terms of air mass contrastand susceptibility to cyclonic disturbance.

POLAR-FRONT THEORY—A theory originated bythe Scandinavian school of meteorologistswhereby a polar front, separating air masses ofpolar and tropical origin, gives rise to cyclonicdisturbances, which intensify and travel along thefront, passing through various phases of acharacteristic life history.

POLAR OUTBREAK—The movement of a cold airmass from its source region; almost invariablyapplied to a vigorous equatorward thrust of coldpolar air, a rapid equatorward movement of thepolar front.

POLAR TROUGH—In tropical meteorology, a wavetrough in the wastrels having sufficient amplitudeto reach the tropics in the upper air. At the surface itis reflected as a trough in the tropical easterlies, butat moderate elevations it is characterized bywesterly winds. It moves generally from west toeast and is accompanied by considerablecloudiness at all levels. Cumulus congests andcumulonimbus clouds are usually found in andaround the trough lines. The early and late seasonhurricanes of the western Caribbean frequentlyform in polar troughs.

POTENTIAL ENERGY—The energy that a bodypossesses as a consequence of its position in thefield of gravity; numerically equal to the workrequired to bring the body from an arbitrarystandard level, usually taken as mean sea level, toits given position.

PRE-FRONTAL SQUALL LINE—A squall line orinstability line located in the warm sector of a wavecyclone, about 50 to 300 miles in advance of thecold front, usually oriented roughly parallel to thecold front and moving in about the same manner asthe cold front.

PRESSURE CENTER—On a synoptic chart, a pointof local minimum or maximum pressure; the centerof a low or high. It is also a center of cyclonic oranticyclonic circulation.

PRESSURE GRADIENT—The rate of decrease(gradient) of pressure in space at a fixed time. Theterm is sometimes loosely used to denote simplythe magnitude of the gradient of the pressure field.

PRESSURE GRADIENT FORCE—The force dueto differences of pressure within a fluid mass. Inmeteorological literature the term usually refersonly to horizontal pressure force.

PRESSURE PATTERN—The general geo-metriccharacteristics of atmospheric pressure distributionas revealed by isobars on a constant-height chart,usually the surface chart.

PRESSURE SYSTEM—An individual cyclonic scalefeature of atmospheric circulation; commonly usedto denote either a high or low, less frequently aridge or trough.

PRIMARY CIRCULATION—The prevailingfundamental atmospheric circulation on aplanetary scale that must exist in response to (a)radiation differences with latitude, (b) the rotationof Earth, and (c) the particular distribution of landand oceans; and which is required from theviewpoint of conservation of energy.

PROMINENCE—A filament-like Protuberance fromthe chromosphere of the Sun.

QUASI-STATIONARY FRONT—(Commonlycalled stationary front) A front that is stationary ornearly so. Conventionally, a front that is moving ata speed less than about 5 knots is generallyconsidered to be quasi-stationary. In synoptic chartanalysis, a quasi-stationary front is one that has notmoved appreciably from its position on the last(previous) synoptic chart (3 or 6 hours before).

RADIATION—(1) The process by whichelectromagnetic radiation is propagated throughfree space by virtue of joint undulatory variationsin the electric and magnetic fields in space. Thisconcept is to be distinguished from convection andconduction. (2) The process by which energy ispropagated through any medium by virtue of thewave motion of that medium, as in the propagationof sound waves through the atmosphere, or oceanwaves along the water surface.

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RADIATIONAL COOLING—The cooling ofEarth’s surface and adjacent air, accomplished(mainly at night) whenever Earth’s surface suffers anet loss of heat due to terrestrial radiation.

RADIATION FOG—A major type of fog, producedover a land area when radiational cooling reducesthe air temperature to or below its dew point.

RAINBOW—Any one of a family of circular arcsconsisting of concentric colored bands, arrangedfrom red on the inside to blue on the outside, whichmay be seenona‘‘sheet” of water drops (rain, fog,or spray).

REDUCTION—In general, the transforma-tion ofdata from a “raw” form to some usable form. Inmeteorology, this often refers to the con-version ofthe observed value of an element to the value that ittheoretically would have at some selected orstandard level, usually mean sea level. The mostcommon reduction in observing is that of stationpressure to sea level pressure.

REFLECTION—The process whereby a surface ofdiscontinuity turns back a portion of the incidentradiation into the medium through which theradiation approached.

REFLECTIVITY—A measure of the fraction ofradiation reflected by a given surface; defined asthe ratio of the radiant energy reflected to the totalthat is incident upon that surface. The reflectivity ofa given surface for a specified broad spectral range,such as the visible spectrum or the solar spectrum,is referred to as albino.

REFRACTION—The process in which the directionof energy propagation is changed as the result of achange in density within the propagating medium,or as the energy passes through the interfacerepresenting a density discontinuity between twomedia.

RESULTANT WIND—In climatology, the vectorialaverage of all wind directions and speeds for agiven level at a given place for a certain period, as amonth. It is obtained by resolving each windobservation into components from north and east,summing over the given period, obtaining theaverages, and reconverting the average componentsinto a single vector.

SCATTERING—The process by which smallparticles suspended in a medium of a differentindex of refraction diffuse a portion of the incidentradiation in all directions.

SEA BREEZE—A coastal local wind that blows fromsea to land, caused by the temperature differencewhen the sea surface is colder than the adjacentland. Therefore, it usually blows on relatively calm,sunny, summer days; and alternates with theoppositely directed, usually weaker, night landbreeze.

SEA-BREEZE FRONT—A sea breeze that forms outover the water, moves slowly toward the coast andthen moves inland quite suddenly. Often associatedwith the passage of this type of sea breeze areshowers, a sharp wind shift from seaward tolandward, and a sudden drop in temperature. Theleading edge of such a sea breeze is sometimescalled the sea breeze front.

SEA LEVEL—The height or level of the sea surface.

SEASON—A division of the year according to someregularly recurrent phenomena, usuallyastronomical or climatic. Astronomical seasonsextend from an equinox to the next solstice (or viceversa). Climatic seasons are often based onprecipitation (rainy and dry seasons).

SECONDARY CIRCULATION—Atmos-phericcirculation features of synoptic scale.

SECONDARY FRONT—A front that forms within abaroclinic cold air mass that itself is separated froma warm air mass by a primary frontal system. Themost common type is the secondary cold front.

SHEAR—The variation (usually the directionalderivative) of a vector field along a given directionin space. The most frequent context for this conceptis wind shear.

SHEAR LINE—A line or narrow zone across whichthere is an abrupt change in the horizontal windcomponent parallel to this line; a line of maximumhorizontal wind shear.

SHORT-WAVE RADIATION—A term used looselyto distinguish radiation in the visible andnear-visible portions of the electromagneticspectrum (roughly 0.4 to 1.0 micron in wavelength)from long-wave radiation.

SIBERIAN HIGH—A cold-core high--pressure areathat forms over Siberia in winter, and which isparticularly apparent on mean charts of sea-levelpressure.

SINGULAR POINT—In a flow field, a point at whichthe direction of flow is not uniquely determined,hence a point of zero speed, e.g., a col.

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SMOOTHING—An averaging of data in space ortime, designed to compensate for random errors orfluctuations of a scale smaller than that presumedsignificant to the problem at hand; the analysis of asea-level weather map smoothes the pressure fieldon a space-scale more or less systematicallydetermined by the analyst by taking each pressureas representative not of a point but of an area aboutthe point.

SOLAR CONSTANT—The rate at which solarradiation is received outside Earth’s at-mosphereon a surface normal to the incident radiation, and atEarth’s mean distance from the Sun.

SOLSTICE—(1) Either of two points on the Sun’sapparent annual path where it is displaced farthest,north or south, from Earth’s equator. The Tropic ofCancer (north) and Tropic of Capricorn (south) aredefined as the parallels of latitude that lie directlybeneath a solstice. (2) Popularly, the time at whichthe Sun is farthest north or south; the “time of thesolstice.” In the Northern Hemisphere, the summersolstice falls on or about 21 June, and the wintersolstice on or about 22 December. The reverse istrue in the southern latitudes.

SPECIFIC HEAT—The heat capacity of a system perunit mass. That is, the ratio of the heat absorbed (orreleased) by unit mass of the system to thecorresponding temperature rise (or fall).

SPECIFIC HUMIDITY—In moist air, the ratio of themass of water vapor to the total mass of the system.For many purposes it may be approximated by themixing ratio.

SQUALL LINE—Any non-frontal line or narrowband of active thunderstorms.

STANDARD ATMOSPHERE—A hypotheticalvertical distribution of atmospheric temperature,pressure, and density which, by internationalagreement, is taken to be representative of theatmosphere for purposes of pressure altimetercalibrations, aircraft performance calculations,aircraft and missile design, ballistic tables, etc. Theair is assumed to obey the perfect gas law and thehydrostatic equation, which, taken together, relatetemperature, pressure, and density variations in thevertical. It is further assumed that the air containsno water vapor and that the acceleration of gravitydoes not change with height.

STRATOSPHERE—The atmospheric shell above thetroposphere and below the mesosphere. It extends,therefore, from the tropopause to the height wherethe temperature begins to increase in the 20- to25-km region.

SUBLIMATION—The transition of a substance fromthe solid phase directly to the vapor phase, or viceversa, without passing through an intermediateliquid phase.

SUBSIDENCE—A descending motion of air in theatmosphere, usually with the implication that thecondition extends over a rather broad area.

SUBSIDENCE INVERSION—A temperatureinversion produced by the adiabatic warming of alayer of subsiding air. Vertical mixing of the airlayer below the inversion enhances this inversion.

SUBTROPICAL HIGH—One of the semi-permanenthighs of the subtropical high-pressure belt. Theyappear as centers of action on mean charts ofsurface pressure. They lie over oceans and are bestdeveloped in summer.

SUBTROPICAL HIGH-PRESSURE BELT—Oneof the two belts of high atmospheric pressure thatare centered, in the mean, near 30°N and 30°Slatitudes.

SUNSPOT—A relatively dark area on the surface ofthe Sun, consisting of a dark central umbrasurrounded by a penumbra, which is intermediatein brightness between the umbra and thesurrounding photosphere.

SUPERADIABATIC LAPSE RATE—Anenvironmental lapse rate greater than thedry-adiabatic lapse rate, such that potentialtemperature decreases with height.

SUPERCOOLING—The reduction of temperature ofany liquid below the melting point of thatsubstance’s solid phase, that is, cooling beyond itsnominal freezing point.

SUPERIOR AIR—An exceptionally dry mass of airformed by subsidence and usually found aloft butoccasionally reaching Earth’s surface duringextreme subsidence processes.

SUPERIOR MIRAGE—A spurious image of anobject formed above its true position by abnormalrefractive conditions; opposite of inferior mirage.

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SUPERSATURATION—The condition existing in agiven portion of the atmosphere (or other space)when the relative humidity is greater than 100percent, that is, when it contains more water vaporthan is needed to produce saturation with respect toa plane surface of pure water or pure ice.

SURFACE BOUNDARY LAYER—That thin layer ofair adjacent to Earth’s surface, extending up to theso-called anemometer level (the height above theground at which an anemometer is exposed;usually 10 meters to 100 meters).

SURFACE CHART—(also called surface map,sea-level chart, sea-level pressure chart) Ananalyzed synoptic chart of surface weatherobservations. It shows the distribution of sea levelpressure (positions of highs, lows, ridges, andtroughs) and the location and nature of fronts andair masses. Often added to this are symbols ofoccurring weather phenomena, analysis of pressuretendency (isallobars), indications of the movementof pressure systems and fronts, and perhaps others,depending on the use of the chart.

SURFACE INVERSION—A temperature in-versionbased at Earth’s surface; that is, an in-crease oftemperature with height beginning at ground level.

SURFACE OF DISCONTINUITY—A surfaceseparating two fluids across which there is adiscontinuity of some fluid property, such asdensity, velocity, etc., or of some derivative of oneof these properties in a direction normal to theinterface. An atmospheric front is representedideally by a surface of discontinuity of velocity,density, temperature, and pressure gradient; thetropopause is represented ideally by a surface ofdiscontinuity of, for example, the derivatives: lapserate and wind shear.

SYNOPTIC—In general, pertaining to or affording anoverall view. In meteorology, this term has becomesomewhat specialized in referring to the use ofmeteorological data obtained simultaneously overa wide area for the purpose of presenting acomprehensive and nearly instantaneous picture ofthe state of the atmosphere.

SYNOPTIC CHART—In meteorology, any chart ormap on which data and analyses are presented thatdescribe the state of the atmosphere over a largearea at a given moment in time.

SYNOPTIC SCALE—The scale of the migratoryhigh- and low-pressure systems (or cyclonic

waves) of the lower troposphere, with wavelengthsof 1,000 to 2,500 km.

SYNOPTIC SITUATION—The general state of theatmosphere as described by the major features ofsynoptic charts.

TEMPERATURE INVERSION—A layer in whichtemperature increases with altitude.

TERTIARY CIRCULATION—The generally small,localized atmospheric circulations. They arerepresented by such phenomena as the local winds,thunderstorms, and tornadoes.

THERMAL—(1) Pertaining to temperature or heat.(2) A relatively-small-scale rising current of airproduced when the atmosphere is heated enoughlocally by Earth’s surface to produce absoluteinstability in its lower layers. The use of this term isusually reserved to denote those currents either toosmall and/or too dry to produce convective clouds;thus, thermals are a common source of low-levelclear-air turbulence.

THERMAL GRADIENT—The rate of variation oftemperature either horizontally or vertically.

THERMAL HIGH—An area of high pressureresulting from the cooling of air by a coldunderlying surface, and remaining relativelystationary over the cold surface.

THERMAL LOW—An area of low atmosphericpressure resulting from high temperatures causedby intense surface heating. They are stationary witha generally weak and diffuse cyclonic circulation.They are non-frontal.

THERMOSPHERE—The atmospheric shellextending from the top of the mesosphere to outerspace. It is a region of more or less steadilyincreasing temperature with height, starting at 70or 80 km.

TORNADO—A violently rotating column of air,pendant from a cumulonimbus cloud, and nearlyalways observable as a “funnel cloud” or tuba.

TRIPLE POINT—Term commonly used to denote theapex of an occlusion.

TROPICAL AIR—A type of air whose characteristicsare developed over low latitudes. Maritime tropicalair (mT) is produced over the tropical andsubtropical seas, while continental tropical air isproduced over subtropical arid regions.

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TROPOPAUSE—The boundary between thetroposphere and stratosphere, usually characterizedby an abrupt change of lapse rate.

TROPOSPHERE—That portion of Earth’satmosphere extending from the surface to thetropopause; that is, the lowest 10 to 20 km of theatmosphere.

TROUGH—An elongated area of low atmosphericpressure; the opposite of a ridge.

TRUE NORTH—The direction from any point onEarth’s surface toward the geographic North Pole;the northerly direction along any projection ofEarth’s axis upon Earth’s surface, for example,along a longitude line. Except for much ofnavigational practice (which uses magnetic north),true north is the universal 0° (or 360°, mappingreference).

UPSTREAM—In the direction from which a fluid isflowing.

UPWIND—In the direction from which the wind isblowing.

VECTOR—Any quantity, such as force, velocity, oracceleration, that has both magnitude and directionat each point in space, as opposed to a scalar, whichhas magnitude only. Geometrically, it isrepresented by an arrow of length proportional toits magnitude, pointing in the assigned direction.

VEERING—A change in wind direction in aclockwise sense in the Northern Hemisphere andcounterclockwise direction in the SouthernHemisphere.

VERNAL EQUINOX—For either hemisphere, theequinox at which the Sun’s most direct raysapproach from the opposite hemisphere. Innorthern latitudes, this occurs approximately on 21March; the Sun’s most direct rays are centered overthe equator and moving north.

VIRTUAL TEMPERATURE—In a system of moistair, the temperature of dry air having the samedensity and pressure as the moist air. It is alwaysgreater than the actual temperature.

WARM-CORE HIGH—At a given level in theatmosphere, any high that is warmer at its centerthan at its periphery.

WARM-CORE LOW—At a given level in theatmosphere, any low that is warmer at its centerthan at its periphery.

WARM FRONT—Any non-occluded front or portionthereof that moves in such a way that warmer airreplaces colder air.

WARM SECTOR—That area within the circulation ofa wave cyclone where the warm air is found. It liesbetween the cold front and the warm front of thestorm; and, in the typical case, the warm sectorcontinually diminishes in size and ultimatelydisappears (at the surface) as the result ofocclusion.

WAVE CYCLONE—A cyclone that forms and movesalong a front.

WAVE THEORY OF CYCLONES—A theory ofcyclone development based upon the principles ofwave formation on an interface between two fluids.In the atmosphere, a front is taken as such aninterface.

WEATHER—The state of the atmosphere, mainlywith respect to its effect upon life and humanactivities.

WESTERLIES—(also known as circumpolarwesterlies, counter-trades, middle-latitudewesterlies, midlatitude westerlies, polar westerlies,subpolar westlies, subtropical westerlies,temperate westerlies, zonal westerlies, and zonalwinds) Specifically, the dominant west-to-eastmotion of the atmosphere, centered over the middlelatitudes of both hemispheres. At the surface, thewesterly belt extends, on the average, from about35° to 65° latitude. At upper levels, the westerliesextend farther equatorward and poleward. Theequatorward boundary is fairly well defined by thesubtropical high-pressure belt; the polewardboundary is quite diffuse and variable.

WIND-CHILL FACTOR—The cooling effect of anycombination of temperature and wind, expressed asthe loss of body heat, in kilogram calories per hourper square meter of skin surface. The wind-chill:factor. is based on the cooling rate of a nude body inthe shade; It is only an approximation, because ofindividual body variations in shape, size, andmetabolic rate.

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WIND ROSE—Any one of a class of diagramsdesigned to show the distribution of wind directionexperienced at a given location over a considerableperiod; it thus shows the prevailing wind direction.The most common form consists of a circle fromwhich 8 or 16 lines emanate,one for each compasspoint. The length of each line is proportional to the

frequency of wind from that direction, and thefrequency of calm conditions is entered in thecenter.

WINTER SOLSTICE—For either hemisphere, thesolstice at which the Sun is above the oppositehemisphere. In northern latitudes, the time of thisoccurrence is approximately 22 December.

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APPENDIX II

ANSWERS TO REVIEW QUESTIONS

CHAPTERS 1 THROUGH 6

CHAPTER 1

A1-1. The metric (CGS centimeter-gram-second) system measures length, weight andtime respectively.

A1-2. Weight is a function of gravitational force. Mass is a functioninertia/acceleration.

A1-3. A dyne is a measure of force.

A1-4. Sunspots are regions of strong localized magnetic fields and indicate relativelycool areas in the photosphere.

A1-5. The Southern Hemisphere receives the greatest amount of incoming solarradiation around December 22.

A1-6. Land and water surfaces absorb 51 percent of the earth's insolation.

A1-7. An air column over the poles is thinner than an air column over the equator.

A1-8. Pressure is the force per unit area.

A1-9. With a sea level pressure reading of 1000 MB, the approximate pressure at18,000 ft will be 500 MB.

A1-10. Temperature change has the biggest effect on pressure change.

A1-11. Temperature is the measure of molecular motion.

A1-12. 20 degrees C converted to Fahrenheit is 68 degrees.

A1-13. The earth's meteorological atmospheric zones in ascending order are thetroposphere, tropopause, stratosphere, stratopause, mesosphere, mesopause,thermosphere, and the exosphere.

A1-14. The four methods of heat transfer are conduction, advection, convection, andradiation.

A1-15. Advection is the horizontal transport of heat.

A1-16. The three states in which moisture in the atmosphere is found are solid, liquid andgaseous.

A1-17. The primary sources of atmospheric moisture are the oceans.

A1-18. The difference between relative humidity and absolute humidity is that relativehumidity is the ratio (in percent) between the water vapor actually present andthe water vapor necessary for saturation at a given temperature. Absolutehumidity is the amount of water vapor present per unit volume of space.

A1-19. The mixing ratio is defined as the ratio of the mass of water vapor to the mass ofdry air.

A1-20. The dew point is the temperature that the air must be cooled, at a constantpressure and constant water vapor content, in order for saturation to occur.

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CHAPTER 2

A2-1. Speed is the rate at which something moves in a given amount of time.

A2-2. The amount of work done is the product of the magnitude of the force and thedistance moved. W=F·d.

A2-3. The two types of forces that AGs deal with are contact force and action at adistance forces.

A2-4. The two basic particles that make up the composition of matter are the atom andthe molecule.

A2-5. The correct formula for density is DM

V(or D M V)= = = ÷ , whereas the density

equals the mass divided by its volume.

A2-6. Fusion is the change of state from a solid to a liquid at the same temperature.

A2-7. The behavior of gases depend on the variations in temperature, pressure, anddensity.

A2-8. According to Boyle's Law, the volume of a gas is inversely proportional to itspressure, provided the temperature remains constant.

A2-9. According to Charles' Law, if the volume of an enclosed gas remains constant,the absolute temperature is directly proportional to the pressure.

A2-10. The universal gas law states that the product of the initial pressure, initialvolume, and new temperature (absolute scale) of an enclosed gas is equal to theproduct of the new pressure, the new volume and the initial temperature. PVT' =P'V'T

A2-11. The two basic kinds of atmospheric energy important to AGs are kinetic energyand potential energy.

A2-12. The definition of lapse rate is the rate of decrease in the value of anymeteorological element with elevation.

A2-13. The dry adiabatic lapse rate is 5 1/2°F per 1,000 feet, or 1°C per 100 meters.

A2-14. The two types of conditional instability are real latent and pseudolatent.

CHAPTER 3

A3-1. The length of day and the angle of the Sun's rays influences the Earth'stemperature.

A3-2. The unequal heating of Earth's surface due to it's tilt, rotation, and differentialinsolation, results in the wide distribution of pressure over Earth's surface.

A3-3. The rotation of Earth causes a force that affects thermal circulation, causing it tobe deflected to the right of the direction of movement in the northern hemisphere.

A3-4. According to the 3-cell theory, Earth is divided into six circulation belts.

A3-5. According to the 3-cell theory, subsidence or high pressure is usually found at 30degrees north latitude.

A3-6. The trade winds are the predominant winds in the tropics.

A3-7. Two types of pressure gradients are horizontal and vertical.

A3-8. The difference between centripetal force and centrifugal force is that centripetalforce is directed toward the center of rotation, and centrifugal force is directedoutward from the center of rotation.

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A3-9. The difference between gradient wind and geostrophic wind is that gradient windflow is parallel to the curved portion of the analysis. Geostrophic wind is thewindflow that is parallel to that portion of the analysis showing straight flow.

A3-10. The relationship between centrifugal force and pressure gradient force aroundanticyclones is that the centrifugal force acts with the pressure gradient force.

A3-11. Anticyclogenesis is the term defined as the formation of an anticyclone or theintensification of an existing one.

A3-12. The direction of windflow around a cyclone is counterclockwise in the northernhemisphere.

A3-13. The temperatures in a cold core low decrease toward the center.

A3-14. Low pressure due to intense heating in the southwestern United States is anexample of a warm core low.

A3-15. Monsoon winds are caused by the unequal heating and cooling of land and watersurfaces.

A3-16. Land and sea breezes are caused by the diurnal (daily) contrast in the heating oflocal water and land areas.

A3-17. Bernoulli's theorem states that pressures are least where velocities are greatest,and pressures are greatest where velocities are least.

A3-18. A valley breeze usually reaches it's maximum strength in the early afternoon.

A3-19. An eddy is caused when the wind flows over or adjacent to rough terrain,buildings, mountains or other obstructions.

A3-20. Foehn winds are caused by adiabatic heating of descending air on the lee sides ofmountains.

CHAPTER 4

A4-1. An air mass is a body of air extending over a large area (usually 1,000 miles ormore across).

A4-2. The two primary factors necessary to produce an air mass are a surface whoseproperties are relatively uniform and a large divergent flow.

A4-3. Maritime tropical air that is colder than the surface over which it is moving iswritten as mTk.

A4-4. The modifying factors on air mass stability are thermodynamic and mechanical.

A4-5. Superior air is the warmest air mass observed in the United States at its altitude.

A4-6. A frontal surface is the surface that separates the air masses.

A4-7. The frontal zone is located between the air masses of different densities.

A4-8. The difference between a stable wave and an unstable wave is that a stable waveneither develops nor occludes. An unstable wave develops along the polar frontand usually occludes.

A4-9. Frontogenesis occurs where there is a concentration of isotherms with thecirculation to sustain that concentration.

A4-10. The polar front in winter is usually found off east coasts of continents between 30and 60 degrees latitude.

AII-3

A4-11. The pressure tendency with the passage of a slow moving cold front is indicatedby a steady or unsteady fall prior to frontal passage, followed by weak rises afterpassage.

A4-12. The slope of a fast moving cold front is usually 1:40 to 1:80 miles.

A4-13. Prefrontal squall lines form about 50 to 300 miles in advance of fast-moving coldfronts.

A4-14. The average speed of a warm front is usually between 10 and 20 knots.

A4-15. The cloud types in advance of a warm front, in order, are cirrus, cirrostratus,altostratus, nimbostratus, and stratus.

A4-16. The difference between warm and cold occlusions is that warm occlusions formwhen the air in advance of the warm front is colder than the air to the rear of thecold front. A cold occlusion forms when the cold air in advance of a warm front iswarmer than the cold air to the rear of the cold front.

A4-17. The most violent weather associated with an occlusion occurs near the apex ortip of the occlusion.

A4-18. When a stationary front moves, the speed is normally less than 5 knots.

A4-19. The weather associated with an unstable stationary front depends on the frontalslope. Severe thunderstorms and heavy rain showers usually occur with steepslopes. Broad or extensive areas of showers, fog, and reduced visibility occurwith shallow slopes.

A4-20. The modifications of fronts are caused by movement and orographic effects.

A4-21. When a cold front moves off the eastern coast of the United States, it intensifiesand waves develop along the frontal boundary.

CHAPTER 5

A5-1. Rain is precipitation that reaches the ground as water droplets and the dropletsize measures .5 mm or greater. Drizzle is very small and appears to float withthe air currents and the droplet size measures less than .5 mm.

A5-2. The altitude range of cloud occurrence in the tropics is from sea level to 60,000feet.

A5-3. The altitude range of middle clouds, in the temperate regions, is from 6,500 to25,000 feet.

A5-4. Sea fog occurs when the wind brings moist, warm air over a colder oceancurrent. Steam fog is caused by saturation of the air through the evaporation ofwater, when cold air moves over warm water.

A5-5. Blowing spray occurs when the water droplets are lifted in such quantities thatthey reduce visibility to six miles or less at eye level.

A5-6. Haze appears as a bluish tinge when viewed against a dark background and adirty yellow or orange tinge when viewed against a bright background. Smokeappears as a reddish tinge when viewed against the solar disk during sunrise andsunset.

A5-7. Dust devils are usually observed on clear, hot afternoons in desert regions.

A5-8. The two sources of light are natural and artificial.

AII-4

A5-9. Natural light is received from the sun and electric lamps, fire, or fluorescent tubesproduce artificial light.

A5-10. Reflection occurs when light waves that are neither transmitted nor absorbed,but are thrown back from the surface of the medium they encounter. Refractionoccurs when a ray of light passes at an oblique angle from one transparentsubstance into another substance of different density.

A5-11. Mirages are images of objects that are made to appear displaced from theirnormal positions because of refraction.

A5-12. The diameter range of a mature thunderstorm cell is 1 to 6 miles.

A5-13. Rain is observed at the surface during the mature stage.

A5-14. A macroburst is a larger scale downburst with winds that can last 5 to 20 minuteswith speeds that reach 130 knots. Microbursts are smaller scale downbursts withwinds that last 2 to 5 minutes with speeds that may reach 130 knots.

A5-15. Two types of thunderstorms are air mass and frontal.

CHAPTER 6

A6-1. Climate is the average or collective state of Earth's atmosphere at any givenlocation or area over a long period of time.

A6-2. Descriptive climatology is typically oriented in terms of geographic regions.

A6-3. Microclimatology is measured in small-scale areas such as golf courses orplowed fields.

A6-4. The most important climatic element is temperature.

A6-5. Wind is the climatic element that transports heat and moisture into a region.

A6-6. The mean or average is the climatological parameter that is determined byadding all values together and dividing by the number of values calculated.

A6-7. Absolute is the term that is usually applied to the extreme highest or lowest valueever recorded at a location.

A6-8. A degree day is the number of degrees the mean daily temperature is above orbelow a standard temperature base.

A6-9. The climatic belts or zones are the torrid or tropical zone, the two temperatezones, and the two polar zones.

A6-10. The three climatic classification types are C.W. Thornthwaite, W. Köppen, andG.T. Trewartha.

A6-11. The five climatic types according to Köppen are tropical rain, dry, warmtemperate rainy, cool snow forest (Boreal), and polar.

A6-12. Latitude is the climatic control that has the biggest effect on climatic elements.

A6-13. Coastal areas assume the temperature characteristics of the land or water that ison their windward side. Therefore, in the middle latitudes, the western coast ofthe United States will normally receive maritime temperature characteristicsfrom the Pacific Ocean, and the eastern coast will normally receive continentaltemperature characteristics from the mainland.

A6-14. Ocean currents transport heat by moving cold polar water equatorward intowarmer waters and moving warm equatorial water poleward into cooler waters.

AII-5

A6-15. Requests for climatic support should be made to the Oceanography Facility orCenter in your chain of command. Requests that cannot be fulfilled areforwarded to FMOD Asheville, N.C.

A6-16. The Local Area Forecaster's Handbook contains climatic information for aparticular weather station.

A6-17. The two climatic extremes that relate to water and land distribution are overEarth are maritime and continental.

A6-18. The southwest pacific coastal area experiences a Mediterranean-type climate.

A6-19. The cause of the summer monsoon is the major warm low-pressure center overAsia (Asiatic Low) during the summer, and the cause of the winter monsoon isthe major cold high-pressure center over Asia during the winter (Siberian High).

A6-20. South America lacks the severe weather of its North American counterpartbecause of the absence of continental polar air. This is due to the tapering of thecontinent toward Antarctica.

AII-6

INDEX

A

Absolute scale (Kelvin), 1-13Adiabatic process, 2-12African weather, 6-29Air masses, 4-1 to 4-20

air masses, 4-1 to 4-20air mass classification, 4-4 to 4-6

geographic origin, 4-4moisture content, 4-4thermodynamic process, 4-4

air mass modification, 4-5 to 4-7age, 4-6

modifying influences on air mass, 4-6stability, 4-6surface moisture, 4-6surface temperature, 4-5topography of surface, 4-6trajectory, 4-6

air mass source regions, 4-2 to 4-3characteristics of air masses, 4-3southern hemisphere air masses, 4-3

air masses in the southern hemisphere, 4-3air masses over Asia, 4-14 to 4-16

continental polar (cP) air, 4-16equatorial (E) air, 4-16maritime tropical (mT) air, 4-16

air masses over Europe, 4-18 to 4-20continental arctic (cA) and continental polar (cP)

air in summer, 4-18continental arctic (cA) and continental polar (cP)

air in winter, 4-18continental tropical (cT) air in summer, 4-19continental tropical (cT) air in winter, 4-19

maritime arctic (mA) air in summer, 4-18maritime arctic (mA) air in winter, 4-18maritime polar (mP) air in winter, 4-18maritime tropical (mT) air in summer, 4-19maritime tropical (mT) air in winter, 4-19

conditions necessary for air mass formation, 4-1effects of circulation on air mass formation,

4-1 to 4-2North American air masses, trajectories,

and weather (summer), 4-13 to 4-16continental polar (cP) air in summer, 4-13 to 4-14continental tropical (cT) air in summer, 4-16

maritime polar (mP) air Atlantic in summer,4-15

maritime polar (mP) air Pacific in summer,4-14

maritime tropical (mT) air Atlantic insummer, 4-14

maritime tropical (mT) air Pacific in summer,4-14

superior (S) air in summer, 4-15North American air masses, trajectories,

and weather (winter), 4-7 to 4-13cPk and cAk air in winter, 4-7 to 4-9

Air masses—Continuedmaritime polar (mP) air Atlantic in winter,

4-11maritime polar (mP) air Pacific in winter, 4-9

to 4-10maritime tropical (mT) air Atlantic in winter,

4-11 to 4-13maritime tropical (mT) air Pacific in winter,

4-10 to 4-13Air masses and fronts, 4-1 to 4-50

air masses, 4-1 to 4-20cold front, 4-31 to 4-36fronts, 4-20 to 4-50modifications of fronts, 4-45 to 4-50occluded fronts, 4-21, 4-39 to 4-43quasi-stationary front, 4-21, 4-43 to 4-45warm front, 4-21, 4-37

Airglow, 5-23Arctic and Antarctic weather, 6-21 to 6-25Asiatic weather, 6-28Atmospheric circulation, 3-1 to 3-30

general circulation, 3-10 to 3-14secondary circulation, 3-14 to 3-20tertiary circulation, 3-20 to 3-29

Atmospheric energy, 2-11 to 2-23atmospheric energy, 2-11 to 2-23

adiabatic process, 2-12 to 2-13adiabatic heating and cooling, 2-12 to 2-13terms, 2-12first law of thermodynamics, 2-11 to 2-12stability and instability, 2013 to 2-23

autoconvection, 2-15, 2-20conditional instability, 2-19convection stability and instability, 2-20 to 2-21determining bases of convective type clouds

equilibrium of dry air, 2-16 to 2-18equilibrium of saturated air, 2-18 to 2-19lapse rate, 2-14 to 2-15stability in relation to cloud type, 2-22 to 2-23types of stability, 2-16

Atmospheric moisture, 1-18 to 1-21Atmospheric optical phenomena, 5-17 to 5-18Atmospheric phenomena, 5-1 to 5-23

electrometers, 5-18 to 5-23hydrometeors, 5-1 to 5-12Lithometeors, 5-12 to 5-13photometeors, 5-13 to 5-17

Atmospheric physics, 2-1 to 2-24atmospheric energy, 2-11 to 2-23gas laws, 2-8 to 2-11matter, 2-4 to 2-8motion, 2-1 to 2-4

Atmospheric radiation, 1-10Auroras, 5-23

B

Boyle’s law, 2-8 to 2-9

INDEX-1

C

Candlepower and foot candles, 5-15Celsius scale, 1-13Centrifugal force, 3-9 to 3-10Charles’ law, 2-9Classification of climate

classification of climate, 6-6 to 6-7climatic types, 6-6 to 6-7climatic zones, 6-6

Climate and climatologyclimate and climatology, 6-1 and 6-2climate, 6-1climatology, 6-1 to 6-2climatology as related to other sciences, 6-2descriptive climatology, 6-1dynamic climatology, 6-1physical climatology, 6-1ecology, 6-2

Climatic controls, 6-7 to 6-13climatic factors, 6-12 to 6-13

land and water distribution, 6-10influence on air circulation, 6-10influence on air temperature, 6-10

latitude, 6-9 to 6-10ocean currents, 6-11 to 6-12

effects on the east coasts, 6-12effects on the west coasts, 6-11 to 6-12other effects, 6-12topography, 6-10 to 6-11

Climatic elements, 6-2 to 6-3climatic elements, 6-2 to 6-3precipitation, 6-3temperature, 6-2wind, 6-3

Climatological data, 6-13 to 6-17application to weather prediction, 6-16 to 6-17

availability of data, 6-14 to 6-15cross-wind summary, 6-14frequency SMOS, 6-14local climatological data summary, 6-14near coastal zone studies, 6-14summary of meteorological

observations, radiosonde(SMOR), 6-14

summary of synoptic meteorologicalobservations (SSMO), 6-14

worldwide airfield summary, 6-15climatological references, 6-15 to 6-16

local area forecaster’s hand-books, 6-15miscellaneous publications, 6-16Naval Intelligence Survey (NIS), 6-16publications, 6-16

climatological services, 6-16additional climatic sources, 6-16methods of presentation, 6-13 to 6-14

Climatology and world weather 6-1 to 6-30classification of climate, 6-6 to 6-7climate and climatology, 6-1 to 6-2climatic controls, 6-7 to 6-13expression of climatic elements, 6-3 to 6-5world weather, 6-17 to 6-30

Cold front, 4-20 to 4-21, 4-31 to 4-37cold fronts aloft, 4-35 to 4-36

surface characteristics, 4-32upper air characteristics, 4-33weather, 4-32 to 4-33

fast-moving cold fronts (inactive cold front), 4-33to 4-35

instability and squall lines, 4-36 to 4-37great plains squall lines, 4-37instability line, 4-36prefrontal squall lines, 4-36squall line, 4-36secondary cold fronts, 4-35slow-moving cold fronts (active cold front), 4-31

to 4-33surface characteristics, 4-32upper air characteristics, 4-33weather, 4-32 to 4-33

Coriolis effect, 3-9Cyclostrophic wind, 3-12

D

Descriptive climatology, 6-1Dynamic climatology, 6-1

E

Earth, 1-5 to 1-8Earth-Sun relationship, 1-2 to 1-11

depletion of solar radiation, 1-9 to 1-10Earth

insolation, 1-8 to 1-10motions, 1-5 to 1-6solstices and equinoxes, 1-6 to 1-8

greenhouse effect, 1-10radiation, 1-10radiation (heat) balance in the

atmosphere, 1-10 to 1-11atmospheric radiation, 1-10

terrestrial (Earth) radiation, 1-10Sun, 1-2 to 1-5

flares, 1-5plages, 1-5solar composition, 1-3 to 1-4solar prominences/filaments, 1-4sunspots, 1-4 to 1-5

Ecology, 6-2Electrometeors, 5-18 to 5-23

electrometeors, 5-18 to 5-23airglow, 5-23auroras, 5-23lightning, 5-22 to 5-23thunderstorms, 5-18 to 5-22

classifications, 5-18formation, 5-18structure, 5-18 to 5-20thunderstorm weather, 5-20 to 5-21vertical development, 5-20

Equation of state, 2-10Equatorial and tropical weather, 6-19Equilibrium of dry air, 2-16 to 2-18

INDEX-2

Equinoxes and solstices, 1-6 to 1-8European weather, 6-27 to 6-28Expression of climatic elements, 6-3 to 6-6

expression of climatic elements, 6-3 to 6-6absolute, 6-3 to 6-4average and standard deviations, 6-4 to 6-6average deviation, 6-4 to 6-5

standard deviation, 6-5 to 6-6degree-day, 6-4extreme, 6-4frequency, 6-4mean (average), 6-3median, 6-4mode, 6-4normal, 6-3range, 6-4

F

Fahrenheit scale, 1-13Flares, 1-5Foehn winds, 3-29Frontogenesis, conditions necessary for, 4-24 to 4-25Frontolysis, 4-26 to 4-30Fronts

Conditions necessary for frontogenesis, 4-24definitions and classifications, 4-20 to 4-21frontal elements, 4-26 to 4-30

pressure, 4-30temperature, 4-27 to 4-28wind, 4-28 to 4-30

frontolysis, 4-25general characteristics of fronts, 4-30 to

4-31clouds and weather, 4-30

frontal intensity, 4-30 to 4-31frontal slope, 4-30 to 4-31

speed, 4-31wind component, 4-31

relation of fronts to air masses, 4-21relation of fronts to cyclones, 4-22 to 4-24stable waves, 4-22unstable waves, 4-23 to 4-24

world frontogenetical zones, 4-25 to 4-26Fundamentals of meteorology, 1-1 to 1-21

Earth-Sun relationship, 1-2 to 1-12moisture, 1-18 to 1-21pressure, 4-30system of measurement, 1-1 to 1-2temperature, 1-13 to 1-18

G

Gas laws, 2-8 to 2-11gas laws, 2-8 to 2-11

Boyle’s law, 2-8 to 2-9Charles’ law, 2-9equation of state, 2-10

hydrostatic equation, 2-10 to 2-11Kinetic theory of gases, 2-8universal gas law, 2-9

General circulation, 3-1 to 3-14Coriolis effect, 3-9

pressure gradient, 3-7 to 3-9pressure gradient force, 3-9 to 3-10

wind forces, 3-7wind types, 3-10 to 3-14

cyclostrophic wind, 3-12geostrophic and gradient

wind, 3-10 to 3-12geostrophic and gradient

wind scales, 3-13 to 3-14movement of wind around

anticyclones, 3-12movement of wind around

cyclones, 3-12 to 3-13elements of circulation, 3-5 to 3-6

rotating Earth, 3-6static Earth, 3-5

pressure over the globe, 3-53-cell theory, 3-6wind theory, 3-7 to 3-10

centrifugal force, 3-9 to 3-10variations, 3-13world temperature gradient, 3-1 to 3-4world winds, 3-6 to 3-7

Geostrophic and gradient wind, 3-10 to 3-12Geostrophic and gradient wind scales, 3-13 to 3-14Glossary, AI-1 to AI-11

H

Heat balance and transfer in the atmosphere, 1-10 to1-11

Hydrometeors, 5-1 to 5-12hydrometeors, 5-1 to 5-12

other hydrometeors, 5-2 to 5-12clouds, 5-2dew, 5-10drifting and blowing snow, 5-11fog, 5-6 to 5-10frost, 5-10glaze (clear icing), 5-11rime (rime icing), 5-10 to 5-11spray and blowing spray, 5-11tornadoes, 5-11 to 5-12waterspouts, 5-12

precipitation, 5-1drizzle, 5-1hail, 5-2ice crystals (ice prisms), 5-2ice pellets, 5-2rain, 5-1snow, 5-1snow grains, 5-1snow pellets, 5-1

precipitation theory, 5-2Hydrostatic equation, 2-10 to 2-11

K

Kinetic theory of gases, 2-8

INDEX-3

L

Laws of motion, 2-1 to 2-2Light, 5-13 to 5-13Lightening, 5-22 to 5-23Lithometeors, 5-12 to 5-13

lithometeors, 5-12 to 5-13dust, 5-13dust devils, 5-13haze, 5-12 to 5-13sand, 5-13smoke, 5-13

M

Matter, 2-4 to 2-8matter, 2-4 to 2-8

changes of state, 2-6 to 2-8heat energy, 2-6liquid to gas and vice versa, 2-7liquid to solid and vice versa, 2-6solid to gas and vice versa, 2-7 to 2-8

definitions, 2-5physical properties, 2-5 to 2-6

density, 2-6gravitation, 2-5 to 2-6inertia, 2-5mass, 2-5volume, 2-6weight, 2-6states, 2-5

Migratory Systems, 3-16 to 3-17Modifications of fronts, 4-45 to 4-50

modifications of fronts, 4-45 to 4-50effects caused by movement, 4-46effects of underlying surfaces, 4-46land to water migration, 4-46movement over land surfaces, 4-46orographic effects, 4-46 to 4-50

cold fronts, 4-46 to 4-47occluded fronts, 4-48 to 4-50warm fronts, 4-48

Moisture, 1-18 to 1-21moisture, 1-18 to 1-21

atmospheric moisture, 1-18terms, 1-20 to 1-21

absolute humidity, 1-20dewpoint, 1-21mixing ratio, 1-21relative humidity, 1-20specific humidity, 1-20 to 1-21water vapor characteristics, 1-18 to 1-20

condensation, 1-19pressure (Dalton’s Law), 1-19 to 1-20temperature, 1-19

Monsoon winds, 3-20 to 3-22Motion, 2-1 to 2-4

motion, 2-1 to 2-4energy, 2-2

application of vectors and resultantforces, 2-4

composition of forces, 2-4

Motion—Continuedforce, 2-2 to 2-4

vectors, 2-3laws of motion, 2-1 to 2-2Newton’s first law, 2-2Newton’s second law, 2-2Newton’s third law, 2-2terms, 2-1 to 2-2

acceleration, 2-1direction, 2-1inertia, 2-1

speed, 2-1velocity, 2-1work, 2-2

N

Naval Intelligence Survey (NIS) publications, 6-16Newton’s laws, 2-2North American air masses, trajectories, and weather

(summer), 4-13 to 4-16North American air masses, trajectories, and weather

(winter), 4-7 to 4-13

O

Occluded fronts, 4-39 to 4-43occluded fronts, 4-39 to 4-43

cold occlusions, 4-40 to 4-42surface characteristics, 4-41

upper air characteristics, 4-42weather, 4-41warm occlusions, 4-42 to 4-43

surface characteristics, 4-42upper air characteristics, 4-42 to 4-43weather, 4-42

Ocean currents, 6-11 to 6-12Oceanic weather, 6-17 to 6-21

P

Pascal’s Law, 1-13Photometeors, 5-13 to 5-17

photometeors, 5-13 to 5-17atmospheric optical phenomena, 5-17 to 5-18coronas, 5-17crepuscular rays, 5-18fogbows, 5-17halos, 5-17

looming, 5-18mirages, 5-17 to 5-18rainbows, 5-17scintillation, 5-18

light, 5-13 to 5-15candlepower and footcandles, 5-15characteristics, 5-15

sources of light, 5-13theory, 5-14wavelength, 5-14

reflection, 5-15refraction, 5-16 to 5-17

INDEX-4

Physical climatology, 6-1Plages, 1-5Precipitation, 5-1 to 5-2Pressure, 1-12 to 1-13

pressure, 1-12definition, 1-12

Pascal’s Law, 1-13standard atmosphere, 1-12standards of measurement, 1-12vertical distribution, 1-12

Pressure gradient force, 3-9

Q

Quasi-stationary front, 4-43 to 4-45quasi-stationary front, 4-43 to 4-45

characteristics, 4-44stable stationary front, 4-44unstable stationary front, 4-44 to 4-45

S

Secondary circulation, 3-14 to 3-20secondary circulation, 3-14 to 3-20

centers of action, 3-14 to 3-16dynamic high, 3-20dynamic low, 3-20migratory systems, 3-16 to 3-17

anticyclones, 3-18 to 3-17cyclones, 3-17vertical structure of high-pressure

systems, 3-18 to 3-19cold core highs, 3-18

warm core highs, 3-18vertical structure of low-pressure

systems, 3-19 to 3-20cold core lows, 3-19warm core lows, 3-19 to 3-20

vertical structure of secondarycirculations (pressure centers), 3-17 to3-18

Solar composition, 1-3 to 1-4Solar prominences/filaments, 1-4Solstices and equinoxes, 1-6 to 1-8South American weather, 6-28 to 6-29

System of measurement, 1-1 to 1-2Sun, 1-2 to 1-5Sunspots, 1-4 to 1-5

system of measurement, 1-1 to 1-2area and volume, 1-1density, 1-2force, 1-2length, 1-1time, 1-2weight, 1-2

T

Temperaturetemperature, 1-13 to 1-18

definition, 1-13

Temperature—Continuedheat transfer, 1-17 to 1-18methods, 1-17specific heat, 1-17 to 1-18temperature scales, 1-13 to 1-14

absolute scale (Kelvin), 1-13Celsius scale, 1-13Fahrenheit scale, 1-13mathematical methods, 1-13scale conversions, 1-13

vertical distribution, 1-15 to 1-17layers of the atmosphere, 1-13 to 1-15

Terrestrial (Earth) radiation, 1-10Tertiary circulation, 3-20 to 3-29

tertiary circulation, 3-20 to 3-29Foehn winds, 3-29funnel effect, 3-24 to 3-26

winds due to local cooling, 3-23 to 3-29winds due to local heating, 3-23 to 3-29

induced or dynamic tertiary circulations, 3-26to 3-29

eddies, 3-26 to 3-27large-scale vertical waves (mountain waves),

3-27 to 3-28turbulence, 3-27

land and sea breezes, 3-22 to 3-23monsoon winds, 3-20 to 3-22

winds due to local cooling and heating, 3-23 to3-29

Thermodynamics, first law of, 2-11 to 2-123-cell theory, 3-6Thunderstorms, 5-18 to 5-22Topography, 6-10 to 6-11Tornadoes, 5-11 to 5-12

W

Warm front, 4-37 to 4-39warm front, 4-37 to 4-39

surface characteristics, 4-37 to 4-38upper air characteristics, 4-39

warm fronts aloft, 4-39weather, 4-38 to 4-39

Waterspouts, 5-12World weather, 6-17

world weather, 6-17 to 6-30African weather, 6-29 to 6-30

equatorial region, 6-29north central region, 6-29northern region, 6-29southeast coastal region, 6-29 to 6-30southeastern interior region, 6-30southwestern region, 6-29sub-equatorial region, 6-29

Arctic and Antarctic weather, 6-21 to 6-25Antarctic weather, 5-25Arctic weather, 6-22 to 6-25Asiatic weather, 6-28European weather, 6-27 to 6-28

oceanic weather, 6-17 to 6-21air-mass weather, 6-20

INDEX-5

World weather—Continuedeffects of air-sea interchange, 6-17

to 6-19equatorial and tropical weather,

6-19North Atlantic and NorthPacific oceans, 6-19 to 6-20

oceanic weather, 6-17 to 6-21South American weather, 6-28 to 6-29

northeastern climate, 6-28southern climate, 6-28 to 6-29west coast climate, 6-29

United States weather, 6-25 to 6-27central plains area, 6-27

World weather—Continuedintermountain west central area, 6-26 to

6-27North Atlantic coastal area, 6-27

northwest Pacific coast area, 6-26southeast and gulf states area, 6-27southwest desert and mountain area,

6-27southwest Pacific coast area., 6-26

Wind theory, 3-7 to 3-10Wind types, 3-10 to 3-14World temperature gradient, 3-1 to

3-5

INDEX-6

Assignment Questions

Information: The text pages that you are to study areprovided at the beginning of the assignment questions.

ASSIGNMENT 1

Textbook Assignment: "Fundamentals of Meteorology"; "Atmospheric Physics." Chapters 1 and 2, Pages 1-1through 2-11.

1-1. The metric (cgs) system has been adopted bymeteorologists to measure units of

1. gravity, density, and force2. length, weight, and time3. centimeters, grams, and seconds4. circular motion, gravity, and speed

1-2. Approximately how many inches are there in25 centimeters?

1. 0.984 in.2. 9.840 in.3. 98.400 in.4. 984.000 in.

1-3. Weight and mass are synonymous, except oneis English and the other metric.

1. True2. False

1-4. A dyne is a measure of

1. length2. force3. area4. density

1-5. Earth receives the majority of its heat from theSun. What percent is NOT received from theSun?

1. 1.0 %2. 0.1 %3. 0.3 %4. 0.5 %

1-6. What are solar winds?

1. Winds generated on Earth by the Sun’sappearance above the horizon

2. Streams of solar particles emitted from theSun’s surface

3. Winds generated by the pressuredifferences between hot and cool spots onthe Sun’s surface

4. Interplanetary winds created by theconstellations

1-7. What are sunspots?

1. Hot spots known as solar flares2. Irregular bright patches on the Sun’s

surface3. Areas where the convective zone is

exposed4. Regions of strong localized magnetic

fields

1-8. It takes Earth approximately 365 1/4 days tocircle the Sun. Approximately how many timeswill Earth rotate about its own axis during thistime?

1. 15 1/42. 30 1/23. 182 1/24. 365 1/4

1-9. In the Southern Hemisphere, on or about whatdate will the greatest amount of incoming solarradiation be received?

1. January 212. March 213. June 214. December 22

1-10. The Sun’s most direct rays reach theirpoleward limit twice in the year. What datesand names mark these occurrences?

1. March 21 and September 22; the springand autumnal equinoxes

2. June 21 and December 22; the summer andwinter equinoxes

3. March 21 and September 22; the springand autumnal solstices

4. June 21 and December 22; the summer andwinter solstices

1-11. Which of the following statements is/arecorrect concerning latitude 23 1/2 N?

1. It is known as the Tropic of Cancer2. It is the northern extent of the Sun’s most

direct rays3. It represents one-half of the total range of

motion of the Sun’s most direct rays4. All of the above

1

1-12. The temperate zone in the SouthernHemisphere receives sunshine all year, butreceives more sunshine when winter is beingexperienced in the Northern Hemisphere.

1. True2. False

1-13. If the Sun’s radiation (Earth’s incoming solarradiation) was not dispersed or filtered, Earthwould eventually become too hot for life toexist as we now know it. Which of thefollowing factors plays the major role indispersing Earth’s insolation?

1. Scattering2. Earth’s inclination3. Earth’s rotation4. Reflection

1-14. Earth’s average albedo is between 36 and 43percent. Which of the following terms mostaccurately defines albedo as it pertains to Earthand its atmosphere?

1. Sky cover2. Scattering capability3. Absorption capability4. Reflective capability

1-15. What percentage of Earth’s insolation isabsorbed by land and water?

1. 13 %2. 36 %3. 43 %4. 51 %

1-16. Through its atmosphere’s ability to absorb andlose heat, Earth enjoys an average temperatureof 15°C/59°F. If it failed to absorb short-waveradiation and radiate long-wave radiation,Earth’s average temperature would be

1. -04°F2. -04°C3. -35°F4. -35°C

1-17. The poles receive far less incident radiationthan the equator. What is the effect on a polarair column in relation to a column of air overthe equator?

1. It is more shallow and lighter2. It is more shallow and heavier3. It is thicker and heavier4. It is thicker and lighter

1-18. Which of the following sea level pressure(s)is/are the standard used by the InternationalCivil Aeronautical Organization?

1. 1013.25 millibars2. 29.92 inches of mercury3. 14.7 lbs. per square inch4. All of the above

1-19. With a sea-level pressure reading of 1000 mb,one would expect the pressure at 18,000 feet toread

1. 200 mb2. 300 mb3. 500 mb4. 750 mb

1-20. Pressure readings vary to the greatest degreewith changes in

1. latitude2. altitude3. temperature4. humidity

1-21. Convert 18° Celsius to Fahrenheit.

1. 85°F2. 74°F3. 64°F4. 57°F

1-22. Convert 91° Fahrenheit to Celsius.

1. 27°C2. 30°C3. 33°C4. 36°C

1-23. Minus 5° Celsius equates to what Kelvin (K)scale temperature?

1. 250°K2. 268°K3. 278°K4. 283°K

1-24. Which of the following statements is/arecorrect concerning the troposphere?

1. Temperature inversions are not uncommon2. Its thickness varies with latitude3. Its thickness varies with the seasons4. All of the above

2

1-25. For meteorological purposes, Earth’satmosphere is classified into zones or layers byits thermal structure. Working upward throughthe atmosphere, which of the following lists ofzones is correct in its vertical order?

1. Troposphere, tropopause, mesosphere,mesopause

2. Troposphere, stratosphere, mesosphere,exosphere

3. Mesosphere, mesopause, thermosphere,exosphere

4. Stratosphere, mesosphere, exosphere,thermosphere

1-26. On some atmospheric soundings, it issometimes possible to have more than onetropopause recorded.

1. True2. False

1-27. The rise in temperature in the upper portion ofthe stratosphere is attributed to

1. the absence of water vapor2. excessive amounts of water vapor3. the presence of ozone4. its relative closeness to the Sun

1-28. Which of the following zones marks the outerlimit of Earth’s atmosphere?

1. Mesosphere2. Troposphere3. Stratosphere4. Exosphere

1-29. Which of the following zones is an electricalclassification?

1. Exosphere2. Mesosphere3. Troposphere4. Thermosphere

1-30. At night, the Earth’s surface radiates some ofthe heat it gains during the day, and it cools. bywhat process does the layer of air in contactwith the Earth’s surface cool?

1. Radiation2. Conduction3. Convection4. Advection

1-31. The specific heat of water is 1. What does 1represent?

1. The temperature of the water2. The calorie requirement to raise the

temperature of 1 gram of water 1° Celsius3. The weight of the substance used in the

ratio4. The time requirement (in minutes) to raise

the temperature of 1 gram of water 1°Celsius

1-32. The horizontal transport of heat is known as

1. radiation2. conduction3. convection4. advection

1-33. Earth’s atmosphere is capable of holding morewater vapor (water in its gaseous state) atwhich of the following latitudes?

1. 5°N2. 30°N3. 60°N4. 80°N

1-34. Which of the following statements concerningsaturation is correct?

1. Water vapor does not exist in a volume ofthe atmosphere that is saturated

2. To saturate air at the Earth’s surfacerequires less water vapor per unit massthan at 500 mb.

3. The degree of saturation is dependent onpressure

4. If equal amounts of water vapor areinjected into the atmosphere, saturation ismore likely to occur in polar regions beforeit occurs in equatorial regions

1-35. Which of the following occurrences will resultin the condensation of water vapor?

1. Air moved over a colder surface2. Air lifted mechanically3. Air-cooled by the radiational cooling

process4. Each of the above

3

1-36. Someone says that it is very humid outside(high relative humidity). What does thisstatement imply?

1. It is very hot out2. Precipitation will occur3. Water vapor content is diminished4. The air is very moist

1-37. To determine the degree of saturation of the air,you must compute

1. absolute humidity2. specific humidity3. relative humidity4. mixing ratio

1-38. In a given mass of dry air, the ratio of the massof water vapor to the given mass of dry air isexpressed in grams per gram or grams perkilogram and is known as the

1. mixing ratio2. specific humidity3. saturation mixing ratio4. relative humidity

1-39. Under which of the following conditions willthe specific humidity of unsaturated airchange?

1. Temperature changes2. Pressure changes3. Air is compressed4. Water vapor content changes

1-40. Knowing that the mixing ratio of a parcel of airis 6.3 g/kg and the saturation mixing ratio is 9.0g/kg, what is the relative humidity of theparcel?

1. 56 %2. 63 %3. 70 %4. 90 %

1-41. Which of the following statements concerningdew point is most correct?

1. Saturation only occurs if the airtemperature is cooled to its dew point

2. Saturation only occurs if the airtemperature is increased to its dew point

3. Saturation occurs if the air temperature iscooled to its dew point and there arecorresponding changes in the pressure andwater vapor content

4. Saturation occurs if the air temperature iscooled to its dew point and the pressureand water vapor content do not change

1-42. A tropical depression moved 360 nauticalmiles in 24 hours. This movement is referred toas

1. acceleration2. speed3. velocity4. inertia

1-43. A low-pressure center is stationary over thesouth central North Atlantic Ocean for threedays. On day four, the low moves 250 milesnorth. Which law of motion applies to the low’schange in position?

1. Newton’s first law2. Newton’s second law3. Newton’s third law4. Dalton’s law

1-44. A stationary high-pressure center begins tomove, and in 12 hours, the upper-level windsmove the center 60 nautical miles. Whatproperty did the high exhibit when it wasstationary, and what was necessary to move it60 nautical miles?

1. Acceleration and inertia2. Inertia and acceleration3. Inertia and work4. Kinetic energy and potential energy

1-45. A destroyer is dead in the water. Which of thefollowing forces is NOT acting upon the ship?

1. Gravity2. A contact force3. An at-a-distance force4. A resultant force

4

1-46. A line that represents magnitude and directionis known as a

1. force2. composite force3. vector4. contact force

1-47. Your ship is moving south (180°) at 15 knots,and the apparent wind reads 090 degrees at 05knots. What is the true wind, and what namedefines the forces used to compute it?

1. 160/16, component2. 160/16, resultant3. 340/20, component4. 340/20, resultant

1-48. What two basic particles make up thecomposition of all matter?

1. The atom and molecule2. The molecule and element3. The compound and mixture4. The element and atom

1-49. When elements and compounds exist togetherwithout forming new compounds, they areknown as a

1. mixture2. compound3. compounded element4. state

1-50. Which of the following forms of matter arecalled fluids?

1. Solids only2. Liquids only3. Gases only4. Liquids and gases

1-51. Air density can be critical to a pilot whoseaircraft must take of f on a short runway and/orwhose aircraft is heavily loaded. Which, if any,of the following factors affects the density ofthe air at a given location?

1. Pressure only2. Temperature only3. Pressure and temperature4. None of the above

1-52. What is the name given to heat that is given offor absorbed in a substance’s change of state?

1. Energy2. Fusion3. Freezing4. Latent

1-53. Water molecules in the oceans are more apt tomove into the atmosphere at which of thefollowing latitudes?

1. 5°S2. 25°S3. 25°N4. 60°N

1-54. You are with someone who is wearing glassesin an air-conditioned space. When you leavethe space and go outside into much warmer air,the person’s glasses fog over. What process hastaken place?

1. Evaporation2. Condensation3. Sublimation4. Fusion

1-55. Just after reveille, you go up on deck and findthe rails and outer bulkheads wet. There hasbeen no precipitation or fog, and the winds andsea have been relatively calm. What do youattribute morning dampness to?

1. Humidity only2. Humidity and condensation3. Evaporation only4. Humidity and evaporation

1-56. All cirriform clouds form through the processof sublimation.

1. True2. False

1-57. Which, if any, of the following relationshipsconcerning enclosed gases is correct?

1. Increasing the temperature decreases thepressure

2. Increasing the temperature and decreasingthe volume decreases the pressure

3. Decreasing the volume decreases thepressure

4. None of the above

5

1-58. Boyle’s law and the Universal gas law are verysimilar except

1. temperature is not considered in theUniversal gas law

2. pressure is not considered in Boyle’s law3. the Universal gas law applies to the free

atmosphere vice enclosed gases4. Boyle’s law is dependent on a constant

temperature

1-59. The molecular weight of dry air is greater thanmoist air. How do their densities compare?

1. Moist air is more dense than dry air2. Moist air is less dense than dry air3. Moist air is occasionally more dense than

dry air4. Moist air and dry air do not differ in their

density

1-60. What is/are the purpose(s) of the hypso-metricequation?

1. To reduce pressure2. To determine the thickness between two

layers3. Both 1 and 2 above apply4. To determine pressure and temperature

variations

1-61. What is the approximate thickness of the1000-500-mb layer when the layer has a meantemperature of -10°C?

1. 5,140 meters2. 5,097 meters3. 4,878 meters4. 4,778 meters

6

ASSIGNMENT 2

Textbook Assignment: “Atmospheric Physics" (Continued); “Atmospheric Circulation.” Chapters 2 and 3, Pages2-11 through 3-29.

2-1. When a parcel of air rises in the atmosphere,what happens to the parcel and the surroundingair?

1. The parcel expands due to lesseningpressure, and its temperature, pressure, anddensity increase

2. The parcel contracts due to increasingpressure, and its temperature, pressure, anddensity decrease

3. The parcel expands due to lesseningpressure, and its temperature, pressure, anddensity decrease

4. The parcel contracts due to lesseningpressure, and its temperature, pressure, anddensity increase

2-2. With regards to Earth’s atmosphere, which ofthe following definitions pertains totemperature lapse rate?

1. The rate at which temperatures decrease orincrease with altitude

2. The rate at which temperatures decrease atnight

3. The rate of temperature decrease withlatitude

4. The rate of temperature decreasehorizontally

2-3. What is an inversion?

1. A decrease in temperature with height2. An isothermal lapse rate3. An increase in temperature due to

subsidence4. An increase in temperature with height

2-4. If a parcel of air is lifted and remainsunsaturated, it will cool at which of thefollowing rates?

1. 1°C per 100 meters2. 2° or 3°C per 100 meters3. 5°C per 100 meters4. 10°C per 100 meters

2-5. When the actual lapse rate of a column of air isless than the dry adiabatic lapse rate but greaterthan the moist adiabatic lapse rate, what can wesay about the air?

1. It is absolutely stable2. It is absolutely unstable3. It is conditionally stable, only4. It may be conditionally stable or unstable

2-6. A maritime polar air mass moves into westernCanada and is forced aloft by the mountains ofBritish Columbia. Prior to being lifted by themountains, the layer of air between 850 mb and500 mb was guite moist up to 600 mb and dryabove. What should you expect concerning thestability of this layer?

1. Instability to remain the same2. Instability to decrease3. Instability to increase4. Stable conditions to prevail throughout the

layer

2-7. Where would you most likely be able todetermine the bases of convective clouds usingsurface temperatures and dew points?

1. Adak, AK2. San Antonio, TX3. San Diego, CA4. South China Sea

2-8. Stratified cloud layers on the western slope ofthe Appalachian Mountains of Virginia wouldbe an indication of which of the followingconditions?

1. Little or no turbulence2. Unstable air3. Hazardous flying conditions along the

mountains due to strong vertical currents4. All of the above

2-9. The unequal heating of Earth’s surface is due towhich of the following factors?

1. Its axis (inclination)2. Its rotation3. Differential isolation4. All of the above

7

2-10. Incoming solar radiation is greatest at theequator and least at the poles. What affect, ifany, does this have on the atmospheric pressurein these areas?

1. Pressure is high in both areas2. Pressure is higher at the poles than at the

equator3. Pressure is lower at the poles than at the

equator4. Incoming solar radiation has no effect on

pressure in these locations

2-11. If Earth did not rotate and its surface wasuniform, in the Northern Hemisphere itssurface winds would blow in what direction?

1. West to east2. East to west3. North to south4. South to north

2-12. Coriolis force is an apparent force created by

1. temperature variations between the polesand equator

2. the tilt of the Earth’s axis3. the Earth’s rotation4. pressure variation between the poles and

equator

2-13. How does Coriolis force affect movingobjects?

1. It produces positive temperature changeson them

2. It lessens the pressure gradient on them3. It increases and decreases their speed4. It forces objects to the right of their

intended path in the Northern Hemisphere

2-14. The three cells of the tri-cellular theory are the

1. tropical, subtropical, and polar2. equatorial, subtropical, and polar3. tropical, midlatitude, and polar4. equatorial, midlatitude, and polar

2-15. The surface wind generated by the Earth’sgeneral circulation pattern is

1. westerly at all latitudes2. northeasterly in the tropics and poleward

of 60°N/S and westerly in the midlatitudes3. northwesterly in the tropics and poleward

of 60°N/S and westerly in the midlatitudes4. northwesterly poleward of 60°N/S,

northeasterly in the midlatitudes andeasterly in the tropics

2-16. Which of the following regions feature(s) lightand variable winds?

1. The doldrums2. The horse latitudes3. The regions near 30°N and 30°S4. All of the above

2-17. What force moves air in a straight line fromareas of high pressure to areas of low pressure?

1. Friction2. Centrifugal3. Pressure gradient4. Coriolis

2-18. What is inferred from horizontal pressuregradients classified as flat or weak?

1. Isobars are closely spaced2. Isobars are widely spaced3. The winds are light4. Both 2 and 3 above are correct

2-19. The latest upper-air sounding shows the1000-700 mb layer over your station hasdecreased in thickness over the last 24 hours.What does this change in thickness tell you, ifanything, about the vertical pressure gradientwithin this stratum?

1. It has increased2. It has decreased3. The gradient remains unchanged because

the pressures have not changed4. Nothing without height figures

2-20. Which of the following forces has the greatesteffect on wind speed?

1. Centrifugal2. Pressure gradient3. Friction4. Coriolis

2-21. Which of the following forces causes the windto begin moving from areas of high pressuretoward areas of low pressure?

1. Centrifugal2. Pressure gradient3. Friction4. Coriolis

8

2-22. What effect does centrifugal force have oncyclonic circulation?

1. It forces air out away from the center2. It pulls air toward the center3. It pushes air toward the center4. It forces air from high to low pressure

2-23. What effect, if any, does the wind speed haveon the centrifugal force in a high-pressuresystem?

1. The higher the wind speed, the greater theforce

2. The higher the wind speed, the smaller theforce

3. The force is inversely proportional to thewind speed

4. None, the force is independent of the windspeed

2-24. Friction affects wind velocities at what levels?

1. The surface only2. All levels3. All levels up to the gradient level4. The gradient level only.

2-25. At 40,000 feet, which of the following balanceof forces causes the wind to blow parallel tocurved isoheights?

1. The centrifugal force and Coriolis forceare in balance

2. The centrifugal force and pressure gradientforces are in balance

3. The centrifugal and centripetal forces arebalanced

4. The pressure gradient force and centripetalforce are in balance

2-26. A low-pressure system over the Virginia Capesmoves northeast without any changesoccurring in the density of the air or to thepressure gradient. What happens to thegradient wind speed?

1. It decreases due to the easterly movement2. It increases due to the northerly movement3. It decreases due to the northerly movement4. It remains the same

2-27. An extratropical low-pressure system isstationary 200 n. mi. south of Kainchatka. Withthe density of the air remaining the same andthe pressure gradient decreasing, what happensto the gradient wind speed associated with thislow?

1. It decreases2. It increases3. It remains the same4. Both 2 and 3 are possible

2-28. Around high-pressure systems, Coriolis forceopposes the

1. gradient force only2. centrifugal force only3. pressure gradient force and centrifugal

force4. centripetal force

2-29. Coriolis force always opposes the pressuregradient force around cyclones andanticyclones.

1. True2. False

2-30. When measuring the gradient winds aroundlow- and high-pressure systems using ageostrophic wind scale, how do geostrophicwind speeds compare to gradient wind speeds?

1. Geostrophic winds are stronger than thegradient winds around both systems

2. Geostrophic winds are weaker around lowsand stronger around highs

3. Geostrophic winds are stronger aroundlows and weaker around highs

4. They do not differ

2-31. What are the most common geostrophic windscale increments?

1. 2 mb and 15 meters2. 4 mb and 30 meters3. 4 mb and 60 meters4. 8 mb and 120 meters

9

2-32. Which of the following statements definesEarth’s secondary circulation?

1. The circulation is created and maintainedby the effect of Earth’s non-uniformsurface and composition

2. The circulation is created by thermaldifferences in the atmosphere

3. It is that portion of the tertiary circulationcaused by thermal differences betweenland and water

4. The circulation is created and maintainedby the effects of Earth’s non-uniformsurface and composition and Earth’sthermal differences

2-33. Centers of action are created by

1. wind2. seasonal temperature differences3. temperature differences between land and

water4. pressure belts

2-34. What is the name given to the permanent andsemi-permanent high- and low-pressure cells?

1. Thermal cells2. Migratory cells3. Centers of action4. Primary circulations

2-35. Some centers of action disappear at certaintimes of year.

1. True2. False

2-36. In winter, what pressure systems are found inthe Northern Hemisphere over Siberia, theeastern Pacific Ocean, and the eastern AtlanticOcean?

1. High pressure at all three locations2. Low pressure covers Siberia, while high

pressure is found over the eastern Pacificand Atlantic

3. Low pressure at all three locations4. High pressure covers Siberia, while low

pressure is found over the eastern Pacificand Atlantic

2-37. How are the subtropical high-pressure systemsaffected, if at all, by seasonal changes?

1. They are weaker in summer and fartherpoleward

2. They are stronger in summer and fartherpoleward

3. They are stronger in summer and nearerthe equator

4. They are not affected

2-38. Which of the following pressure systems isNOT classified as a center of action?

1. Aleutian low2. Bermuda high3. Polar high4. Hatteras low

2-39. Where is the largest individual secondarycirculation cell in the Northern Hemispherelocated?

1. North American continent2. Asian continent3. African continent4. European continent

2-40. Migratory wind circulations are not classifiedas centers of action. Why?

1. They are seasonal2. They are not as intense3. They are found only in midlatitudes4. They are not persistent in location or

intensity

2-41. Pressure-system movement, shape andintensity are dependent on what factor?

1. Circulation2. Temperature3. Height4. Thickness

2-42. An anticyclonic circulation in the SouthernHemisphere whose temperature pattern is suchthat colder temperatures are located at thecirculation center is known as a

1. warm-cored low2. warm-cored high3. cold-cored low4. cold-cored high

10

2-43. Which of the following systems have thegreatest vertical extent?

1. Cold-cored lows and highs2. Warm-cored lows and highs3. Warm-cored lows and cold-cored highs4. Cold-cored lows and warm-cored highs

2-44. Well-developed cyclonic and anticyclonicclosed circulations at the surface may or maynot be evident on upper-level charts, and thesame type circulations may appear onupper-level charts and not be evident at thesurface.

1. True2. False

2-45. How does a closed cyclonic circulation in theNorthern Hemisphere with warmertemperatures toward the circulation center’differ from a similar circulation with coldertemperatures toward the center?

1. It does not extend as far into theatmosphere

2. Its intensity lessens with height3. It is classified as a warm-cored low4. All of the above

2-46. A migratory closed circulation that extendswell into the atmosphere is classified as

1. warm-cored2. cold-cored3. dynamic4. vertically axised

2-47. Tertiary circulations are small, localizedcirculations created by which of the followingconditions?

1. Local heating and cooling2. Adjacent heating and cooling3. Induction4. All of the above

2-48. The monsoons of India and southeast Asia areseasonal in nature, and in winter, the monsoonwinds are normally accompanied by whatweather conditions?

1. Constant heavy rain2. Heavy rain showers and thunderstorms3. Both 1 and 2 are correct4. Mostly clear skies

2-49. A sea breeze can be expected to reach itsmaximum intensity between what hours?

1. 0600 to 0800 local2. 0900 to 1100 local3. 1400 to 1600 local4. 2000 to 2200 local

2-50. Sea breezes are most pronounced during whichof the following seasons?

1. Winter2. Late winter and early spring3. Late spring, summer and early autumn4. Late autumn to early spring

2-51. Mountains act as barriers to wind; however, ifthere are valleys or passages through themountains, the wind may pass through at greatspeeds. Which of the following factors controlsthe wind speeds through such openings?

1. The orientation of the mountain range2. The pressure difference on each side of the

mountain3. The pressure pattern on each side of the

mountain4. Each of the above

2-52. Which of the following names applies to thecold dense air of the Greenland ice cap (10,000feet above sea level) when it is set in motionand rushes down the cap to sea level?

1. Glacier wind2. Mountain wind3. Gravity wind4. Each of the above

2-53. What is a thermal?

1. A warm dry wind that begins at the base ofa mountain and ascends the mountainslope

2. A warm moist wind that begins at the baseof a mountain and ascends the mountainslope

3. A relatively small 1-scale convectivecurrent produced by strong local heating

4. Turbulence created by moderate to strongairflow over rough or hilly terrain

11

2-54. Which of the following types of rotation is/areinduced in eddies, dust devils and waterspouts?

1. Cyclonic in the Northern Hemisphere;anticyclonic in the Southern Hemisphere

2. Anticyclonic in the Northern Hemisphere;cyclonic in Southern Hemisphere

3. Cyclonic only4. Cyclonic or anticyclonic, indeperxlent of

the hemisphere

2-55. When winds in excess of 20 knots blowperpendicular to a mountain range, what windconditions might be expected on the lee side?

1. Updrafts only2. Strong downdrafts3. Very turbulent conditions4. Both 2 and 3

2-56. Under which of the following wind conditionsmay turbulence be expected?

1. Winds blow in the same direction but atdifferent speeds

2. Wind currents blow past each other inopposite directions

3. Winds blow over uneven surfaces4. Each of the above

2-57. Mountain waves are an example of

1. large-scale vertical eddies2. small-scale vertical eddies3. large-scale horizontal eddies4. small-scale horizontal eddies

12

ASSIGNMENT 3

Textbook Assignment: “Air Masses and Fronts.” Chapter 4, Pages 4-1 through 4-31.

3-1. What two factors are necessary to produce anair mass?

1. Anticyclonic circulation and non-homogeneous properties of temperature,lapse rate and moisture

2. Large divergent flow and a widespreadbody of relatively uniform air

3. Uniform surface and relative humidity4. Moisture and heat

3-2. Why are anticyclonic circulations most favor-able for air mass development?

1. The horizontal outflow of air affects amuch larger area

2. The air moves slowly or is stagnant,making it easier for the air to assume thecharacteristics of the underlying surface

3. The subsidence associated with thesecirculations is favorable for lateral mixing,thereby bringing about horizontalhomogeneity

4. All of the above are reasons

3-3. The properties an air mass acquires in itssource region are dependent on a number offactors. Which of the following is NOT a factorin determining air mass properties?

1. Time of year2. Type of surface (land, water, ice)3. Length of time the air mass remains over

the region4. Circulation pattern

3-4. Which air mass has its source region between10°N lat. and 10°S lat.?

1. T2. A3. P4. E

3-5. Monsoon air is REALLY one of two air massesdepending on the time of year. Which two airmasses make up monsoon air?

1. cP and mT2. cP and E3. mP and E4. mP and mT

3-6. On what factors are air mass classificationsbased?

1. Temperature, humidity, and wind2. Season, latitude, and source region3. Geographic origin, moisture content, and

thermodynamic process4. Geographic origin, temperature, and

humidity

3-7. A cP air mass moves south out of its sourceregion in Canada and invades the south centralU.S. How would this air mass most likely beclassified thermodynamically?

1. Moist Cm)2. Cold (k)3. Warm (w)4. Cool (c)

3-8. How is the stability of an air mass affected, if atall, when it leaves source region?

1. It is increased only2. It is decreased only3. It may be increased or decreased4. It is not affected

3-9. It is February, and a very cold continental polarair mass pushes south over tropical waters.Which, if any, of the following changes is mostlikely to occur within the air mass?

1. A decrease in water vapor content takesplace

2. The lower layers are cooled by conduction3. An increase in stability occurs4. None of the above

13

3-10. Air mass stability can be changedthermodynamically or mechanically.

1. True2. False

3-11. Over the midlatitudes of North America inwinter, an air mass exhibiting surfacetemperatures of -18°C or colder is generallyclassified as

1. cPk2. mPk3. cAk4. cPw

3-12. Much can be gained from knowing the path cPand cA air masses take on leaving their sourceregions in North America. Which of thefollowing statements pertains to the winteroutbreaks of these air masses when their path iscyclonic?

1. Good flying conditions are the rule2. Cloud cover lingers along the Atlantic

coast until the air mass clears theAppalachian mountains

3. Frequent and widespread snow squalls canbe expected on the leeward side of theGreat Lakes

4. Unrestricted visibilities are common onthe windward side of the Appalachianmountains

3-13. Weather along the U.S. west coast in winter ispredominantly the result of which air mass?

1. mP2. cP3. cA4. mT

3-14. What air mass is generally responsible forrelatively mild weather across the U.S. inwinter, and is often incorrectly referred to asmT?

1. cP with a short cyclonic trajectory over thePacific

2. cP with a long cyclonic trajectory over thePacific

3. mP with an anticyclonic trajectory alongthe northern border of the Pacific high

4. Highly modified equatorial (E) air

3-15. The heaviest precipitation recorded inSouthern California in winter is produced fromwhat air mass?

1. mP2. cP3. mT4. cT

3-16. Maritime polar (mP) air is far more frequentalong the west coast of the U.S. than the eastcoast. Why?

1. mP air of the Atlantic and Pacific are bothmore apt to move in an easterly direction,and in the Atlantic, this movement takesmP air away of the U.S. east coast

2. cP air is heavier and more dense than mPair, and it acts as a barrier over NorthAmerica

3. The Greenland ice cap blocks mP air frommoving west to the North Americancontinent (U.S. east coast)

4. The very warm water temperatures of theGulf Stream rapidly modify mP airrequiring it to be reclassified as mT air

3-17. After a week of colder than averagetemperatures, the southeastern U.S. comesunder the influence of much warmer mT airover the Gulf of Mexico. Which of thefollowing types of weather will likely beproduced by the mT air?

1. Snow flurries2. Thunderstorms and tornadoes3. Widespread advection fog4. Copious rain

3-18. True mT air does not have dew pointtemperatures below what value?

1. 60°F2. 65°F3. 70°F4. 75°F

3-19. Which of the following air masses dominatesmost of the U.S. during the summer season?

1. mT or cP2. mT or mP3. mT or S4. E or mP

14

3-20. Which of the following air masses dominatesthe U.S. Pacific coast during summer?

1. mT2. mP3. cP4. S

3-21. In summer, east of the Rocky Mountains, mPair and cP air exhibit the same properties.

1. True2. False

3-22. When mT air moves north over the GrandBanks area of Newfoundland in summer,which of the following types of weather is mostlikely to occur?

1. Fog2. Heavy rain3. Convective thunderstorms4. Mechanical thunderstorms

3-23. Which of the following air masses is NOTfound over the North American continentduring winter?

1. cT2. S3. mT4. cPk

3-24. Which of the following statements concerningsuperior air is correct?

1. It is found in the northwest U.S.2. It is an exceptionally moist air mass3. It is rarely found at the surface4. Each of the above

3-25. In winter, Japan’s weather is primarilyinfluenced by which of the following airmasses?

1. cP2. mT3. mP4. cT

3-26. The summer monsoon of India and Burma isthe result of what air mass?

1. mT2. cP3. mP4. E

3-27. In winter, Great Britain occasionallyexperiences the effects of cA and cP air. Wheredo these air masses originate?

1. Iceland2. North America3. Greenland and Spitsbergen4. Siberia, Finland and Lapland

3-28. Which of the following air masses originatesover the Atlantic Ocean but moves over landand is classified as a continental air mass?

1. cP2. cT3. cA4. S

3-29. Australian weather is dominated by cT air;however, mT air is more of a factor along oneof its four coasts. Which coast is most affectedby mT air?

1. East2. West3. North4. South

3-30. Which of the following air masses is thecoldest on record and where is it found?

1. cP - North America2. cA - Arctic3. cA - Antarctica4. mP - Weddell Sea

3-31. In the Southern Hemisphere, which of thefollowing air masses is the most important inproviding relief from the oppressive summerheat?

1. cP2. cA3. mP4. mA

3-32. What is a front?

1. A boundary between two air masses2. A zone of transition between two adjacent

air masses bounded by a frontal surface3. A zone of transition between two air

masses of different densities4. A point where two air masses touch

15

3-33. What determines frontal classification?

1. Density differences2. Temperature differences3. Frontal movement4. Involved air masses

3-34. What classification is given to the front thatseparates a warm air mass from a retreatingmass of cold air?

1. Warm2. Cold3. Quasi-stationary4. Occluded

3-35. What are the primary frontal zones in theNorthern Hemisphere?

1. Polar and tropical2. Polar and arctic3. Arctic and tropical4. Polar and equatorial

3-36. Cold air being heavier than warm air will eitherunderrun the warm air or be overrun by warmair.

1. True2. False

3-37. Which of the following statements refer(s) tofrontal slope?

1. A front’s position along the Earth’s surface2. The zone of discontinuity between air

masses3. The ratio of a frontal surface’s elevation to

horizontal extent4. All of the above

3-38. Which of the following statements concerningthe relationship between fronts and cyclones(low-pressure centers) is correct?

1. All surface fronts develop a closedcyclonic circulation at the surface and aloft

2. Upper-level cyclones that lower to theEarth’s surface always contain fronts

3. Every front is associated with a cycloneand travels with it

4. Fronts can occur anywhere but cyclonescannot

3-39. What is the average speed of wave cyclonesalong the polar front?

1. 10 - 15 knots2. 15 - 20 knots3. 20 - 25 knots4. 25 - 30 knots

3-40. When is a frontal wave most intense?

1. When a cyclonic circulation develops2. When a cyclonic circulation causes the

cold air to overtake the warm air and formsan occlusion

3. When the pressure in the wave cyclonebegins to lower

4. When the pressure in the wave cyclonereaches its lowest point

3-41. When two air masses having different densitiesare brought together by the prevailing windfield, what takes place?

1. A front forms2. There’s a decrease in the temperature

grient3. The windflow parallels the isotherms4. Each of the above

3-42. What is cross-isothermal flow?

1. The flow of wind across isobars2. The flow of wind across isotherms3. The flow of wind across fronts4. A col.

3-43. Frontogenesis is most likely to occur wherethere is a concentration of isotherms and acirculation that sustains the concentration.

1. True2. False

3-44. Which of the following statements concerningfrontolytical processes is correct?

1. They are most effective in the lower layersof the atmosphere

2. They are more common thanfrontogenetical processes

3. They bring about frontal dissipation4. Each of the above

16

3-45. During summer in the Northern Hemisphere,where would you most likely find the Arcticfront?

1. In the North Atlantic2. In the North Pacific3. North of Europe4. Northeastern Asia

3-46. Which of the following statements concerningpolar fronts is correct?

1. They separate polar air from tropical aironly

2. They are stronger in summer than winter3. They are more common along eastern

coasts of continents in summer4. They are present throughout the year

3-47. What three elements are used to determinewhether or not a front actually exists?

1. Visibility, temperature, and pressure2. Clouds, temperature, and wind3. Temperature, pressure, and wind4. Present weather, temperature, and pressure

3-48. The temperature increase within a frontalinversion and the thickness of the inversionlayer provide a rough indication of

1. frontal slope2. frontal intensity3. turbulent mixing4. precipitation within a frontal zone

3-49. Which of the following statements concerningfrontal inversions is correct?

1. Cold fronts generally show strongerinversions than warm fronts

2. They normally show up as a decrease in thelapse rate below 400 millibars

3. Double inversions are often evident withoccluded fronts

4. Each of the above

3-50. Which of the following occurrences causes afront to exhibit a strong inversion layer andlittle or no weather activity?

1. Subsidence in the warm air above thefrontal surface

2. Subsidence in the cold air beneath thefrontal surface

3. Adiabatic warming of the cold air beneaththe frontal surface

4. Upward vertical motion in the warm air

3-51. In the Northern Hemisphere when a frontpasses your station, what change takes place inthe wind direction?

1. It veers2. It backs3. It shifts in a counterclockwise direction4. It shifts in a clockwise direction

3-52. In a frontal zone, what, if anything, normallyhappens to the wind speeds?

1. They increase with height only2. They decrease with height only3. They may increase or decrease with height4. They vary on either side of the frontal

zone, but maintain a steady state throughthe zone

3-53. At the surface, when a front moves beyond itsassociated pressure trough, how, if at all, arethe winds across the front affected?

1. The wind speeds do not change, but thewind shift becomes far more apparent

2. The wind speeds show a drastic change,and the wind shift becomes far moreapparent

3. The wind speed difference across the frontcontinues, but the wind shift can becomealmost undetectable

4. Movement out of the pressure troughaffects neither wind speeds or direction

3-54. Which of the following factors causes frontalclouds, condensation, and weather?

1. Low pressure2. Friction between front and Earth’s surface3. Vertical displacement of air along the front4. Each of the above

3-55. Which of the following frontal slopes isclassified as being the steepest?

1. 1:352. 1:503. 1:1504. 1:300

3-56. What factor(s) contribute(s) to a steep frontalslope?

1. High wind velocity difference across thefront

2. Small temperature contrast across the front3. High latitude4. All of the above

17

3-57. In the warm air ahead of a cold front, thetemperatures average 82°F/28°C. In the coldair 100 miles to the rear of the cold front, thetemperatures average 64°F/l8°C. What is thefront’s intensity based on temperaturegradient?

1. Very weak2. Weak3. Moderate4. Strong

3-58. Frontal movement is determined by the

1. temperature gradient behind the front2. pressure difference across the front3. wind speed component ahead of the front4. wind velocity component behind the front

18

ASSIGNMENT 4

Textbook Assignment: “Air Masses and Fronts” (continued); “Atmospheric Phenomena.” Chapters 4 and 5, Pages4-31 through 5-6.

4-1. What is the average slope of a slow-movingcold front?

1. 1:502. 1:1003. 1:1504. 1:300

4-2. Which of the following indications is repre-sentative of the passage of a slow-moving coldfront?

1. Wind backs2. Sharp rise in pressure3. Marked temperature rise4. Sharp drop in the dew point

4-3. What is the average range of speed of a slow-moving cold front?

1. 5 to 10 knots2. 10 to 15 knots3. 15 to 20 knots4. 20 to 25 knots

4-4. When the cold air to the rear of a slow-movingcold front is moist and stable, and the warm airthat it is displacing is also moist and stable,which of the following weather conditions ismost likely to occur in the vicinity of the front?

1. Thunderstorms at and ahead of the front2. Thunderstorms at and behind the front3. Rapid clearing with the frontal passage4. Low ceilings of stratus and fog

4-5. Which of the following statements describes acharacteristic of slow-moving cold fronts?

1. The frontal inversion is usually veryevident

2. Isotherms parallel the front and areconcentrated in the cold air

3. Cloudiness and precipitation normallyextend back into the cold air as far as thewind and isotherms parallel the front

4. Each of the above

4-6. The winds that push a slow-moving cold frontare more parallel to the front at lower levelsthan aloft.

1. True2. False

4-7. Where do squall lines develop?

1. In advance of a slow-moving cold front2. To the rear of a slow-moving cold front3. In advance of a fast-moving cold front4. To the rear of a fast-moving cold front

4-8. Which of the following factors determines thetype of weather associated with a fast-movingcold front?

1. The moisture content of the cold air only2. The stability of the cold air only3. The moisture content and stability of the

cold air4. The moisture content and stability of the

warm air

4-9. A fast-moving cold front has an average rangeof speed of

1. 15 to 20 knots2. 20 to 25 knots3. 25 to 30 knots4. 30 to 35 knots

4-10. Which of the following indications is/areassociated with the passage of a fast-movingcold front?

1. The dew point changes little if at all2. The temperature changes little, if at all,

until the front is well past3. Rapid clearing4. Answers 2 and 3 are both correct

19

4-11. Which of the following upper aircharacteristics is associated with the passage ofa fast-moving cold front?

1. Slight backing of the wind with height2. A double inversion; the frontal inversion

and a subsidence inversion some distanceto the rear of the front

3. Isotherms are well spaced and cross thefront at an angle of about 30 degrees

4. Each of the above

4-12. What is a secondary cold front?

1. A fresh outbreak of very cold air to the rearof a fast-moving cold front

2. A trough of low pressure3. The classification given to any

summertime cold front4. Any cold front that is classified as

unimportant meteorologically

4-13. Which of the following occurrences leads tothe formation of a cold front aloft?

1. The mP air to the rear of a cold frontcrosses a mountain range and rides atopwarm moist mT air

2. Cool air overtakes colder more dense airand rides up over it

3. Cold dense air overtakes cooler less denseair and forces it aloft

4. Each of the above

4-14. A squall line is an instability line, but aninstability line is NOT necessarily a squall line.

1. True2. False

4-15. Which of the following statements concerningprefrontal squall lines is correct?

1. They form about 50 to 300 miles inadvance of fast-moving cold fronts

2. Their speed is roughly equal to 40% of the500-mb wind speed

3. They are most common in spring andsummer in the United States

4. Each of the above

4-16. Which of the following weather changesoccurs with the passage of a prefrontal squallline?

1. The temperature rises significantly2. The pressure falls3. The wind shifts cyclonically4, The wind shifts anticyclonically

4-17. What air mass(es) is/are involved in thedevelopment of Great Plains squall lines?

1. mT only2. mT and mP3. mT and cP4. mP and cP

4-18. What is the average speed of warm fronts?

1. 5 to 10 knots2. 10 to 20 knots3. 15 to 25 knots4. 20 to 30 knots

4-19. In the Northern Hemisphere, how are thesurface winds affected before and after thepassage of a warm front?

1. They are generally southeasterly ahead ofthe front and shift to southwesterly afterpassage

2. They are strongest after passage3. They shift in a counterclockwise direction

4-20. Where is nimbostratus and its accompanyingprecipitation most frequently found in relationto the warm front?

1. Within 300 miles of the front in the coldsector

2. Within 300 miles of the front in the warmsector

3. 500 miles in advance of the front4. 500 miles to the rear of the front

4-21. What is produced when the windsperpendicular to a warm front increase withheight?

1. Strong overrunning of the warm air acrossthe top of the retreating cold air mass

2. Pronounced prefrontal cloudiness3. Precipitation4. Each of the above

4-22. When overrunning occurs, and the air is moistand unstable, which of the following weatherphenomena occurs?

1. Clear skies2. High and mid clouds only3. Stratus and fog4. Thunderstorms

20

4-23. When a warm front crosses a mountain rangeand encounters colder air on the lee side of themountain, which of the following phenomenamay occur?

1. The warm front moves across the top of thecold air as an upper warm front

2. Overrunning3. Inversions are wiped out4. Each of the above

4-24. Occluded fronts are classified as which of thefollowing types?

1. Cold only2. Warm only3. Cold or warm4. Cold, warm or cool

4-25. What is the primary difference between a warmand cold occlusion?

1. The temperature of the warm air2. The temperature of the cold air3. The temperature of the cool air4. The location of the associated upper front

in relation to the surface front

4-26. Which of the following occurrences takesplace in the cold-occlusion process?

1. Cold air displaces the warm air to the rearof a warm front and then undercuts therelatively cooler air in advance of the warmfront

2. Cool air displaces the warm air to the rearof a warm front and then rides up over thecolder retreating air ahead of the warmfront

3. Warm air displaces the cold air in advanceof the warm front and rides up over cool airbehind the cold front

4. Cold air replaces warm air and thenoverruns relatively cooler air ahead of thewarm front

4-27. How is a cold occlusion designated that crossesthe Rocky Mountains and encounters deep,cold air over the Plateau or Western Plains?

1. As an occlusion2. As a cold front3. As an upper cold front4. As a warm front

4-28. Where does MOST of the precipitation occurwith a cold occlusion?

1. Ahead of the occlusion, if the occlusion isold

2. To the rear of the occlusion in theocclusion’s initial stages of development

3. Just ahead of the occlusion4. Just to the rear of the occlusion

4-29. How are the isotherms affected as an occlusionmatures?

1. They become more parallel to theocclusion on the cold air side

2. They become more parallel to theocclusion on the warm air side

3. They become more perpendicular as theycross the front

4. Warm and cold pockets form, and noisotherms cross the front

4-30. Which of the following situations is conduciveto the formation of a warm occlusion?

1. The presence of a cPk air mass in the Gulfof Mexico

2. The invasion of mPk air into the GreatPlains

3. The presence of cpk air over Canada, awarm front along its western periphery,and an approaching mPk air mass

4. The development of a low at the southerntip of the Appalachian Mountains

4-31. Where are the pressure falls associated with awarm occlusion located?

1. In advance of the upper warm front2. In advance of the occlusion’s surface

position only3. In the pressure trough behind the occlusion4. In advance of the upper cold front and the

surface occlusion

4-32. In a warm occlusion, where is the most severeweather located?

1. At the apex during the developmental stage2. At the point where the warm air is at its

highest altitude3. In the warm sector equatorward of the apex

of the occlusion4. At the northernmost extension of a mature

occlusion

21

4-33. The wind shift across a quasi-stationary front ison the order of how many degrees?

1. 452. 903. 1354. 180

4-34. Which of the following statements is trueconcerning the winds above the friction levelover a quasi-stationary front?

1. They parallel the front2. They are more or less perpendicular to the

front3. They are non-existent4. They parallel the front in the warm air only

4-35. The weather along a quasi-stationary front isdependent on which of the followingconditions?

1. The steepness of the frontal slope2. The stability of the warm air3. Undulations of the front toward the warm

air4. All of the above conditions

4-36. Which, if any, of the following types ofweather is associated with a quasi-stationaryfront when stable, warm air is advected up asteep frontal slope?

1. Tornadoes2. Thunderstorms3. Embedded showers4. None of the above

4-37. The most violent weather associated withquasi-stationary fronts occurs when

1. stable warm air is advected up a steepfrontal slope

2. unstable warm air is advected up a shallowfrontal slope

3. unstable warm air is advected up a steepfrontal slope

4. stable warm air is advected up a shallowfrontal slope

4-38. Which, if any, of the following effects offrontal speed on weather is correct?

1. Fast-moving fronts usually produce themost violent weather

2. The return to favorable weather conditionstakes place much quicker with aslow-moving front

3. A front whose speed is erratic createsvarying weather conditions, and is mucheasier to forecast

4. None of the above

4-39. Which of the following aspects of a front isaffected by mountain ranges?

1. Speed2. Slope3. Weather4. Each of the above

4-40. With regard to precipitation, a cold front thatapproaches and crosses a mountain range willgenerally

1. show a decrease in precipitation intensity2. have its area of precipitation narrowed3. produce greater amounts of precipitation

on the leeward side4. produce greater amounts of precipitation

on the windward side

4-41. Which of the following occurrences takesplace when a warm front encounters amountain range?

1. The warm air above the frontal surface ismechanically lifted producing severethunderstorms

2. The frontal slope is drastically changed athigher altitudes

3. The front becomes more or less stationaryon the leeward side

4. The cold air beneath the frontal surfacegets cut off on the windward side

4-42. Mountain ranges prolong warm frontalprecipitation and widen the precipitation area.

1. True2. False

22

4-43. Which of the following statements concerningskagerraking and occlusions is correct?

1. Skagerraking occurs most frequently onthe west coast of mountainous continents

2. The new low develops very rapidly3. Skagerraking can occur with either cold or

warm occlusions4. Each of the above

4-44. When an air mass leaves its source region, itmay be modified by the underlying surface inwhich of the following manners?

1. Moisture may be added and taken away2. Temperatures may be increased or

decreased3. Frontal characteristics may be completely

destroyed4. All of the above

4-45. In the western Atlantic and Pacific Oceans,cold fronts of fall and winter are of greaterconcern, to shipping than at other times of theyear. Why?

1. Air mass contrast is magnified therebyproducing more severe weather

2. Gale force winds are common in the coldair to the rear of these fronts

3. Low-pressure systems are often spawnedand develop over the warm northerlyflowing waters of these regions

4. All of the above

4-46. To be classified as rain, the water droplets thatreach the Earth’s surface will have a diameterof

1. 001 to .01 inch2. .005 to .02 inch3. .010 to .02 inch4. .020 inch and greater

4-47. How is precipitation that falls from convectiveclouds classified?

1. Rain2. Snow3. Showery4. Steady

4-48. Which of the following hydrometeors appearsas a fine mist, floats rather than falls throughthe air, and is frequently accompanied by fogand restricted visibilities?

1. Light rain2. Snow3. Drizzle4. Rain

4-49. Which of the following hydrometeors is con-sidered to be the frozen equivalent of drizzle?

1. Snow grains2. Snow pellets3. Ice pellets4. Ice crystals

4-50. What is another name for sleet?

1. Snow grains2. Snow pellets3. Ice pellets4. Ice crystals

4-51. How, if at all, does sleet differ from small hail?

1. Sleet rebounds on striking the ground, haildoes not

2. Sleet is composed of snow encased in anice layer, and hail is the exact opposite

3. Sleet is a continuous type of precipitation,while small hail is showery

4. They are both ice pellets and do not differ

4-52. Hail forms in what type of cloud?

1. Cumulus mediocris2. Altocumulus castellanus3. Nimbostratus4. Cumulonimbus

4-53. Which of the following hydrometeors iscommon in polar regions and mainly visible insunlight?

1. Ice prisms2. Ice pellets3. Snow pellets4. Snow grains

4-54. What occurs when water droplets in a cloudevaporate and then sublimate directly onto icecrystals within the cloud?

1. The ice crystals always melt2. Precipitation begins3. Nothing until the ice crystals melt, then the

original droplets will have grown in size4. Turbulence

23

4-55. In order for water vapor to condense and formclouds, which of the following conditions isNOT necessary?

1. Sufficient moisture2. Hygroscopic or sublimation nuclei3. Turbulent air currents4. A cooling process

4-56. Why are hygroscopic and sublimation nucleiso important in the cloud formation process?

1. They determine the type of cloud that willform

2. Cloud formation is all but impossiblewithout them

3. They trigger the precipitation process4. All of the above

4-57. What clouds are believed to be the result ofdirect sublimation?

1. Cirriform2. Stratiform3. Cumuliform4. Nacreous

4-58. What are the upper limits of cirriform clouds(based on etage classification) in the tropics,middle latitudes, and polar regions?

1. 80,000, 45,000, and 25,000 feet2. 60,000, 45,000, and 25,000 feet3. 60,000, 30,000, and 16,600 feet4. 20,000, 16,500, and 10,000 feet

4-59. Which of the following clouds is classified asbelonging to one etage but may extend intoother etages?

1. Altocumulus2. Altostratus3. Nimbostratus4. Stratus

4-60. The cloud species castellanus applies mainly towhich of the following cloud genera?

1. Cumulus2. Stratus3. Altocumulus4. Cirrus

4-61. A cumulonimbus cloud that produces hangingpouchlike protuberances is known as

1. tuba2. castellanus3. mammatus4. congestus

4-62. Elongated cloud masses in the shape of lensesor almonds are classified as

1. humilis2. stratiformis3. fractus4. lenticularis

4-63. The fair weather cumulus clouds of the tropicshave little vertical extent and are classified as

1. humilis2. mediocris3. fractus4. castellanus

24

ASSIGNMENT 5

Textbook Assignment: “Atmospheric Phenomena” (continued); “Climatology and World Weather.” Chapters 5and 6, Pages 5-6 through 6-6.

5-1. Which of the following facts about fog is incor-rect?

1. Fog is most easily described as a cloud atthe Earth’s surface

2. All fogs are composed of minute waterparticles only

3. Fog depth and density are quite variable4. Local geography and topography can play

a major role in the formation anddissipation of fog

5-2. Where and when is the formation of radiationfog most common?

1. Over cold waters at night2. Over land at night3. Over land in the early afternoon4. Over coastal waters in the early morning

5-3. How does wind speed affect radiation fog?

1. Calm winds cause a shallow fog layer toform

2. Winds of 5 to 10 knots create turbulentcurrents that increase the depth of the fog

3. Winds greater than 10 knots usually causethe fog to lift, thereby forming low scud,stratus, or stratocumulus

4. All of the above

5-4. Which of the following conditions is most con-ducive to the formation of radiation fog?

1. Low pressure, light winds, and overcastskies

2. Low pressure, light winds, and clear skies3. High pressure, light winds, and clear skies4. High pressure, light winds, and overcast

skies

5-5. What are advection fogs?

1. Fogs produced by the movement of warmair over a colder land or water surface

2. Fogs that form in the clear night air overwarm waters

3. Fogs produced across air mass frontalboundaries

4. Fogs of the tropics

5-6. Which of the following types of fog is not clas-sified as advection fog?

1. Sea fog2. Arctic sea smoke3. Upslope fog4. Steam fog

5-7. Most fog is destroyed (lifted) when the windspeed over a fog enshrouded area increases.Which of the following classifications/types offog is most likely to persist in wind up to 26knots?

1. Land advection fog2. Sea fog3. Upslope fog4. Radiation fog

5-8. Which of the following classifications/types offog is most likely to occur in winter, when anarctic outbreak pushes off the U.S. east coastover warm Gulf Stream waters?

1. Sea fog2. Steam fog3. Land advection fog4. Radiation fog

5-9. Which of the following statements concerningfrontal fog is correct?

1. Frontal fog is the result of evaporation offalling rain

2. It forms in the cold air mass3. This fog begins as low clouds that

eventually lower to the ground4. Each of the above

5-10. On some mornings, grass, plants, and possiblyyour car will be wet with dew while the roadand some large objects will be dry. Why dosome surfaces remain dry?

1. Micro air temperature differences2. Micro dew point variations3. Some surfaces retain heat longer and fail to

cool to the dew point4. Some surfaces cool far too fast for the

moisture to accumulate on them

25

5-11. With regard to classification, how does spraydiffer from blowing spray?

1. Wind speed2. Visibility3. Wave heights4. Droplet size

5-12. Tornadoes travel at what average range ofspeed?

1. 0 to 5 knots2. 7 to 15 knots3. 12 to 20 knots4. 22 to 34 knots

5-13. Which of the following areas is most conducivefor the formation of tornadoes?

1. Cols2. 30 miles to the rear of short-wave troughs3. 75 to 180 miles in advance of fast-moving

cold fronts4. In areas of warm air overrunning cold air

5-14. Which of the following conditions is NOTindicative of tornado formation?

1. Strong convergent winds at the surface2. Suppressed convection up to the minus

10°C isotherm3. Marked convective instability4. Strong horizontal wind shear

5-15. Upon observing the development of a water-spout, how can an observer tell, if it is of thelocal or tornadic variety?

1. Size2. Stability index3. Development process4. Vertical extent of convective clouds

5-16. Which of the following lithometeors reduce(s)visibility in a veil-like cover?

1. Smoke2. Dust storms3. Haze4. Sand storms

5-17. Your station’s visibility markers are set at 1/8,1/4, 3/8, 1/2, 3/4, 1, 1 1/2, 2, 2 1/2, 3, 4, 5, 6, 7,and 15 miles. What is the maximum distance(by marker) that your observer will be able tosee in a severe dust storm?

1. 1/8 mi2. 1/4 mi3. 3/8 mi4. 1/2 mi

5-18. Which of the following statements is NOT acharacteristic of photometeors?

1. They appear as luminous patterns in thesky

2. Many are cloud related3. They help in describing the state of the

atmosphere4. They are all precursors of bad weather

5-19. When light encounters any substance, which ofthe following occurrences might take place?

1. Refraction only2. Reflection or refraction3. Absorption or refraction4. Absorption, reflection, or refraction

5-20. Visible light occupies that portion of theelectromagnetic spectrum between

1. 4000 and 7000 angstroms2. 2500 and 4000 angstroms3. 1200 and 2500 angstroms4. 400 and 1100 angstroms

5-21. How does the Moon produce moonlight?

1. It is a luminous body and produces its ownlight

2. It absorbs light from the Sun andregenerates it at night

3. It reflects the light it receives from the Sun4. Through a combination of reflection,

absorption, and refraction

5-22. A substance permits the passage of lightthrough it, but the light appears clouded, andviewing things through such a substance isimpaired. This substance is described as being

1. transparent2. translucent3. opaque4. fluorescent

26

5-23. An object that allows virtually 100 percent ofthe light striking it to pass through exhibits theproperty of

1. opacity2. translucency3. transparency4. absorptivity

5-24. When none of the light waves that strike amedium pass through it, the medium is termed

1. opaque2. absorbent3. translucent4. transparent

5-25. A ray of light striking a mirror perpendicularlyis referred to as the

1. angle of reflection2. angle of refraction3. normal4. reflected light

5-26. What is the term given to the angle between areflected light ray and a perpendicular lightray?

1. Angle of incidence2. Angle of reflection3. Angle of refraction4. The normal angle

5-27. When light passes through a medium thatchanges the direction of the light, the light isbeing

1. refracted only2. reflected only3. reflected or refracted4. absorbed and reflected

5-28. When a light ray passes from one medium intoanother of greater density at an angle of 45degrees, how is the light ray affected?

1. It slows and bends away from the normal2. It slows and bends toward the normal3. It is reflected at a 45-degree angle4. It slows, but its path is not altered

5-29. What are the six distinct colors of the visiblespectrum?

1. Red, orange, yellow, green, blue, andbrown

2. Yellow, green, blue, orange, violet, and red3. Blue, green, yellow, orange, black, and

white4. White, black, gray, yellow, blue, and red

5-30. Halos are almost exclusively associated withwhich of the following cloud forms?

1. Cumuliform2. Stratiform3. Cirriform

5-31. Which of the following differences dis-tinguishes coronas from halos?

1. Coronas are usually much larger than halos2. The outer ring of a corona is red, while a

halo’s is violet3. Coronas are formed by refraction of light

through ice crystals, while halos are causedby the diffraction of light by water droplets

4. Coronas form around the Sun and Moonwhile halos form only around the Sun

5-32. What color is usually seen on the outer arc of arainbow?

1. Blue2. Red3. Yellow4. Green

5-33. Mirages are produced when light is

1. absorbed in a very dense cold air mass2. reflected off a very hot surface such as a

desert3. refracted when passing through layers of

air with highly different densities4. reflected, refracted, and diffracted in hot

air

5-34. What is the term given to the phenomena thatcauses stars near the horizon to twinkle andchange color?

1. Iridescence2. Looming3. Superior mirage4. Scintillation

27

5-35. What is “looming”?

1. An atmospheric phenomenon that causesobjects over the horizon, which wouldotherwise not be seen, to become visible

2. A phenomenon that causes stars to twinkleand change color near the horizon

3. An inferior mirage4. A form of iridescence

5-36. A luminous beam of sunlight passing through abreak in the clouds and extending to the Earthlike a spotlight is known as

1. iridescence2. scintillation3. a crepuscular ray4. a sunstroke

5-37. Which of the following atmospheric conditionsis necessary for the formation ofthunderstorms?

1. High temperatures and contrasting airmasses

2. Conditionally stable air and high humidity3. Moist, conditionally unstable air and a

lifting mechanism4. A weak horizontal temperature gradient,

low-level turbulence, and high humidity

5-38. Which of the following statements is NOT trueconcerning the makeup of thunderstorms?

1. In the initial stages of developmentupdrafts prevail throughout the cell

2. A cell’s life cycle usually lasts 1 to 3 hours3. There are three distinct stages in the life

cycle of a cell4. They consists of only one convective cell

5-39. Which of the following lapse rates would mostlikely NOT be found in a thunderstorm?

1. .45/100 meters2. .75/100 meters3. 7.0/1000 meters4. 7.5/1000 meters

5-40. What is considered to be the most hazardouslevel for flying in a thunderstorm?

1. The base2. The middle level3. The upper level4. The freezing level

5-41. Which of the following statements concerningthe winds associated with thunderstorms iscorrect?

1. Microbursts, macrobursts, and first gustsoccur in all convective cells

2. Microbursts are produced by violentupdrafts

3. The wind speed of the first gust is usuallythe highest recorded in a storm

4. Macrobursts normally last 2 to 3 hours

5-42. What is the Earth’s normal electrical field?

1. Ground negative and air positive2. Ground positive and air negative3. Ground and air both positive4. Ground and air both negative

5-43. Within a thunderstorm cloud, where islightning most frequently encountered?

1. Several thousand feet below the freezinglevel

2. At the freezing level3. Between the freezing level and 15°F4. Between the freezing level and the base of

the cloud

5-44. Auroras most commonly occur

1. in thunderstorms2. near the Earth’s magnetic poles3. when rarefied gases invade the lower

atmosphere4. near the equator

5-45. Which of the following factors distinguishesairglow from an aurora?

1. Airglow is fainter2. Airglow does not shimmer as much as an

aurora3. Airglow appears in middle and lower

altitudes, while auroras are a feature ofhigh altitudes

4. Each of the above

5-46. Which of the following definitions bestdescribes climate?

1. The scientific study of the weather of aregion

2. The sum total of the Earth’s atmosphericvariables

3. The average state of the Earth’satmosphere over any given location over along period of time

4. The general weather of a region

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5-47. Which approach to climatology provides themost useful information to Aerographer’sMates in their travels around the world?

1. Physical climatology2. Descriptive climatology3. Dynamic climatology4. Mesoclimatology

5-48. Which of the following types of climaticstudies is usually likely be used to positionrunways for a new naval air station?

1. Microclimatology2. Mesoclimatology3. Macroclimatology4. Physical climatology

5-49. Of the following climatic elements, which isconsidered to be the most important?

1. Pressure2. Temperature3. Wind4. Precipitation

5-50. Moisture modifies temperature, while, at thesame time, it is also influenced by temperature.

1. True2. False

5-51. In most countries of the world, the amount ofprecipitation in climatic studies is expressed inwhat increments?

1. Inches2. Centimeters3. Millimeters4. Centiliters

5-52. What are resultant winds?

1. The wind directions and speeds for a givenlevel in the atmosphere

2. The vectorial average of all winddirections and speeds for a given period oftime

3. The vectorial average of all winddirections and speeds for a given period oftime, at a specific place

4. The wind directions and speeds for aspecific place

5-53. Which of the following climatic terms is beingdetermined when the highest and lowesttemperatures of the day are added together anddivided by 2?

1. Mean2. Mode3. Median4. Normal

5-54. The extreme lowest temperature ever recordedat your station is -22°F. Which of the followingclimatic terms applies to this temperature?

1. Extreme low2. Absolute low3. Absolute minimum4. Extreme absolute minimum

5-55. What temperature is normally used as thestandard base temperature in computingheating degree days?

1. 85°F2. 75°F3. 65°F4. 60°F

5-56. On the first day of your local power company’sheating season, five heating degree days aremeasured. What does this number represent?

1. The number of kilowatts of energy usedabove the average number required to coolto a standard temperature

2. The difference between the first day’smean temperature and a temperaturestandard

3. An index of required energy4. Standard deviation

5-57. Which of the following statements is correctwith regard to average and standard deviation?

1. (+ or -) signs are critical in thesecomputations

2. Average deviations use arithmeticaverages of data, while standard deviationsuse actual measurements

3. A standard deviation is the square root ofan average of squared mean deviations

29

IN ANSWERING QUESTIONS 5-58 THROUGH5-65, USE THE FOLLOWING MONTHLYINFORMATION. (HIGHS AND LOWS AREDEGREES FAHRENHEIT).

High Low February High Low

1 41 21 15 27 112 39 21 16 25 093 39 19 17 25 104 29 15 18 26 115 27 12 19 18 056 30 13 20 16 037 32 15 21 16 048 37 19 22 17 089 37 21 23 19 13

10 40 23 24 23 1411 40 26 25 26 1612 41 27 26 29 1813 39 19 27 32 2114 37 16 28 33 22

5-58. What is the mean high temperature (roundedoff) for the month?

1. 37°2. 32°3. 30°4. 26°

5-59. What is the range of the high temperatures?

1. 24° to 26°2. 41° to 29°3. 30°4. 25°

5-60. What is the extreme mean monthlytemperature?

1. 15°2. 22°3. 31°4. 32°

5-61. What is the mode of the low temperatures?

1. 15°2. 19°3. 21°4. 27°

5-62. What are the medians of the high and lowtemperatures?

1. 29.0 and 15.0°2. 29.5 and 15.5°3. 30.0 and 15.5°4. 32.0 and 16.0°

5-63. When you use 41°F as the standard, what is thenumber of degree days for the first seven daysof the month?

1. 712. 863. 1094. 133

5-64. What is the average daily temperaturedeviation?

1. 6°2. 7°3. 8°4. 9°

5-65. What is the standard deviation (rounded off) ofthe temperature for the month?

1. 6°2. 7°3. 8°4. 9°

30

ASSIGNMENT 6

Textbook Assignment: “Climate and Climatology.” Chapter 6, Pages 6-6 through 6-30.

6-1. Which of the following lists represents theclimatic zones?

1. Arctic, Antarctic, Polar, Mid-latitudes,Tropical, and Equatorial

2. Arctic, Polar, Midlatitudes, and Tropicalonly

3. Arctic, Temperate, Equatorial4. Polar, Temperate, and Tropical

6-2. Which of the following factors is most com-monly used to limit the extent of each climaticzone?

1. Lines of latitude based on solar (light)zones

2. Isotherms3. Precipitation lines4. Lines depicting plant growth differences

6-3. Climatic classifiers use the same factors whenclassifying types of climate.

1. True2. False

6-4. Which of the following climatic classifiersplaces a great deal of emphasis on therelationship between precipitation andevaporation?

1. C. W. Thornthwaite2. W. Koppen3. G. T. Trewartha4. Each of the above

6-5. Koppen’s five climatic types are based on

1. temperature only2. precipitation amounts only3. the effectiveness of precipitation4. the effect of temperature and precipitation

on plant growth

6-6. Which of the following climatic controls hasthe greatest effect on climatic elements?

1. Latitude2. Ocean currents3. Topography4. Lard and water distribution

6-7. Compared to water, approximately how manytimes faster does lard heat aid cool?

1. 62. 23. 84. 4

6-8. Air temperature contrasts over oceans are rela-tively minimal between day and night andwinter and summer because of

1. water’s higher absorption rate of insolation2. the subtropical anticyclones’ positions3. the constancy of sea surface temperatures

due to mixing processes4. Earth’s land and water distribution

6-9. The seasonal change in the worldwide tem-perature gradient is greater in the NorthernHemisphere than in the Southern Hemisphere.Why?

1. The differences in the land and waterdistribution between the two hemispheres

2. The Southern Hemisphere’s longersummers

3. The absence of cP air in-the SouthernHemisphere

4. All of the above

6-10. Mountains affect which climatic element themost?

1. Wind2. Temperature3. Precipitation4. Cloud cover

6-11. Why are climates cooler along west coasts ofcontinents than along east coasts of continents?

1. Prevailing westerly winds2. Presence of mountain ranges which

impede cold air3. Cold ocean currents flow along the west

coasts, while warm ocean currents flowalong east coasts

4. Higher albedoes

31

6-12. The infamous fog that invades San FranciscoBay during the summer is caused by

1. upwelling2. contrasting temperatures between the Bay

and the California current3. radiational cooling4. warm, moist air being advected over the

cold California current

6-13. Climatically, the Grand Banks ofNewfoundland and the Kamchatka Peninsulaof eastern Asia are well known for

1. upwelling2. their extensive fogs3. extremely cold summers4. cyclogenesis

6-14. When comparing climates of heavily woodedareas to nearly open areas in the same region,how do the heavily wooded areas differ, if atall?

1. They have lower humidities2. The wind speeds are considerably higher3. The maximum and minimum temperatures

are higher4. They will not differ if in the same region

6-15. Worldwide climatological records aremaintained at which of the followingcommands?

1. NAVLANTMETOCCEN Norfolk, VA.2. COMNAVMETOCCOM Stennis, Space

Center, MS3. FLENUMOCEANCEN Monterey, CA.4. FLENUMOCEANCEN Asheville, NC

6-16. Which of the following climatic information isavailable aid produced only upon request?

1. Summary of Meteorological Observations(SMOS)

2. Cross-Wind Summary3. Local Climatological Data Summary

(LCD)4. Worldwide Airfield Summary

6-17. How often is the SMOS updated?

1. Annually2. Biannually3. Triennially4. Every 5 years

6-18. A complete listing of climatological referencesis available in which of the followingpublications?

1. Climatic publications prepared forCommander, Naval OceanographyCommand

2. Guide to Standard Weather Summaries(NAVAIR 50-lC-534)

3. Navy Stock List of Forms andPublications, NAVSUP 2002

4. All of the above

6-19. A prospective graduate of AG C-l has orders toGuantanamo Bay, Cuba. Which of thefollowing publications provides a limitedamount of climatology but provides valuableinformation on local and area weather withregard to this station?

1. U.S. Navy Marine Climatic Atlas of theWorld

2. Guantanamo Bay’s Local AreaForecasters s Handbook

3. Naval Intelligence Survey4. Worldwide Airfield Summaries

6-20. Forecaster’s guides for data-sparse areas andareas of high naval interest may be availablefrom

1. NAVLANTMETOCDET Asheville, NC2. COMNAVMETOCCOM3. Naval Environmental Prediction Research

Facility, Monterey, CA4. Air Weather Service Environmental

Technician Application Center

6-21. In two months, your ship is scheduled toembark on a 6-month Mediterraneandeployment. The meteorological officer wantsclimatic data on each port that is scheduled tobe visited. What step(s) do you take to get thisdata?

1. Task the nearest NAVLANTMETOCFAC2. Draft a request for climatic support to

COMNAVMETOCCOM through yourchain of command

3. Use your ship’s climatic publications;then, if required, request assistance fromthe nearest Naval OceanographyCommand activity

4. Request the data fromCOMNAVMETOCCOM Stennis, SpaceCenter, MS

32

6-22. Climatology should always come into play inoperational planning that extends beyond therange of forecasting techniques.

1. True2. False

6-23. Which of the following characteristics isassociated with maritime climates?

1. Minimal cloudiness2. Little precipitation3. Small diurnal temperature range4. Large annual temperature range

6-24. The amount of radiant energy absorbed by thesea when the Sun is directly overhead isapproximately what percent?

1. 32. 63. 254. 91

6-25. Which, if any, of the following statements ischaracteristic of the interchange of radiationbetween Earth’s oceans and the atmosphere?

1. The interchange is a short-wave radiationexchange

2. The interchange is primarily dependent onthe sea-surface temperature and theamount of water vapor in the atmosphere

3. The interchange is solely dependent on thetime of day and season of the year

4. None of the above apply

6-26. Convective activity is most likely to occurwhen

1. warm air moves over cold ocean waters2. cold air moves over warm ocean waters3. warm air moves over warm ocean waters4. cold air moves over cold ocean waters

6-27. When is evaporation of Earth’s surface watersmost intense?

1. When the vapor pressure of the atmosphereis greater than that of the surface water

2. When the vapor pressure of the atmosphereand the surface water coincide

3. When the vapor pressure of the surfacewater exceeds the vapor pressure of theatmosphere

4. When the air temperature exceeds thewater temperature

6-28. Oceans are an abundant source of moisture, butprecipitation occurs much more frequentlyover land than over the oceans for which of thefollowing reasons?

1. Orographic influences2. Stronger temperature contrasts3. Greater vertical mixing4. All of the above

6-29. Atmospheric soundings show that a layer ofmoist air exists in the tropics. During favorableweather, what is the mean depth of this layer?

1. 2,000 to 3,000 feet2. 3,000 to 5,000 feet3. 5,000 to 8,000 feet4. 5,000 to 12,000 feet

6-30. Within the temperate latitudes of the NorthAtlantic and Pacific Oceans, where are themost active frontal systems found?

1. Along the west coasts of North Americaand Asia

2. Along the east coasts of North Americaand Asia

3. Along the northern boundary of thesubtropical high-pressure systems

4. Along the eastern boundary of thesubtropical high-pressure systems

6-31. In winter in the North Atlantic Ocean, what isthe average number of days that passesbetween polar outbreaks?

1. 3 1/22. 5 1/23. 34. 4

6-32. What are cyclone families?

1. Polar outbreaks2. A series of midwestern tornadoes3. The fronts associated with polar outbreaks4. A series of cyclonic waves that form along

the polar front

6-33. Which of the following occurrences issynonymous with the splitting of the Pacificsubtropical high in winter?

1. A more vigorous polar-front off the Asiaticeast coast

2. Severe cyclones in the Gulf of Alaska3. Two polar fronts coexist in the North

Pacific4. The northeast trade winds are reinforced

33

6-34. What is the primary flight hazard associatedwith mT air on the east side of a subtropicalhigh?

1. Coastal fog2. Turbulence3. Thunderstorms4. Heavy rain and low ceilings

6-35. Which, if any, of the following factors is theprimary controller of Arctic weather andclimate?

1. Land-sea-ice distribution2. Mountain barriers3. Insolation4. None of the above

6-36. During the Arctic summer, the distinctionbetween Arctic and polar air masses almostdisappears.

1. True2. False

6-37. Which of the following statements is correctconcerning Arctic air masses in winter?

1. Humidity is high2. Cloudiness and precipitation increase3. Temperatures are usually between 0° and

10°C4. A large temperature inversion exists in the

lower few thousand feet over land

6-38. Which of the following statements ischaracteristic of the flying weather in theArctic?

1. It is worst during the transition periodbetween the seasons

2. Fog is a major problem over land insummer

3. Low ceilings and visibilities are mostfrequent in winter

4. High winds, blowing snow, and turbulenceare more frequent in summer

6-39. The summers of the Canadian Archipelago arebest classified as

1. hot and long2. cold and long3. cool and short4. warm and short

6-40. Strong surface winds are most likely to occurwithin the interior of the Arctic region duringwhich of the following seasons?

1. Winter2. Fall and winter3. Spring and fall4. Summer and fall

6-41. Which of the following annual precipitationamounts is representative of Arctic coastalareas and the Arctic ice pack?

1. 3 to 7 in.2. 5 to 15 in.3. 8 to 17 in.4. 10 to 20 in.

6-42. Ice fog is most likely to occur when the airtemperature is around how many degreesCelsius?

1. 02. -153. -304. -45

6-43. Diamond dust is a name that applies to

1. Arctic smoke2. Arctic sea smoke3. Arctic haze4. ice fog

6-44. In the Arctic, the Sun, Moon, and other objectsnear the horizon often appear distorted. Why?

1. Aurora borealis2. Inversion induced mirages3. The highly transparent air4. Whiteouts

6-45. In addition to equal amounts of skylight andreflected light, what other conditions arenecessary to bring about a whiteout?

1. Broken snow cover, and clear sky2. Broken snow cover, and an overcast sky3. Unbroken snow cover, and clear sky4. Unbroken snow cover, and a uniformly

overcast sky

6-46. The lowest recorded temperature in the worldwas observed in

1. Siberia2. Greenland3. Canadian Archipelago4. Antarctica

34

6-47. Which of the following areas of the UnitedStates is favorable for the development ofstorm (low-pressure) centers?

1. Ohio Valley2. Tennessee Valley3. Central Idaho4. Great Plains

6-48. Which of the following regions of the UnitedStates has a cold, dry climate in winter and awarm, dry climate in summer?

1. Central Plains2. Intermountain West Central3. Southwest Pacific Coast4. Southeast and Gulf States

6-49. The chief flight hazard in the southwesterndesert and mountain area of the United States is

1. high level turbulence2. spring and summer thunderstorms3. haze4. dust devils

6-50. Tornadoes are a climatic feature of which of thefollowing areas of the United States?

1. Central Plains2. Southeast United States3. Intermountain west central area4. Southwest Pacific coast area

6-51. Why is the southeast and Gulf states area of theUnited States an especially difficult area formaking forecasts?

1. Stagnating frontal systems, fog, and Gulfstratus

2. Air-mass thunderstorms3. Rapidly moving squall lines4. Various combinations of all the above

reasons

6-52. The influx of maritime air into western Europeresults in

1. low-temperature extremes2. infrequent precipitation3. high humidity4. mostly clear skies

6-53. Which of the following European areasexperiences the least amount of change in itstemperature extremes between summer andwinter?

1. European Atlantic coast2. The Rhine Valley3. Eastern Europe4. The northern Alpine region

6-54. The Asian continent is dominated by

1. high pressure in winter and low pressure insummer

2. low pressure in winter and high pressure insummer

3. low pressure year round4. high pressure year round

6-55. If a relatively dry excursion into northeastSouth America is planned, which month wouldbe most suitable?

1. January2. June3. October4. November

6-56. Southern Chile experiences a climate similar tothat experienced by what area of the UnitedStates?

1. Northwest coast2. Southwest coast3. Northeast coast4. Southeast coast

6-57. Why do the climatic zones of Africa lack sharpdistinction?

1. Africa is an island continent2. There are no prominent mountain ranges in

Africa3. In Africa, the zones are controlled by the

ITCZ4. Africa is under the influence of only one

air mass

6-58. Which of the following climatic elements is themost important in Africa?

1. Temperature2. Wind3. Precipitation4. Cloud cover

35

6-59. The sub-equatorial region of Africaexperiences marked seasonal rainfall. Whatfive-month period is associated with the rainyseason?

1. Jan - May2. Apr - Aug3. Aug - Dec4. Nov - Mar

6-60. Climatically, where is the wettest region ofAfrica?

1. North central2. Equatorial3. Southwestern4. Southeast coastal

6-61. What is the average variation in maximumtemperatures in the interior of Australiabetween summer and winter?

1. 15°F2. 22°F3. 28°F4. 31°F

6-62. What portion of Australia is under theinfluence of mT air?

1. Northern 1/32. Eastern 1/33. Southern 2/34. Western 3/4

6-63. Climatically, southern New Zealand is wetterthan northern New Zealand.

1. True2. False

36