lecture notes for chapter 12 - air masses and fronts - survey of meteorology at lyndon state college

Lecture Notes for Chapter 12 - Air masses and fronts - Survey of Meteorology at Lyndon State College

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/index.html[9/29/2013 8:31:22 AM]

Chapter 11 - Air masses and fronts

Index of Lecture Notes

Air Masses defined:Air Mass source regionsAir Mass ClassificationNorth America Air Masses

cP air masses - lake effect snowsmP air mass - west coastmP air mass - east coastmT and cT air masses

Fronts - boundaries between air massesStationary FrontsCold frontsCold front cross sectionWarm FrontsWarm front cross sectionWarm front weatherOccluded frontCold occlusionWarm occlusion

Review Questions

Homework

Sample test questions

Air Masses defined

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Air Masses Defined

Large body of air with similar temperature andmoisture characteristics in any horizontaldirection

cover many 1000's of square kilometers

Part of weather forecasting is determining airmass characteristics, how they may bemodified, and their movement

Air Mass Source Regions Defined

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Air Mass Source Regions Defined

Air masses originate in source regionssource regions:

flat, uniform compositionlight winds

So, where are the good source regions? ANSWERCheck out the topography of Canada withGoogle Earth....Air masses tend to clash and interact in themiddle latitudes.

Air Mass Classification

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/airmass_classification.html[9/29/2013 8:31:44 AM]

Air Mass Classification

four general categories according to sourceregion (see table)

extremely cold cP air is sometimes denoted atcA

extremely hot, humid mT air is sometimedenoted by mE

Air Masses on the move:

if an air mass is colder than the surface overwhich it is moving, "k" is added

if an air mass is warmer than the surface overwhich it is moving, "w" is added

Example - a cP air mass moving over the greatlakes in December becomes cPk

SourceRegion Polar Tropical

Land -continental(c)

cP (cold,dry, stable)

cT (hot, dry, stable airaloft; unstable surfaceair

Water -maritime(m)

mP (cool,moist,unstable)

mT (warm, moist;usually unstable)

Airmasses affecting North America

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Air Masses of North America

Continental Polar/Arctic -

Stable, cold, dry air massesoriginating over N. Canada andAlaskaeventually plunge southward tointerior of U.S. as a shallow domeof high pressure - why???can reach Gulf of Mexico andFlorida - freeze cropsProduce lake effect snows as theymove over the great lakes.Usually do not move west ofRocky mountains - mountainsconfine cold air to the east.Upslope precipitation is commoneast of the rockies as the cP airmass slides to the south.During the summer, cP air masscan bring relief to hot, humidregions.

The Maritime Polar Air Mass:

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/mp.html[9/29/2013 8:32:13 AM]

mP Air Mass:

West Coast:

originate over Asia as cPtends to be unstableheavy rains as cool moist airflows over mountains alongwest coastmP is modified (how???) bytime it reaches interior ofUS, though is milder that cP

The Martitime Polar air mass - east coast

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/mp_east.html[9/29/2013 8:32:18 AM]

mP Air Mass:

East Coast:

not as common as westcoast mPcolder than west coastmPusually brought onshoreby high pressure to thenorth of us and/or lowpressure to the southmoving up the coast.

QUESTIONS FORTHOUGHT:

1. Suppose an mP air massmoving eastward from thePacific Ocean travels acrossthe United States. Describeall of the modifications thatcould take place as this airmass moves eastward inwinter. In summer.

2. Explain how ananticyclone during autumncan bring record-breakinglow temperatures and cP airto the south-eastern statesand only several days latervery high temperatures andmT air.

mT and cT air masses - The Dryline

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/mt.html[9/29/2013 8:32:25 AM]

mT Air Mass:

largely originates in Gulf ofMexico, western Atlantic -affecting eastern 2/3 of countryalso originates in tropical easternpacific (SW monsoons insummer)warm, moist, unstableconfined to southern US in winterimportant source of moisturefeeding storms all year round

cT Air Mass:

originates over Mexican Plateauregion and desert SWhot, dry, unstable at low levels,stable at upper levelsboundary between cT and mT isoften called the dryline

The dryline is often seen insurface and satellite data andis a favored location forstorm initiation:Examples: | static visiblesatellite image |satellite/surface loop |

Atmospheric Fronts - Introduction

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/fronts_intro.html[9/29/2013 8:32:32 AM]

AtmosphericFronts -

Introduction

Front -boundary,transition zonebetween twodifferent airmassesthe two airmasses havedifferentdensities.Frequently,they arecharacterizedby differenttemperaturesand moisturecontentsfront hashorizontal andvertical extentfrontalboundary/zonecan be 1-100km wide!!types ofsynoptic-scalefronts:

stationaryfrontscoldfrontswarmfrontsoccludedfronts

Stationary Fronts

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/stationary_fronts.html[9/29/2013 8:32:45 AM]

Stationary Fronts

has little/no movementdenoted by alternating cold/warm frontalsymbolsAssociated weather:

clear, partly cloudy, cloudy, light precipusually nothing severe

If the stationary front starts moving north inthe example to the right it will become aANSWERIf the stationary front starts moving south inthe example to the right it will become aANSWER

Cold Front Characteristics

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/cf.html[9/29/2013 8:32:58 AM]

Cold Fronts

Zone/boundary between warmer, more moist,unstable air (usually mT) being replaced bycolder, drier, more stable air (usually cP).Location of cold front:

leading edge of sharp temperature changemoisture content (dew point) changesdramaticallywind shift (direction and speed)pressure trough (pressure tendency isuseful!!!)oftencloudy/showers/thunderstorms/sometimessevere

What does a vertical cross section through afront look like??

image from Meteorology Today by C. Donald Ahrens 1994 West Publishing Company

Cold Front-Cross Section

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/cf_xsect.html[9/29/2013 8:33:06 AM]

Cold Front-Cross Section

warm air ahead of front islifted up and overcan get intenseshowers/thunderstorms atfrontal boundaryCs and Ci clouds are blownahead of the front by upperlevel windscloud base is generally lowerbehind the front.... why???further behind the front, the airis quite dry, few cloudssteep frontal boundary, slopesbackward into the cold airfrontal speed averages 15-25knotstemperature and wind profileson either side of cold front???

fronts can weaken with time -frontolysisfronts can strengthen with time- frontogenesis

what processes can leadto frontogenesis? ANSWER

from MeteorologyToday by D. Ahrens

BeforePassing While Passing

AfterPassing

Winds south-southwest gusty; shifting west-

northwest

Temperature warm sudden drop steadilydropping

Pressure fallingsteadily minimum, then sharp rise rising

steadily

Clouds increasing:Ci, Cs and

Cb Cb Cu

Precipitation short periodof showers heavy rains, sometimeswith hail, thunder and

lightning

showersthen

clearing

Visibility fair to poorin haze poor, followed by

improving good,

except inshowers

Dew Point high;

remainssteady

sharp drop lowering

Warm Fronts

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Warm Fronts

Zone/boundary between advancing warmer,more moist air (usually mT) and cooler, drierair (usually mP)average speed is about 10 knots

What does a vertical cross section through a frontlook like??

warm front cross section

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/wf_xsect.html[9/29/2013 8:33:29 AM]

Warm Fronts

Often associated with"overrunning"frontal passage:Clouds associated with warmfrontsfrontal surface has a muchsmaller slope than for coldfrontsoften produces wide-spreadnimbostratus precip near frontoverrunning in motion:


http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/wf_xsect2.html[9/29/2013 8:33:50 AM]

Warm Fronts

Typical weather associated with warm frontalpassage:

BeforePassing While

Passing After

Passing

Winds south-southeast variable south-

southwest

Temperature cool-cold,

slowwarming

steadyrise warmer,

then steady

Pressure usuallyfalling leveling

off slight rise,

followed byfall

Clouds

in thisorder: Ci,

Cs, As, Ns,St, and fog;occasionally

Cb insummer

stratus-type

clearingwith

scatteredSc;

occasionallyCb in

summer

Precipitation

light-to-moderate

rain, snow,sleet, ordrizzle

drizzle ornone

usuallynone,

sometimeslight rain or

showers

Visibility poor poor, butimproving fair in haze

Dew Point steady rise steady rise, thensteady

Overrunning associated with a warm front:


http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/wf_xsect2.html[9/29/2013 8:33:50 AM]

Occluded Fronts

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/of.html[9/29/2013 8:34:01 AM]

Occluded Fronts (Occlusion)

Why do they form ANSWERThere are two types of occluded fronts:

cold occlusionwarm occlusion

Cold Occlusion

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/of_cold.html[9/29/2013 8:34:06 AM]

Cold Occlusion

cold front "lifts" the warm front up and overthe very cold air

Associated weather is similar to a warm frontas the occluded front approaches

once the front has passed, the associatedweather is similar to a cold front

vertical structure is often difficult to observe

Warm Occlusion

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/of_warm.html[9/29/2013 8:34:12 AM]

Warm Occlusion

cold air behind cold front is not denseenough to lift cold air ahead of warmfront

cold front rides up and over the warmfront

upper-level cold front reached stationbefore surface warm occlusion

QUESTIONS FOR THOUGHT:

1. Explain why freezing rain more commonlyoccurs with warm fronts than with coldfronts.

2. Sketch the relative positions of a low and itsattendant cold and warm fronts that would beobserved in the southern hemisphere. Howwould the winds shift during the passage ofthe cold front?

3. Why does the same cold front produce morerain over Kentucky than over westernKansas?

4. Sketch representative soundings (temperatureand winds) ahead and behind a cold front

5. Sketch representative soundings (temperatureand winds) ahead and behind a warm front

6. You are in Ithaca, NY and observe the windshifting from East to South accompanied by asudden rise in both the air temperature anddew point temperature. What type of frontpassed?

7. If Lake Erie froze over in January, is it stillpossible to lave lake-effect snows off LakeErie in February? Why or why not?

8. In winter, cold frontal weather is typicallymore violent than warm-front weather. Why?Explain why this is not necessarily true insummer.

9. Thunderstorms have formed along a coldfront producing rain showers on the cold-airside of the front. Assuming that the air on thecold-air side of the front is initially subsaturated, will the rain showers increase thetemperature difference across the front(frontogenesis) or decrease the temperaturedifference across the front (frontolysis)?Explain your answer.

Review Questions for Chapter 12 - Air Masses and Fronts - Survey of Meteorology at Lyndon State College

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/ch12_review_questions.html[9/29/2013 8:34:13 AM]

Review Questions for Chapter 12 - Air Masses and Fronts

What is an air mass?What and where are good source regions for air masses?How are air masses classified?What are the characteristics of the air masses that commonly affect weather over the U.S.?How can air masses be modified?What are the fundamental, physical processes involved with generating lake effect snows?What is a front?What are the types of fronts? How are they analyzed (what symbol) on a surface weather map?What are the characteristics of each kind of front?What is observed during frontal passage for each kind of front?What kind of weather and clouds are produced by each kind of front?What is "frontogenesis" and "frontolysis"?What is the physical interpretation of the 1000-500 thickness values? Which value can be used tohelp forecast rain/snow here at LSC?

Homework for Chapter 12- Air Masses and Fronts - Survey of Meteorology at Lyndon State College

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/hw/ch12_homework.html[9/29/2013 8:34:15 AM]

Homework Questions for Chapter 11 - Air Masses and Fronts

Consult Syllabus for Due Date

Questions 1-3 will be turned in for a grade.

SHOW ALL WORK , CIRCLE THE CORRECT ANSWER, PLEASE BE NEAT AND STAPLE YOURHOMEWORK!

ALSO, PLEASE USE A SPREADSHEET FOR ALL GRAPHS

Follow the Problem solving steps discussed in class

1. Create a vertical cross section of temperature from northern Canada down past the Gulf of Mexico. Label the MTand CP air mass locations in this cross section. Also pencil in the location of the tropopause. Create another crosssection of isotahs and isotherms through the polar jet. Label the jet location.

2. One measure of the strength of a front is the magnitude of the temperature difference across the front (delta T). Ifwe assume that the frontal orientation is N-S, as in the case of an eastward moving cold front, the frontal strength (FS)is related to the magnitude of the temperature difference by:

where delta T is the change in temperature over a distance delta X across the front.

(a) What are the units of frontal strength?

(b) Assume that one observes a cold front oriented N-S. If the temperature at the front is 20 degrees C and decreasesto 10 degrees C 100 km to the northwest of the front, what is the frontal strength? Assume that the angle between theisotherms and the front is 45 degrees.

EXTRA CREDIT:

Find an example of a cold frontal passage in a forecast meteogram for Burlington, VT. Indicate:

1. which model produced the meteogram2. the time of cold frontal passage on the forecast meteogram. 3. the time when the front was observed to pass through Burlington.

Sample Test Questions for Chapter 11

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/sample-test-ch11.htm[9/29/2013 8:34:22 AM]

____ 1. A good source region for an air mass would be:a. mountains with deep valleys and strong surface windsb. generally flat areas of uniform composition with light surface windsc. hilly with deep valleys and light windsd. generally flat area of uniform composition with strong surface winds

____ 2. Which of the following statements is most plausible?a. In winter, cP source regions have higher temperatures than mT source regionsb. In summer, mP source regions have higher temperatures than cT source regionsc. In winter, cA source regions have lower temperatures than cP source regionsd. In summer, mT source regions have lower temperatures than mP source regionse. They are all equally plausible

____ 3. The greatest contrast in both temperature and moisture will occur along the boundary separatingwhich air masses?a. cP and cTb. mP and mTc. mP and cTd. mT and cPe. cT and mT

____ 4. An air mass is characterized by similar properties of ____ and ____ in any horizontal direction.a. temperature, pressureb. pressure, moisturec. winds, moistured. temperature, moisture

____ 5. Which air mass would show the most dramatic change in both temperature and moisture content as itmoves over a large body of very warm water?a. cT in summerb. cP in winterc. mP in winterd. mT in summer

____ 6. The coldest of all air masses is:a. mTb. mPc. cTd. cFe. cA

____ 7. Cumuliform cloud development would be most likely in which of the following?a. cT air mass moving over a mountain rangeb. cP air mass moving over warm waterc. mT air mass moving over cold land surfaced. cT air mass moving over cold water

____ 8. Wintertime mP air masses are less common along the Atlantic coast of North America than along thePacific coast mainly because:a. the water is colder along the Pacific coastb. the prevailing winds aloft are westerlyc. the source region for mP air on the Atlantic coast is western Europed. the water is warmer along the Atlantic coaste. the land is colder along the Atlantic coast

____ 9. What type of air mass would be responsible for snow showers on the western slopes of the Rockies?a. mTb. cPc. mPd. cA

____ 10. What type of air mass would be responsible for persistent cold, damp weather with drizzle along theeast coast of North America?a. mP

Sample Test Questions for Chapter 11

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/sample-test-ch11.htm[9/29/2013 8:34:22 AM]

b. mTc. cPd. cTe. cA

____ 11. The air mass with the highest actual water vapor content is:a. mTb. cTc. mPd. cP

____ 12. The word "frontogenesis" on a weather map would mean that:a. a front is in the process of dissipatingb. one front is about to over take another frontc. a front is regenerating or strengtheningd. severe thunderstorms will form along a front

____ 13. Fronts are associated witha. low pressureb. high pressure

____ 14. A dryline isa. a stalled cold frontb. a stalled warm frontc. a dew point frontd. a boundary marking a strong horizontal change in atmospheric moisturee. both c and d

____ 15. A true cold front on a weather map is always:a. associated with precipitationb. associated with a wind shiftc. followed by drier aird. followed by cooler air

____ 16. Occluded fronts may form as:a. a cold front overtakes a warm frontb. a warm front overtakes a cold frontc. a cold front overtakes a squall lined. overrunning occurs along a warm front

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Great Dryline Example

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Great Dryline ExampleYou need a Java-enabled browser to view this applet

Animation

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/graphics/59_Cold_Front/59.html[9/29/2013 8:39:01 AM]

Animation

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/graphics/58_Warm_Front/58.html[9/29/2013 8:42:40 AM]

vsc.eduLecture Notes for Chapter 12 - Air masses and fronts - Survey of Meteorology at Lyndon State CollegeAir Masses definedAir Mass Source Regions DefinedAir Mass ClassificationAirmasses affecting North AmericaThe Maritime Polar Air Mass:The Martitime Polar air mass - east coastmT and cT air masses - The DrylineAtmospheric Fronts - IntroductionStationary FrontsCold Front CharacteristicsCold Front-Cross SectionWarm Frontswarm front cross sectionwarm front cross sectionOccluded FrontsCold OcclusionWarm OcclusionReview Questions for Chapter 12 - Air Masses and Fronts - Survey of Meteorology at Lyndon State CollegeHomework for Chapter 12- Air Masses and Fronts - Survey of Meteorology at Lyndon State CollegeSample Test Questions for Chapter 11http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/graphics/namussfcwbg.gifhttp://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/graphics/cold_surge_1_6_98_surf.gifhttp://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/graphics/lakeeffect_sat.gifhttp://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/graphics/cp_upslope_schem.jpghttp://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/graphics/mt_subtropical_satellite.jpghttp://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/graphics/dryline_vis.gifGreat Dryline ExampleAnimationAnimation

lecture notes for chapter 12 - air masses and fronts - survey of meteorology at lyndon state college

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