hses 1ete c18.qxd 9/27/04 11:40 pm page 510 section 18.2...

18.2 Cloud Formation

Reading StrategyIdentifying Main Ideas Copy the table. Asyou read, write the main idea for each topic.

Key ConceptsWhat happens to air whenit is compressed orallowed to expand?

List four mechanisms thatcan cause air to rise.

Contrast movements ofstable and unstable air.

What conditions in airfavor condensation ofwater?

Vocabulary◆ dry adiabatic rate◆ wet adiabatic rate◆ orographic lifting◆ front◆ temperature

inversion◆ condensation

nuclei

Recall that condensation occurs when water vapor changes to aliquid. Condensation may form dew, fog, or clouds. Although thesethree forms are different, all require saturated air to develop. Saturationoccurs either when enough water vapor is added to air or, more com-monly, when air is cooled to its dew point.

Near Earth’s surface, heat is quickly exchanged between the groundand the air above. During evening hours, the surface radiates heat away,causing the surface and adjacent air to cool rapidly. This radiational cool-ing causes the formation of dew and some types of fog. In contrast,clouds, like those shown in Figure 6, often form during the warmest partof the day. Clearly, some other process must cool air enough to generateclouds.

Air Compression and ExpansionIf you have pumped up a bicycle tire, you might have noticed that thepump barrel became warm. The increase in temperature you feltresulted from the work you did on the air to compress it. When air iscompressed, the motion of gas molecules increases and the air temper-ature rises. The opposite happens when air is allowed to escape from abicycle tire. The air expands and cools. The expanding air pushes on thesurrounding air and cools by an amount equal to the energy used up.

Adiabatic Temperature Changes Temperature changesthat happen even though heat isn’t added or subtracted are called adi-abatic temperature changes. They result when air is compressed orallowed to expand. When air is allowed to expand, it cools, andwhen it is compressed, it warms.

Topic Main Idea

Adiabatic temperaturechanges a.

Stability measurements b.

Degrees of stability c. ?

?

?

Figure 6 Clouds form when air iscooled to its dew point.

510 Chapter 18

510 Chapter 18

FOCUS

Section Objectives18.6 Describe what happens to

air when it is compressed orallowed to expand.

18.7 List four mechanisms thatcause air to rise.

18.8 Compare and contrastmovements of stable andunstable air.

18.9 Describe the conditions in airthat favor condensation ofwater.

Build VocabularyLINCS Have students use the LINCSstrategy to learn and review the termsadiabatic, orographic, and inversion.In LINCS exercises, students List whatthey know about each term, Imagine apicture that describes the word, Note a“sound-alike” word, Connect the termsto the sound-alike word by making upa short story, and then perform a briefSelf-test.

Reading Strategya. Adiabatic temperature changes arethose that occur without the additionor subtraction of heat.b. Stability measurements are madeusing meteorological instruments thatmeasure the temperature profile of theatmosphere.c. Degrees of stability relate to thetendency of air to resist verticalmovement (stable) ranging to airthat tends to rise (unstable).

L2

L2

Reading Focus

1

Section 18.2

HSES_1eTE_C18.qxd 9/27/04 11:40 PM Page 510

Moisture, Clouds, and Precipitation 511

Expansion and Cooling As you travel from Earth’s surfaceupward through the atmosphere, the atmospheric pressure decreases.This happens because there are fewer and fewer gas molecules. Anytime a volume of air moves upward, it passes through regions of suc-cessively lower pressure. As a result, the ascending air expands andcools. Unsaturated air cools at the constant rate of 10°C for every 1000meters of ascent. In contrast, descending air encounters higher pres-sures, compresses, and is heated 10°C for every 1000 meters it movesdownward. This rate of cooling or heating applies only to unsaturatedair and is called the dry adiabatic rate.

If a parcel of air rises high enough, it will eventually cool to its dewpoint. Here the process of condensation begins. From this point on asthe air rises, latent heat of condensation stored in the water vapor willbe released. Although the air will continue to cool after condensationbegins, the released latent heat works against the adiabatic coolingprocess. This slower rate of cooling caused by the addition of latentheat is called the wet adiabatic rate. Because the amount of latent heatreleased depends on the quantity of moisture present in the air, thewet adiabatic rate varies from 5–9°C per 1000 meters.

Figure 7 shows the role of adiabatic cooling in the formation ofclouds. Note that from the surface up to the condensation level the aircools at the dry adiabatic rate. The wet adiabatic rate begins at the con-densation level.

What happens to heat stored in water vapor whenit is cooled to its dew point?

Figure 7 Cloud Formation byAdiabatic Cooling Rising aircools at the dry adiabatic rate of10°C per 1000 meters, until the airreaches the dew point andcondensation (cloud formation)begins. As air continues to rise,the latent heat released bycondensation reduces the rateof cooling.Interpreting Diagrams Use thisdiagram to determine theapproximate air temperature at3500 m.

Hei

ght

(m)

Surface

Wetadiabatic rate

(temperature ofrising air drops at5°C/1000 meters)

Condensationlevel

5000

4000

3000

2000

1000

–8°C

–3°C

2°C

12°C

22°C

32°C

Dryadiabatic rate

(temperature ofrising air drops at

10°C/1000 meters)

INSTRUCT

Air Compressionand ExpansionCompression andExpansionPurpose Students observe howcompression and expansion of aircause temperature changes.

Materials bicycle tire, bicycle pump,can of compressed air duster

Procedure Ask students what theythink will happen to the bicycle pumpwhen you use it to add air to the tire.Pump up the tire a bit, then let thestudent feel the barrel of the pump. Askthem why the pump got warm. (Energywas used to compress the air.) Now askstudents what they think will happen tothe can of compressed air duster whenyou release some air from it. Releasesome air, then let students feel the can.Ask them why the can became cold.(Energy was absorbed by the expandingair.)

Expected Outcomes The bicyclepump will get warm, and the can willget cold.Kinesthetic, Logical

Build Science SkillsRelating Cause and Effect Gothrough each step in what happensas a parcel of air rises through theatmosphere. Ask: Why does air coolas it rises? (It expands, a process thatabsorbs heat.) Why does air cool moreslowly after it reaches its dew point?(Above the dew point, condensationbegins. Condensation releases heat, whichsomewhat counteracts the cooling effectof expansion.) Why does the wetadiabatic rate vary? (The amount ofheat released varies with the amount ofmoisture. The more moisture, the moreheat is released and the slower thecooling is.)Logical

L2

L2

2


Customize for English Language Learners

Students who are learning English can benefitfrom real-life examples that relate to sciencecontent. Encourage students to think ofobservations they have made about risingobjects or processes that cause objects to rise.

For example, they likely have seen a heliumballoon rising. They may also have seenwedges such as those used as doorstops.Encourage students to share theirobservations with the class.

Answer to . . .

Figure 7 The temperature is �0.5°C.

Latent heat is released.


512 Chapter 18

Processes That Lift AirIn general, air resists vertical movement. Air located near the surfacetends to stay near the surface. Air far above the surface tends to remainfar above the surface. Some exceptions to this happen when conditionsin the atmosphere make air buoyant enough to rise without the aid ofoutside forces. In other situations, clouds form because there is somemechanical process that forces air to rise. Four mechanisms thatcan cause air to rise are orographic lifting, frontal wedging, conver-gence, and localized convective lifting.

Orographic Lifting When elevated terrains, such as mountains,act as barriers to air flow, orographic lifting of air occurs. Look atFigure 8A. As air goes up a mountain slope, adiabatic cooling oftengenerates clouds and precipitation. Many of the rainiest places onEarth are located on these windward mountain slopes.

By the time air reaches the leeward side of a mountain, much of itsmoisture has been lost. If the air descends, it warms adiabatically. Thismakes condensation and precipitation even less likely. A rain shadowdesert can occur on the leeward side of the mountain. For example,the Great Basin Desert of the western United States lies only a few hun-dred kilometers from the Pacific Ocean, cut off from the ocean’smoisture by the Sierra Nevada Mountains.

Frontal Wedging If orographic lifting was the only mechanismthat lifted air, the relatively flat central portion of North America wouldbe an expansive desert instead of the nation’s breadbasket. Fortunately,this is not the case.

In central North America, masses of warm air and cold air collide,producing a front. Here the cooler, denser air acts as a barrier overwhich the warmer, less dense air rises. This process, called frontalwedging, is shown in Figure 8B. Weather-producing fronts are associ-ated with specific storm systems called middle-latitude cyclones. Youwill study these in Chapter 20.

Airflow

Cold air

Warm airAirflow

Rainshadowdesert

A B

Processes that Lift Air

Figure 8 A Orographic LiftingB Frontal WedgingRelating Cause and Effect Whydoes the warm air mass moveupward over the cold air mass?

512 Chapter 18

Processes ThatLift AirUse VisualsFigure 8: A and B Use this diagramto explain how air can be lifted bymountains or cooler air masses. Ask:What happens as air rises up the sideof a mountain? (It cools, often creatingclouds and precipitation.) What happenswhen warm and cold air massescollide? (The colder, denser air forces thewarmer air up.) How are the processesin A and B similar? (In both cases, air isforced to rise by a barrier, often resultingin clouds and precipitation.)Visual, Logical

Build Reading LiteracyRefer to p. 306D in Chapter 11, whichprovides the guidelines for KWL(Know/Want to Know/Learned).

KWL Teach this independent study skillas a whole-class exercise. 1. Draw athree-column KWL chart on the boardfor students to copy. 2. Have studentscomplete the Know column with facts,examples, and other information thatthey already know about processes thatlift air. Have students use one row foreach of the four processes. 3. Tellstudents to complete the Want toKnow column with questions aboutthese processes. 4. Have students readpp. 512–513 to learn more about theseprocesses. As they read, have them noteanswers in the Learned column, alongwith other facts, examples, and detailsthey learned. 5. Have students draw anInformation I Expect to Use box belowtheir KWL chart. Have them review theinformation in the Learned column andcategorize the useful information inthe box.Verbal

L1

L1

Section 18.2 (continued)

The phrase parcel of air is often used inmeteorology to help understand and simplifydiscussion of principles. It refers to an imaginaryvolume of air isolated from other air by, forexample, a thin elastic cover. (Picture a hot-airballoon.) A parcel is typically considered to bea few hundred cubic meters in volume. It is

assumed to act independently of thesurrounding air. It is also assumed that noheat is transferred into or out of the parcel.Although parcels are imaginary, over shorttime periods, a parcel is a good model for theactual behavior of a volume of air movingvertically through the atmosphere.

Facts and Figures



Convergence Recall that the collision of contrasting air massesforces air to rise. In a more general sense, whenever air in the loweratmosphere flows together, lifting results. This is called convergence.When air flows in from more than one direction, it must go some-where. Because it cannot go down, it goes up, as shown in Figure 8C.This leads to adiabatic cooling and possibly cloud formation.

The Florida peninsula provides an example of how convergence cancause cloud development and precipitation. On warm days, the airflowis from the ocean to the land along both coasts of Florida. This leads toa pileup of air along the coasts and general convergence over the penin-sula. This pattern of air movement and the uplift that results is helpedalong by intense solar heating of the land. The result is that the penin-sula of Florida experiences the greatest number of mid-afternoonthunderstorms in the United States.

Localized Convective Lifting On warm summer days,unequal heating of Earth’s surface may cause pockets of air to bewarmed more than the surrounding air. For example, air above a pavedparking lot will be warmed more than the air above an adjacentwooded park. Consequently, the parcel of air above the parking lot,which is warmer and less dense than the surrounding air, will moveupward, as shown in Figure 8D. These rising parcels of warmer air arecalled thermals. The process that produces rising thermals is localizedconvective lifting. Birds such as hawks and eagles use these thermals tocarry them to great heights where they can gaze down on unsuspect-ing prey. People have learned to use these warm parcels effectively forhang gliding. When warm parcels of air rise above the condensationlevel, clouds form. These clouds may produce mid-afternoon rainshowers.

What are thermals?

Figure 8 C Convergence D Localized Convective Lifting

Condensationlevel

Convergingwinds

Uplifting

Convergingwinds

C D

Use VisualsFigure 8: C and D Use this diagram toexplain convergence and convectivelifting. Ask: What happens when twosimilar air masses converge andcollide? (Air is forced up at the center,possibly causing cloud formation.) Howdoes local convective lifting occur?(A pocket of air may be warmed morethan surrounding air, causing it to rise.)What happens when such a parcelrises above the condensation level?(Water vapor condenses, forming clouds.)Visual, Logical

Integrate BiologyBirds and Thermals Tell studentsthat using thermals is important tomany birds of prey, such as hawks andvultures. Ask: How do birds of preyuse thermals? (Birds use thermals to becarried up to great heights, where theycan look for prey.) What would happenif the birds could not use thermals?What problem would this cause?(Birds would have to flap their wings togain altitude. Doing so would waste a lotof energy.)Logical

L2

L1


Answer to . . .

Figure 8 Warm air is less dense, andtherefore more buoyant, than the coldair mass is.

Thermals are risingparcels of air that are

warmer than surrounding air.


514 Chapter 18

StabilityIf a volume of air was forced to rise, its temperature would dropbecause of expansion. If this volume of air was cooler than the sur-rounding environment, it would be denser, and if allowed to do so, itwould sink to its original position. Air of this type, called stable air,resists vertical movement.

Density Differences If this imaginary volume of rising air waswarmer and therefore less dense than the surrounding air, it wouldcontinue to rise until it reached an altitude where its temperatureequaled that of its surroundings. This is exactly how a hot-air balloonworks. The balloon rises as long as it is warmer and less dense than thesurrounding air, as shown in Figure 9. This type of air is classified asunstable air. Stable air tends to remain in its original position,while unstable air tends to rise.

Stability Measurements Air stability is determined by meas-uring the temperature of the atmosphere at various heights. The rate ofchange of air temperature with height is called the environmental lapserate. This rate is determined from observations made by aircraft andby radiosondes. A radiosonde is an instrument designed to collect

weather data high in the atmosphere. Radiosondesare often carried into the air by balloons. It is impor-tant not to confuse the environmental lapse ratewith adiabatic temperature changes.

Degrees of Stability Air is stable when thetemperature decreases gradually with increasingaltitude. The most stable conditions happen whenair temperature actually increases with height,called a temperature inversion. Temperature inver-sions frequently happen on clear nights as a result ofradiation cooling off Earth’s surface. The inversionis created because the ground and the air immedi-ately above the ground will cool more rapidly thanair higher above the ground. Under these condi-tions, there is very little vertical air movement. Incontrast, air is considered unstable when the airclose to the surface of Earth is significantly warmerthan the air higher above the surface, indicating alarge environmental lapse rate. Under these condi-tions, the air actually turns over, as the warm airbelow rises and is displaced by the colder air higherabove the ground.

Figure 9 Hot-air balloons will riseas long as the air inside them iswarmer than the air in theatmosphere surrounding them.

514 Chapter 18

StabilityBuild Science SkillsDesigningExperiments Askstudents to designexperiments todetermine the effects of temperature onthe buoyancy of a helium balloon. Theyshould compare buoyancy for at leasttwo different temperatures. Studentsmay use materials of their choosing,with the exception of open flames. Theyshould write a hypothesis, develop anexperimental plan, submit the plan toyou for approval, do the experiment,draw conclusions, and report theirresults. (One approach is to heat theballoon, using hot water, and/or cool it,using cold water or ice. Buoyancy can betested by releasing the balloon at a fixeddistance above the floor and using a meterstick and stopwatch to measure how fastthe balloon rises or falls.)Logical, Visual

L2


A hot-air balloon has three main parts: thebasket, the envelope, and the burner. Thebasket holds the balloonist, passengers,propane tanks, navigation equipment, andother needed supplies. The basket is oftenmade out of wicker because it is fairlylightweight but also strong and flexibleenough to absorb some of the energy oflandings. The envelope, which is essentially

a large nylon bag, holds the hot air. Theburner is somewhat like a giant Bunsen burner.It burns propane that is stored as a liquid inlarge tanks. When the balloonist fires theburner, it heats the air inside the balloon andincreases its buoyancy, lifting the balloon. Tolower the balloon, the balloonist pulls a cordthat opens a parachute valve at the top of theballoon and lets some of the hot air out.

Facts and Figures

HSES_1eTE_C18.qxd 9/24/04 9:42 PM 14


Stability and Daily Weather Recall that stable air resists ver-tical movement and that unstable air rises freely. But how do these factsapply to the daily weather?

Because stable air resists upward movement, you might concludethat clouds won’t form when stable conditions are present in theatmosphere. Although this seems reasonable, remember that there areprocesses that force air above Earth’s surface. These include orographiclifting, frontal wedging, and convergence. When stable air is forcedabove Earth’s surface, the clouds that form are widespread and havelittle vertical thickness when compared to their horizontal dimension.Precipitation, if any, is light to moderate.

In contrast, clouds associated with the lifting of unstable air aretowering and often generate thunderstorms and occasionally even atornado. For this reason, on a dreary, overcast day with light drizzle,stable air has been forced above Earth’s surface. During a day whencauliflower-shaped clouds appear to be growing as if bubbles of hotair are surging upward, the air moving up is unstable. Figure 10 showscauliflower-shaped clouds caused by the rising of unstable air.

What types of weather can result when stable airrises?

Figure 10 These clouds provideevidence of unstable conditions inthe atmosphere.

Build Science SkillsComparing and ContrastingCompare the behavior of stable andunstable air parcels. Ask: What factorscause stable air to rise? (orographiclifting, frontal wedging, and convergence)Why are clouds formed from stable airwidespread and thin? (Because the airis stable, it all stays at roughly the samealtitude.) What factors cause unstableair to rise? (localized convective lifting)Why are clouds formed from unstableair often towering? (Strong convectionlifts and spreads out the clouds.)Logical

Condensation

Making a CloudPurpose Students observe how acloud forms.

Materials valve stem from a car orbicycle, 2-hole rubber stopper, glasstubing, rubber tubing, hose clamp,warm water, Erlenmeyer flask, match,bicycle pump, sheet of black paper,flashlight

Procedure In advance, insert a valvestem from a car or bicycle into one holeof a 2-hole rubber stopper. Insert a pieceof glass tubing into the other hole andattach a piece of rubber tubing to theglass tubing, Clamp off the rubbertubing. Pour a small amount of warmwater into an Erlenmeyer flask. In frontof students, light a match and hold itinside the flask for a few seconds. Insertthe stopper into the flask and use abicycle pump to increase the pressureof the flask. Stand up a sheet of blackpaper behind the flask and shine aflashlight through it. Now open theclamp. Ask students what theyobserved. (A cloud forms.) Ask why itformed. (The sudden drop in pressurecooled the air, causing water vapor tocondense and form a cloud of droplets.)Ask what the role of the smoke was.(It provided condensation nuclei onwhich the droplets could form.)

Expected Outcome A small cloud willform inside the flask.Visual, Logical

L2

L2


Answer to . . .

Clear weather mayoccur or light-to-

moderate precipitation.


516 Chapter 18

Section 18.2 Assessment

Reviewing Concepts1. Describe what happens to air temperature

when work is done on the air to compress it.

2. What does stability mean in terms of airmovement?

3. List four mechanisms that cause air to rise.

4. Describe conditions that causecondensation of liquid water in air.

5. What is a temperature inversion?

6. Which types of condensation nuclei areespecially good for condensation to form?

Critical Thinking7. Hypothesizing Study a world map.

Hypothesize about other regions on Earth,other than the Florida peninsula, whereconvergence might cause cloud developmentand precipitation.

CondensationRecall that condensation happens when water vapor in the air changesto a liquid. This may be in the form of dew, fog, or clouds. For anyof these forms of condensation to occur, the air must be saturated.Saturation occurs most commonly when air is cooled to its dew point,or less often when water vapor is added to the air.

Types of Surfaces Generally, there must be a surface for watervapor to condense on. When dew forms, objects at or near the ground,such as grass and car windows, serve this purpose. But when conden-sation occurs in the air above the ground, tiny bits of particulatematter, called condensation nuclei, serve as surfaces for water-vaporcondensation. These nuclei are important because if they are absent, arelative humidity much above 100 percent is needed to produce clouds.

Condensation nuclei such as microscopic dust, smoke, and salt par-ticles from the ocean are abundant in the lower atmosphere. Because ofthese plentiful particles, relative humidity rarely exceeds 100 percent.Some particles, such as ocean salt, are especially good nuclei becausethey absorb water. When condensation takes place, the initial growthrate of cloud droplets is rapid. It diminishes quickly because the excesswater vapor is quickly absorbed by the numerous competing particles.This results in the formation of a cloud consisting of millions uponmillions of tiny water droplets. These droplets are all so fine that theyremain suspended in air. In the next section, you will examine types ofclouds and the precipitation that forms from them.

Air Temperature Review the descriptionof atmospheric temperature changes inSection 17.1. Then write a paragraphexplaining how these differ from adiabatictemperature changes in parcels of air.

516 Chapter 18

ASSESSEvaluateUnderstandingAsk students in turn to explain oneprocess that lifts air. Encourage them todraw diagrams on the board to helpexplain the process.

ReteachUse Figure 7 to review adiabatic coolingand its role in cloud formation. Explainwhy the moist adiabatic rate is slowerthan the dry adiabatic rate.

The general cooling trend with heightin the troposphere is called theenvironmental lapse rate. This isdifferent from parcels of air that rise,expand, and cool adiabatically.

L1

L2

3


5. A temperature inversion is a stable-airsituation in which air temperature increaseswith height.6. Good condensation nuclei are those thatabsorb water, such as ocean salt.7. Sample answer: Yucatan Peninsula; penin-sulas in Northern Australia; New Zealand;Indonesia; tropical islands in general

Section 18.2 Assessment

1. Air temperature rises when work is doneon the air to compress it.2. Stable air tends to remain in its originalposition, while unstable air tends to rise.3. The mechanisms are frontal wedging,orographic lifting, convergence, andlocalized convective lifting.4. Condensation of liquid water in air occurswhen the air is saturated.


hses 1ete c18.qxd 9/27/04 11:40 pm page 510 section 18.2...

Documents