19.2 pressure centers and winds 1hagenc.yolasite.com/resources/este192.pdf · 19.2 pressure centers...

Air Pressure and Wind 537

19.2 Pressure Centersand Winds

Reading StrategyComparing and Contrasting Copy thetable below. As you read about pressurecenters and winds, fill in the table indicatingto which hemisphere the concept applies. UseN for Northern Hemisphere, S for SouthernHemisphere, and B for both.

Key ConceptsDescribe how winds blowaround pressure centers inthe Northern andSouthern Hemispheres.

What are the air pressurepatterns within cyclonesand anticyclones?

How does friction controlnet flow of air around acyclone and an anticylone?

How does the atmosphereattempt to balance theunequal heating of Earth’ssurface?

Vocabulary◆ cyclone◆ anticyclone◆ trade winds◆ westerlies◆ polar easterlies◆ polar front◆ monsoon

Pressure centers are among the most common features on anyweather map. By knowing just a few basic facts about centers of highand low pressure, you can increase your understanding of present andforthcoming weather. You can make some weather generalizationsbased on pressure centers. For example, centers of low pressure are fre-quently associated with cloudy conditions and precipitation. Bycontrast, clear skies and fair weather may be expected when an area isunder the influence of high pressure, as shown in Figure 6.

Highs and LowsLows, or cyclones (kyklon � moving in a circle) arecenters of low pressure. Highs, or anticyclones, arecenters of high pressure. In cyclones, the pres-sure decreases from the outer isobars toward thecenter. In anticyclones, just the opposite is thecase—the values of the isobars increase from theoutside toward the center.

Cyclones rotate a.counterclockwise.

Net flow of air is inward b.around a cyclone.

Anticyclones rotate c.counterclockwise.

Coriolis effect deflects d.winds to the right.

?

?

?

?

Figure 6 These sunbathers atCape Henlopen, Delaware, areenjoying weather associated witha high-pressure center.

FOCUS

Section Objectives19.5 Explain how winds blow

around pressure centers inthe Northern and SouthernHemispheres.

19.6 Describe the air pressurepatterns within cyclones andanticyclones.

19.7 Describe how friction controlsthe net flow of air around acyclone and an anticyclone.

19.8 Explain how the unequalheating of Earth’s surfaceaffects the atmosphere.

Build VocabularyConcept Map Have students make aconcept map using the term global windsas the starting point. All the vocabularyterms in this section except cyclone andanticyclone should be used. Havestudents include the definitions of thevocabulary terms in their concept maps.

Reading Strategya. N b. Bc. S d. N

INSTRUCT

Highs and LowsBuild Science SkillsUse Models Havestudents use a globeto review and demon-strate the Corioliseffect. One student can rotate the globewhile the other uses a pointer to show aflow of air moving straight down from apole to the equator. Students can alsouse the model to compare the directionin which airflow is deflected in theNorthern and Southern Hemispheres.Ask: Seen from Earth’s orbit, what isthe relationship between Earth’ssurface and the line along which theair is flowing? (Earth rotates beneath theline of airflow.) Seen from Earth’ssurface, what appears to behappening? (Earth’s rotation makesit appear that the airflow is deflected toone side—to the right in the NorthernHemisphere, to the left in the SouthernHemisphere.)Visual, Verbal

L2

2

L2

L2

Reading Focus

1


Section 19.2

HSES_1eTE_C19.qxd 5/16/04 2:00 PM Page 537

538 Chapter 19

Cyclonic and Anticyclonic Winds You learned that the twomost significant factors that affect wind are the pressure gradient andthe Coriolis effect. Winds move from higher pressure to lower pres-sure and are deflected to the right or left by Earth’s rotation. Whenthe pressure gradient and the Coriolis effect are applied to pressurecenters in the Northern Hemisphere, winds blow counterclockwisearound a low. Around a high, they blow clockwise. Notice the winddirections in Figure 7.

In the Southern Hemisphere, the Coriolis effect deflects the windsto the left. Therefore, winds around a low move clockwise. Windsaround a high move counterclockwise. In either hemisphere, fric-tion causes a net flow of air inward around a cyclone and a net flowof air outward around an anticyclone.

Weather and Air Pressure Rising air is associated with cloudformation and precipitation, whereas sinking air produces clear skies.

Imagine a surface low-pressure system where the air is spiralinginward. Here the net inward movement of air causes the area occupiedby the air mass to shrink—a process called horizontal convergence.Whenever air converges (or comes together) horizontally, it mustincrease in height to allow for the decreased area it now occupies. Thisincrease in height produces a taller and heavier air column. A surfacelow can exist only as long as the column of air above it exerts less pres-sure than does the air in surrounding regions. This seems to be aparadox—a low-pressure center causes a net accumulation of air,which increases its pressure.

With what type of weather is rising air associated?

Figure 7 This map shows cyclonicand anticyclonic winds in theNorthern Hemisphere.

1004

1016

1020

1012

1008

101210081012 1008 10161012

1008

1004

1000

996992

(Anticyclone)

(Cyclone)

538 Chapter 19

Warm Air RisesPurpose Students see that air risesupward as it is warmed by a heat source.

Materials 2 meter sticks, 2 large papergrocery bags, string, tape, lamp withincandescent bulb

Procedure Tape string to the bottomof each bag. Tie the two strings to ameter stick, as far apart as possible. Thebags should be hanging upside down.Use more string to hang this meter stickat its balance point from another meterstick that rests between two studentdesks. Place the lamp beneath one ofthe bags and switch it on.

Expected Outcomes The lamp heatsthe air inside the bag, causing it to riseabove the other bag and showing thatwarm air rises.Visual, Logical

Students may have the misconceptionthat no outside factor except heat isrequired to cause masses of warm airto rotate as they rise upward in theatmosphere. Point out that the cyclonicmotion in low-pressure cells is primarilya result of the Coriolis effect. As a follow-up, ask: What is the primary cause ofthe anticyclonic motion of high-pressure cells? (Coriolis effect )Verbal, Logical

L2

L2

Section 19.2 (continued)

Customize for Inclusion Students

Visually Impaired Help visually impairedstudents compare cyclones and anticyclonesby using their hands to model the motions ofthese phenomena. Explain that, in the NorthernHemisphere, cyclonic winds blow inward andcounterclockwise around a low-pressurecenter. Have students use one hand to

represent Earth’s surface. This hand remainsstationary. They can use the other hand tomake the counterclockwise, inward motion ofa cyclonic wind. Then have students use thesame hand to make the clockwise, outwardmotion of an anticyclonic wind in theNorthern Hemisphere.



In order for a surface low to exist for very long, converging air atthe surface must be balanced by outflows aloft. For example, surfaceconvergence could be maintained if divergence, or the spreading out ofair, occurred above the low at a rate equal to the inflow below. Figure8 shows the relationship between surface convergence (inflow) anddivergence (outflow) needed to maintain a low-pressure center. Surfaceconvergence around a cyclone causes a net upward movement. Becauserising air often results in cloud formation and precipitation, a low-pressure center is generally related to unstable conditions and stormyweather.

Like cyclones, anticyclones also must be maintained from above.Outflow near the surface is accompanied by convergence in the airabove and a general sinking of the air column, as shown in Figure 8.

Weather Forecasting Now you can see why weather reportsemphasize the locations and possible paths of cyclones and anticy-clones. The villain in these reports is always the low-pressure center,which can produce bad weather in any season. Lows move in roughlya west-to-east direction across the United States, and they require afew days, and sometimes more than a week, for the journey. Theirpaths can be somewhat unpredictable, making accurate estimation oftheir movement difficult. Because surface conditions are linked to theconditions of the air above, it is important to understand total atmos-pheric circulation.

CYCLONIC FLOW ANTICYCLONIC FLOW

Subsidingair

Risingair

Surfacedivergence

Surfaceconvergence

Convergence aloftDivergence aloft

Surfacedivergence

Surfaceconvergence

Subsidingair

Risingair

CYCLONIC FLOW ANTICYCLONIC FLOW

Figure 8 Air spreads out,or diverges, above surfacecyclones, and comes together,or converges, above surfaceanticyclones.Applying Concepts Why is fairweather associated with a high?

Airflow Patterns, Surface and AloftBuild Reading LiteracyRefer to p. 530D, which provides theguidelines for making inferences.

Make Inferences After students havefinished reading this section, ask:What does sunlight have to do withthe occurrence of rainy weather?(Sunlight warms the atmosphere.Differential warming creates pressuregradients, which result in the formationof high- and low-pressure centers.Winds blow toward low-pressure centers,bringing moist air in which clouds canform.)Verbal, Logical

Use VisualsFigure 8 After students have examinedthe illustration, ask: Where does airenter a cyclonic flow? (at Earth’ssurface) Where does air leave acyclonic flow? (aloft) Where does airenter an anticyclonic flow? (aloft)Where does air leave an anticyclonicflow? (at Earth’s surface)Visual, Verbal

L1

L1


Answer to . . .

Figure 8 Convergence aloft causes acolumn of air to sink, creating a zoneof high pressure and fair weather.Sinking air is compressed and warmedas it nears Earth’s surface, whichdiscourages cloud formation.

cloud formation andprecipitation

Accurate forecasts require that meteorologistsnot only predict the movement of low-pressurecenters, but also determine if the airflow aloftwill intensify an embryo storm or act to suppressits development. Surface cyclones would quicklyeradicate themselves—not unlike the incomingrush of air that occurs when a vacuum-packedcan is opened—without divergence in the air

above. As a result, meteorologists must basetheir forecasts on data from upper and loweratmospheric conditions. Because of the closerelationship between conditions at the surfaceand aloft, understanding of total atmosphericcirculation, particularly in the mid-latitudes, isvery important.

Facts and Figures


540 Chapter 19

Global WindsThe underlying cause of wind is the unequal heating of Earth’s sur-face. In tropical regions, more solar radiation is received than isradiated back to space. In regions near the poles the opposite is true—less solar energy is received than is lost. The atmosphere balancesthese differences by acting as a giant heat-transfer system. Thissystem moves warm air toward high latitudes and cool air towardthe equator. On a smaller scale, but for the same reason, ocean currentsalso contribute to this global heat transfer. Global circulation is verycomplex, but you can begin to understand it by first thinking about

circulation that would occur on a non-rotating Earth.

Non-Rotating Earth Model On ahypothetical non-rotating planet with asmooth surface of either all land or all water,two large thermally produced cells wouldform, as shown in Figure 9. The heated airat the equator would rise until it reached thetropopause—the boundary between the tro-posphere and the stratosphere. The

tropopause, acting like a lid, would deflect thisair toward the poles. Eventually, the upper-level

airflow would reach the poles, sink, spread out inall directions at the surface, and move back toward

the equator. Once at the equator, it would be reheatedand begin its journey over again. This hypothetical circula-

tion system has upper-level air flowing toward the pole and surfaceair flowing toward the equator.

Rotating Earth Model If the effect of rotation were added tothe global circulation model, the two-cell convection system wouldbreak down into smaller cells. Figure 10 illustrates the three pairs ofcells that would carry on the task of redistributing heat on Earth. Thepolar and tropical cells retain the characteristics of the thermally gen-erated convection described earlier. The nature of circulation at themiddle latitudes, however, is more complex.

Near the equator, rising air produces a pressure zone known as theequatorial low—a region characterized by abundant precipitation. Asshown in Figure 10, the upper-level flow from the equatorial lowreaches 20 to 30 degrees, north or south latitude, and then sinks backtoward the surface. This sinking of air and its associated heating due

How does the atmospherebalance the unequal heatingof Earth’s surface?Cold

Hot

Cold

Sur

face

flow

Surfa

ceflo

w

Con

vect

ion

cell

Conve

ctio

nce

ll

Figure 9 Circulation on a Non-Rotating Earth A simpleconvection system is produced by unequal heating of theatmosphere. Relating Cause and EffectWhy would air sink after reachingthe poles?

540 Chapter 19

Global WindsUse VisualsFigure 9 After students have examinedthe illustration, ask: What happens tosurface air at the equator? (It risesupward as it is warmed by the sun.) Whydoes air flow from the poles to theequator? (Air at the equator receivesmore heat from the sun, thus making thearea lower in density and pressure thancolder air at the poles. Since higherpressure air moves toward lower pressureair, the net flow is toward the equator.)Visual, Logical

Build Science SkillsRelating Cause and Effect Explain tostudents that, as warm air rises from thesurface at the equator, cooler air comingfrom the poles moves in to fill the space.Ask: Why is the warm air in the upperatmosphere above the equator drawntoward the poles? (The tropopauseprevents the air from rising any higher.The only possible direction of motion istoward the poles.)Verbal, Logical

L2

L1


George Hadley, an English meteorologist of theeighteenth century, first proposed the simpleconvection system pictured in Figure 9. BecauseEarth rotates, however, meteorologists had todevelop a more complex global circulationmodel. This model is pictured in Figure 10 andhas three cells on each side of the equator. TheHadley cells, named after George Hadley andalso called tropical cells, are shown north and

south of the equator. The next cell, which isunlabeled on the diagram, is the mid-latitudecell. It is also called a Ferrel cell after WilliamFerrel, a nineteenth century Americanmeteorologist who helped explain atmosphericcirculation at mid-latitudes. The third type ofcell, also unlabeled, is the polar cell.

Facts and Figures



to compression produce hot, arid condi-tions. The center of this zone of sinkingdry air is the subtropical high, whichencircles the globe near 30 degreesnorth and south latitude. Thegreat deserts of Australia,Arabia, and the Sahara inNorth Africa exist becauseof the stable dry conditionsassociated with the sub-tropical highs.

At the surface, airflowmoves outward from thecenter of the subtropicalhigh. Some of the air travelstoward the equator and isdeflected by the Coriolis effect,producing the trade winds. Tradewinds are two belts of winds thatblow almost constantly from easterlydirections. The trade winds are locatedbetween the subtropical highs and the equator.The remainder of the air travels toward the poles and isdeflected, generating the prevailing westerlies of the middle latitudes.The westerlies make up the dominant west-to-east motion of theatmosphere that characterizes the regions on the poleward side of thesubtropical highs. As the westerlies move toward the poles, theyencounter the cool polar easterlies in the region of the subpolar low.The polar easterlies are winds that blow from the polar high towardthe subpolar low. These winds are not constant winds like the tradewinds. In the polar region, cold polar air sinks and spreads toward theequator. The interaction of these warm and cool air masses producesthe stormy belt known as the polar front.

This simplified global circulation is dominated by four pressurezones. The subtropical and polar highs are areas of dry subsiding (sink-ing) air that flows outward at the surface, producing the prevailingwinds. The low-pressure zones of the equatorial and subpolar regionsare associated with inward and upward airflow accompanied by cloudsand precipitation.

What is the polar front?

Polar easterlies

NEtrade winds

SEtrade winds

Westerlies

Polar high

Polar easterlies

Polar front

Hadley cell

NEtrade winds

SEtrade winds

Westerlies

0°

30°

60°

Hadleycell

Hadleycell

Hadley cell

Subpolarlow

Equatorial low

Subtropical high

Ferrelcell

Ferrelcell

Ferrelcell

Ferrel cell

Polar cell

Figure 10 Circulation on aRotating Earth This model ofglobal air circulation proposesthree pairs of cells.Interpreting Diagrams Describethe patterns of air circulation atthe equatorial and subpolar lows.

Use VisualsFigure 10 After students have readRotating Earth Model and examined theillustration, ask: What happens to warmequatorial air that has risen into theupper atmosphere? (It moves towardthe poles until it reaches latitudes of 20 or30 degrees, then sinks downward.) Whatkind of weather is characteristic ofregions around 20 to 30 degreeslatitude? (dry, hot) What factor isprimarily responsible for the tradewinds? (Coriolis effect) What factorscreate the polar front? (meeting of thewarmer, subpolar westerlies with thecolder polar easterlies) What kind ofweather is characteristic of the polarfront? (stormy)Visual, Logical

Students may think that heat is “lost”from air as it sinks toward Earth’s surfacein a high-pressure center. Explain tostudents that the term adiabatic refers tothe cooling or warming of air causedwhen air is allowed to expand or iscompressed, not because heat is addedto or subtracted from the system. Inother words, no heat enters or leavesthe system. Compression of air as it sinkscreates adiabatic heating. Expansion ofair as it rises creates adiabatic cooling.The air continues to cool as it rises untilit reaches its dew point. Then conden-sation takes place and clouds begin toform. Ask: How does adiabatic coolinghelp explain why precipitation isassociated with low-pressure centersbut not high-pressure centers? (Risingair in a low-pressure center becomes coolenough for condensation. Sinking airin a high-pressure system is heatedadiabatically, preventing condensation.)Verbal

L2

L1


Answer to . . .

Figure 9 Air becomes more dense asit cools, causing it to sink.

Figure 10 Both are zones where twocells, or air masses, converge and airrises, forming zones of low pressure.

The atmospheretransfers heat by moving

warm air toward high latitudes andcool air toward the equator.

the stormy belt wheresubpolar westerlies and

polar easterlies meet

HSES_1eTE_C19.qxd 9/24/04 10:12 PM 41

542 Chapter 19

Section 19.2 Assessment

Reviewing Concepts1. Describe how winds blow around pressure

centers in the Northern Hemisphere.

2. Compare the air pressure for a cyclonewith an anticyclone.

3. How does friction control the net flow ofair around a cyclone and an anticyclone?

4. Describe how the atmosphere balancesthe unequal heating of Earth’s surface.

5. What is the only truly continuous pressurebelt? Why is it continuous?

6. In general, what type of weather can youexpect if a low-pressure system is moving intoyour area?

Critical Thinking7. Identifying Cause and Effect What must

happen in the air above for divergence at thesurface to be maintained? What type ofpressure center accompanies surfacedivergence?

8. Examine Figure 7. What is theapproximate range of barometricpressure indicated by the isobars onthe map? What is the pressure intervalbetween adjacent isobars?

Influence of ContinentsThe only truly continuous pres-sure belt is the subpolar low inthe Southern Hemisphere. Herethe ocean is uninterrupted bylandmasses. At other latitudes,particularly in the NorthernHemisphere where landmassesbreak up the ocean surface, largeseasonal temperature differencesdisrupt the pressure pattern.Large landmasses, particularlyAsia, become cold in the winterwhen a seasonal high-pressuresystem develops. From this high-

pressure system, surface airflow is directed off the land. In the summer,landmasses are heated and develop low-pressure cells, which permit airto flow onto the land as shown in Figure 11. These seasonal changes inwind direction are known as the monsoons. During warm months, areassuch as India experience a flow of warm, water-laden air from the IndianOcean, which produces the rainy summer monsoon. The winter mon-soon is dominated by dry continental air. A similar situation exists to alesser extent over North America.

0°60° 60°120°

30°

120°

30°

0°

60°

1011

1008

1005

1002

1011

101410171020

10201023

10081005

1002

1011

10141017

999

1020

1020

1023

1008

10141017

1014

1020

1017

1026

1011

1011

1017

1014

1014

1023

1017

1002

10081017

1014

1008

1005

999

1020

1020

1023

1017

1026

1011

10171014

1023

10111014

ITCZ

ITCZ

BermudaHigh

PacificHigh

Figure 11 Average SurfacePressure and AssociatedGlobal Circulation for July.The ITCZ line stands for theIntertropical Convergence Zone.

542 Chapter 19

Build Science SkillsApplying Concepts Use a world mapto help students visualize the movementof monsoons in south Asia. Have thempicture a high-pressure system abovethe land that pushes air out to sea. Thenhave them picture how heat from theland during summer causes air to rise,producing a low-pressure system thatpulls in moist air from the ocean.Visual, Logical

ASSESSEvaluateUnderstandingHave students write a paragraphcomparing and contrasting cyclonic andanticyclonic winds. Have students writea second paragraph explaining howeach of the following global winds isformed: trade winds, westerlies, andpolar easterlies.

ReteachDraw a globe on the board or use alaminated world map. On the map,draw in each type of global wind whileexplaining its formation to students. UseFigure 10 as a reference.

Solutions8. 992 (�) to 1020 (�) millibars. In otherwords, the map includes pressures a bitless than 992 millibars and a bit morethan 1020 millibars. Isobar intervalequals 4 millibars.

L1

L2

3

L2


4. The atmosphere acts like a huge heat-transfer system, transporting warm air fromthe equator toward the poles and cold airfrom the poles toward the equator.5. The subpolar low in the SouthernHemisphere; no landmasses break upthe pressure system.6. cloud formation and precipitation7. Convergence must occur aloft in orderfor a surface divergence to be maintained.A surface divergence is associated with ahigh-pressure center.

Section 19.2 Assessment

1. In the Northern Hemisphere, winds blowcounterclockwise and inward around a low,and clockwise and outward around a high.2. In cyclones, air pressure decreases towardthe center of the cell. In anticyclones, air pres-sure increases toward the center of the cell.3. The effect of friction is to cause a net flowof air inward around a cyclone and a net flowoutward about an anticyclone.


19.2 pressure centers and winds 1hagenc.yolasite.com/resources/este192.pdf · 19.2 pressure centers...

Documents