HSES 1eTE C19.qxd 5/16/04 2:01 PM Page 543 19.3 Regional ... · PDF...

Air Pressure and Wind 543

19.3 Regional Wind Systems

Reading StrategyPreviewing Copy the table below. Beforeyou read, use Figure 17 to locate examples ofthe driest and wettest regions on Earth. Afteryou read, identify the dominant wind systemfor each location.

Key ConceptsWhat causes local winds?

Describe the generalmovement of weather inthe United States.

What happens whenunusually strong, warmocean currents flow alongthe coasts of Ecuador andPeru?

How is a La Niña eventtriggered?

Vocabulary◆ prevailing wind◆ anemometer◆ El Niño

C irculation in the middle latitudes is complex and does not fit theconvection system described for the tropics. Between about 30 and 60degrees latitude, the general west-to-east flow, known as the wester-lies, is interrupted by migrating cyclones and anticyclones. In theNorthern Hemisphere, these pressure cells move from west to eastaround the globe.

Local WindsSmall-scale winds produced by a locally generated pressure gradientare known as local winds. The local winds are caused either bytopographic effects or by variations in surface composition—landand water—in the immediate area.

Land and Sea Breezes In coastal areas during thewarm summer months, the land surface is heated moreintensely during the daylight hours than an adjacent body ofwater is heated. As a result, the air above the land surfaceheats, expands, and rises, creating an area of lower pressure.As shown in Figure 12, a sea breeze then develops becausecooler air over the water at higher pressure moves toward thewarmer land and low pressure air. The breeze starts devel-oping shortly before noon and generally reaches its greatestintensity during the mid- to late afternoon. These relativelycool winds can be a moderating influence on afternoon tem-peratures in coastal areas.

Precipitation Location DominantWind System

Extremely low a. b.

Extremely high c. d. ??

??

1004 mb

996 mb

992 mb

988 mb

1000 mb

LL HH

1008 mb

1016 mb HH LL

Cool water Warm land

Figure 12 Sea Breeze Duringdaylight hours, the air above landheats and rises, creating a localzone of lower air pressure.

FOCUS

Section Objectives19.9 Identify the causes of local

winds.19.10 Describe the general movement

of weather in the United States.19.11 Compare and contrast weather

patterns characteristic of ElNiño and La Niña events.

Build VocabularyWord Parts Explain to students thatthe word anemometer is a combinationof the Greek word anemos, meaning“wind,” and the Latin meter. Havestudents use this information to predictthe definition of anemometer. After theyhave studied the section, have themcorrect their predictions if necessary.

Reading StrategySample answers:a. Arabian Desertb. sinking trade winds produce tropicalhighc. Amazon rain forestd. rising trade winds produce equatoriallow

Local WindsUse VisualsFigure 12 After students have examinedthe illustration, ask: Does a sea breezemove toward the sea or away fromthe sea? (away from the sea) Why doesa sea breeze usually reach its highestspeeds in the late afternoon? (Theheat differential between land and waterincreases with the number of hours thesun has been shining.)Visual, Logical

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Reading Focus

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Section 19.3

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544 Chapter 19

At night, the reverse may take place. The land cools morerapidly than the sea, and a land breeze develops, as shown inFigure 13. The cooler air at higher pressures over the landmoves to the sea, where the air is warmer and at lower pres-sures. Small-scale sea breezes also can develop along theshores of large lakes. People who live in a city near the GreatLakes, such as Chicago, recognize this lake effect, especiallyin the summer. They are reminded daily by weather reportsof the cool temperatures near the lake as compared towarmer outlying areas.

Valley and Mountain Breezes A daily wind sim-ilar to land and sea breezes occurs in many mountainous

regions. During daylight hours, the air along the slopes of the moun-tains is heated more intensely than the air at the same elevation overthe valley floor. Because this warmer air on the mountain slopes is lessdense, it glides up along the slope and generates a valley breeze, asshown in Figure 14A. The occurrence of these daytime upslope breezescan often be identified by the cumulus clouds that develop on adja-cent mountain peaks.

After sunset, thepattern may reverse.The rapid cooling ofthe air along themountain slopes pro-duces a layer of coolerair next to the ground.Because cool air isdenser than warm air,it moves downslopeinto the valley. Such amovement of air,

illustrated in Figure 14B, is called a mountain breeze. In the GrandCanyon at night, the sound of cold air rushing down the sides of thecanyon can be louder than the sound of the Colorado River below.

The same type of cool air drainage can occur in places that havevery modest slopes. The result is that the coldest pockets of air are usu-ally found in the lowest spots. Like many other winds, mountain andvalley breezes have seasonal preferences. Although valley breezes aremost common during the warm season when solar heating is mostintense, mountain breezes tend to be more dominant in the cold season.

What type of local wind can form in the GrandCanyon at night?

1004 mb

996 mb

992 mb

988 mb

1000 mb

1008 mb

1016 mb

LLHH

HHLL

Warm water Cool land

H

H

Figure 13 Land Breeze At night,the land cools more rapidly thanthe sea, generating an offshoreflow called a land breeze. Inferring How would the isobarlines be oriented if there was noair pressure change across theland–water boundary?

Cool airWarm air

Figure 14 A Valley BreezeHeating during the day generateswarm air that rises from thevalley floor. B Mountain BreezeAfter sunset, cooling of the airnear mountain slopes can result incool air moving into the valley.

BA

544 Chapter 19

Use VisualsFigure 13 After students have examinedthe illustration, ask: Does a land breezemove toward the land or away fromthe land? (away from the land) Why doland breezes usually take place atnight? (After sunset, both land and waterbegin to cool, but land cools more rapidly.)Visual, Logical

Build Science SkillsComparing and Contrasting Havestudents create a table that contrasts sea,land, valley, and mountain breezes.

Day/Direction Night Cause

sea toward dayland

land toward nightsea

valley upslope day

mountain downslope night

Verbal, Logical

Differential HeatingPurpose Students observe that soil heatsfaster than water.

Materials 2 containers, soil, water,2 thermometers

Procedure Fill the two containers 2/3full, one with water and the other withsoil. Place a thermometer in eachcontainer and place both in a hot, sunnylocation. Read temperature measurementsat the beginning of the experiment andafter about 25 minutes.

Expected Outcome Soil heats fasterthan water.Visual, Kinesthetic

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air alongslopes coolsfaster than airabove valley,sinks downslope intovalley

air along slopesheats fasterthan air abovevalley, glidesup slopebecause it isless dense

land coolsfaster thanwater; airabove landsinks, creatinghigh pressurezone relative toair above water

sun heats landfaster thanwater; air aboveland rises,creating lowpressure zone

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Section 19.3 (continued)

Customize for English Language Learners

Explain that the terms prevailing and pre-dominant are synonyms. They are adjectivesthat mean “having superior strength, influence,or authority.” Have students look up thedefinitions of verb forms of these words anduse each in a sentence. Then ask students to

look up definitions for the related verbdominate and the related adjective dominant.Students also could be asked to compare andcontrast the meanings of all six words, as wellas the suffix pre-.



How Wind IsMeasuredTwo basic wind measurements—direction and speed—are particularlyimportant to the weather observer.Winds are always labeled by the direc-tion from which they blow. A northwind blows from the north toward thesouth. An east wind blows from theeast toward the west. The instrumentmost commonly used to determinewind direction is the wind vane,shown in the upper right of Figure 15.Wind vanes commonly are located onbuildings, and they always point intothe wind. The wind direction is oftenshown on a dial connected to thewind vane. The dial indicates winddirection, either by points of the com-pass—N, NE, E, SE, etc.—or by a scaleof 0° to 360°. On the degree scale, 0°or 360° are north, 90° is east, 180° issouth, and 270° is west.

Wind Direction When the wind consistently blows more oftenfrom one direction than from any other, it is called a prevailing wind.Recall the prevailing westerlies that dominate circulation in the middlelatitudes. In the United States, the westerlies consistently moveweather from west to east across the continent. Along within this gen-eral eastward flow are cells of high and low pressure with thecharacteristic clockwise and counterclockwise flows. As a result, thewinds associated with the westerlies, as measured at the surface, oftenvary considerably from day to day and from place to place. In contrast,the direction of airflow associated with the trade winds is much moreconsistent.

Wind Speed Shown in the upper left of Figure 15, a cup anemome-ter (anemo � wind, metron � measuring instrument) is commonly usedto measure wind speed. The wind speed is read from a dial much like thespeedometer of an automobile. Places where winds are steady and speedsare relatively high are potential sites for tapping wind energy.

Toward which direction does a SE wind blow?

Figure 15 Wind Vane and CupAnemometerInterpreting Photographs Howcan you tell which direction thewind is blowing?

For: Links on winds

Visit: www.SciLinks.org

Web Code: cjn-6193

How Wind IsMeasuredUse VisualsFigure 15 After students have examinedthe photograph, ask: What is thepurpose of the cup-shaped side ofthe device shown in the photograph?(to measure wind speed) What is thepurpose of the opposite end of thedevice? (to indicate wind direction) Whatmight be the purpose of the box-shaped component on the polebeneath the anemometer and windvane? (could contain devices for recordingwind speed and direction data, and/ortransmitting the data to another location)Visual, Logical

Use CommunityResourcesTake students on a field trip to visita weather station maintained by acommunity member or local or govern-ment organization. Ask the person incharge to show students how winddirection and speed are measured atthe station, how records are kept,and how the information is used.Interpersonal, Kinesthetic

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Download a worksheet on windsfor students to complete, and findadditional teacher support fromNSTA SciLinks.

Answer to . . .

Figure 13 Isobars would behorizontal.

Figure 15 Wind vanes point intothe wind.

mountain breeze

NW


546 Chapter 19

El Niño and La NiñaLook at Figure 16. The cold Peruvian current flows toward the equa-tor along the coasts of Ecuador and Peru. This flow encouragesupwelling of cold nutrient-filled waters that are the primary foodsource for millions of fish, particularly anchovies. Near the end of theyear, however, a warm current that flows southward along the coasts ofEcuador and Peru replaces the cold Peruvian current. During the nine-teenth century, the local residents named this warm current El Niño(“the child”) after the Christ child because it usually appeared duringthe Christmas season. Normally, these warm countercurrents last fora few weeks and then give way to the cold Peruvian flow again.

El Niño At irregular intervals of three to seven years, thesewarm countercurrents become unusually strong and replace nor-mally cold offshore waters with warm equatorial waters. Scientistsuse the term El Niño for these episodes of ocean warming that affectthe eastern tropical Pacific.

The onset of El Niño is marked by abnormal weather patterns thatdrastically affect the economies of Ecuador and Peru. As shown inFigure 17, these unusually strong undercurrents accumulate large quan-tities of warm water that block the upwelling of colder, nutrient-filledwater. As a result, the anchovies starve, devastating the local fishingindustry. At the same time, some inland areas that are normally aridreceive an abnormal amount of rain. Here, pastures and cotton fieldshave yields far above the average. These climatic fluctuations have beenknown for years, but they were originally considered local phenomena.It now is understood that El Niño is part of the global circulation andthat it affects the weather at great distances from Peru and Ecuador.

When an El Niño began in the summer of 1997, forecasters pre-dicted that the pool of warm water over the Pacific would displace the

Figure 16 Normal ConditionsTrade winds and strongequatorial ocean currents flowtoward the west.

AsiaPolar jet North

America

Subtropical jet

Strong trade winds

Australia

SouthAmerica

StrongPeruviancurrent

Highpressure

Equatorial currents (strong)Low

pressure

Warmwater

Coolwater

Ecuador

Peru

546 Chapter 19


El Niño and La NiñaBuild Reading LiteracyRefer to p. 246D in Chapter 9, whichprovides the guidelines for relatingcause and effect.

Relate Cause and Effect As they readthrough the text on El Niño and La Niña,have students create a flowchart showingthe cause-effect chain that results in anEl Niño event. (Sample answer: highpressure over eastern Pacific → warmwater pileup in eastern Pacific thatprevents cold water upwelling andpromotes formation of low air pressurezone → inland areas receive abnormalamounts of rain → trade winds weaken ormay reverse direction → subtropical andmid-latitude jet streams are displaced →abnormal rainfall in Florida/southernUnited States and mild winter temperatureswest of the Rocky Mountains)Verbal, Logical

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The seesaw pattern of atmospheric pressurechanges between the eastern and westernPacific is called the Southern Oscillation.During average years, high pressure over theeastern Pacific causes surface winds and warmequatorial waters to flow westward. The resultis a pileup of warm water in the westernPacific, which promotes the lowering of air

pressure. An El Niño event begins as surfacepressure increases in the western Pacific anddecreases in the eastern Pacific. This airpressure reversal weakens or may even reversethe trade winds, and results in an eastwardmovement of the warm waters that hadaccumulated in the western Pacific.

Facts and Figures



paths of both the subtropical and midlatitude jet streams, as shown inFigure 17. The jet streams steer weather systems across North America.As predicted, the subtropical jet brought rain to the Gulf Coast. Tampa,Florida, received more than three times its normal winter precipita-tion. The mid-latitude jet pumped warm air far north into thecontinent. As a result, winter temperatures west of the RockyMountains were significantly above normal.

La Niña The opposite of El Niño is an atmospheric phenomenonknown as La Niña. Once thought to be the normal conditions thatoccur between two El Niño events, meteorologists now considerLa Niña an important atmospheric phenomenon in its own right.

Researchers have come to recognize that when surface temper-atures in the eastern Pacific are colder than average, a La Niña eventis triggered that has a distinctive set of weather patterns. A typical LaNiña winter blows colder than normal air over the Pacific Northwestand the northern Great Plains. At the same time, it warms much of therest of the United States. The Northwest also experiences greater pre-cipitation during this time. During the La Niña winter of 1998–99, aworld-record snowfall for one season occurred in Washington State. LaNiña impact can also increase hurricane activity. A recent study con-cluded that the cost of hurricane damages in the United States is 20times greater in La Niña years as compared to El Niño years.

The effects of both El Niño and La Niña on world climate are wide-spread and vary greatly. These phenomena remind us that the air andocean conditions of the tropical Pacific influence the state of weatheralmost everywhere.

What is an El Niño and what effect does it have onweather?

Figure 17 El Niño Warmcountercurrents cause reversal ofpressure patterns in the westernand eastern Pacific.

AsiaPolar jet

NorthAmerica

Subtropical jet

Weak trade winds

Australia

SouthAmerica

WeakPeruviancurrent

Strong counter current Warmwater

Ecuador

Peru

Warmer thanaverage winter

Pressureincreases

Pressuredecreases

Dryerthan

average

Wetter thanaverage winter

For: Links on La Niña and El Niño

Visit: www.SciLinks.org

Web Code: cjn-6211

Integrate PhysicsThermal Energy Transfer Remindstudents that heat is the transfer ofenergy between two objects of differenttemperatures. Heat can be transferreddirectly, when the objects are in contact.It also can be transferred indirectly, byradiation and convection. Radiation isthe transfer of energy via electro-magnetic waves. Convection is thetransfer of energy by fluid motion. In afluid, convection works when colder,denser fluid sinks below warmer, lessdense fluid. Ask: Which of these threetypes of heat transfer is involved in ElNiño formation and the weatherpatterns that result? (All three; directtransfer takes place between ocean waterand air. Radiation transfers heat from sunto land and atmosphere. Convection is theairflow that occurs between high- and low-pressure regions.)Verbal, Logical

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When winds blow along the surface of theocean, the water is not pushed along directlyin front of the wind. Instead, it is deflected byabout 45 degrees as a result of the Corioliseffect. When surface waters are deflected,colder, deeper water rises to take its place.

This cold water is richer in nutrients thanwarmer surface water and promotes highlyproductive coastal fisheries. In years whencoastal upwelling does not occur, coastal fishpopulations may be reduced or absent.

Facts and Figures

Answer to . . .

An episode—occurringevery three to seven

years—of ocean warming that affectsthe eastern tropical Pacific; warmcountercurrents become unusuallystrong and replace normally coldoffshore waters with warm equatorialwaters.

Download a worksheet on La Niñaand El Niño for students tocomplete, and find additionalteacher support from NSTASciLinks.

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548 Chapter 19

Section 19.3 Assessment

Reviewing Concepts1. What are local winds, and how are they

caused?

2. Describe the general movement ofweather in the United States.

3. What happens when strong, warmcountercurrents flow along the coasts ofEcuador and Peru?

4. How is a La Niña event recognized?

5. What two factors mainly influence globalprecipitation?

Critical Thinking6. Interpreting illustrations Study Figure 17.

How could air pressure changes influenceweather patterns in this region?

Global Distribution of PrecipitationFigure 18 shows that the tropical region dominated by the equatoriallow is the rainiest region on Earth. It includes the rain forests of theAmazon basin in South America and the Congo basin in Africa. In theseareas, the warm, humid trade winds converge to yield abundant rain-fall throughout the year. In contrast, areas dominated by the subtropicalhigh-pressure cells are regions of extensive deserts. Variables other thanpressure and wind complicate the pattern. For example, the interiorsof large land masses commonly experience decreased precipitation.However, you can explain a lot about global precipitation if you applyyour knowledge of global winds and pressure systems.

Global Precipitation

Equator

Tropic of Capricorn

Precipitation in mm< 400400–800800–1600>1600 Antarctic Circle

Tropic of Cancer

Arctic Circle

30°

0°

30°

60°

60°

30°

0°

30°

60°

60°

Figure 18

Regions The map showsaverage annual precipitationin millimeters. Using theMap Key Determine therange of precipitation thatdominates Northern Africa.Identify Causes Whichweather pattern influencesprecipitation in this area?

Compare-Contrast Paragraph Write aparagraph comparing the features andeffects of El Niño and La Niña. Includespecific weather patterns associated witheach phenomenon.

548 Chapter 19

Global Distributionof Precipitation

AnswersUsing the Map Key less than 400 mmIdentify Causes subtropical highpressure cells

ASSESSEvaluateUnderstandingTo assess students’ knowledge of sectioncontent, have them write a shortdescription of each of the four types oflocal winds. Then have students writetwo or three sentences describing theevents that give rise to an El Niño event.

ReteachHave students draw and label their owndiagrams illustrating sea, land, valley,and mountain breezes. Allow studentsto use Figures 12, 13, and 14 asreferences, if needed.

Student paragraphs should contrast theocean water temperature conditionsthat trigger each phenomenon anddescribe how these in turn influence airpressure. Specific examples of weatherassociated with El Niño and La Niña alsoshould be included in the paragraph.

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on weather patterns of regions that are greatdistances from Ecuador and Peru.4. La Niña is triggered when surface tempera-tures in the eastern Pacific are colder thanaverage.5. moisture content of air and distribution ofland and water6. Warm water near Ecuador and Peru wouldheat air and cause it to rise, producing lowerpressure and precipitation. Regions north ofAustralia that normally receive more warmwater would experience higher pressure,possibly leading to drought.

Section 19.3 Assessment

1. Local winds are small-scale winds causedby local variations in air pressure, which arecaused by topography and unequal heatingof land and water, or unequal heating of airabove slopes and valleys.2. from west to east3. An El Niño event blocks normal upwellingof cold, nutrient-laden waters off the shores ofthese countries, and causes heavy precipitationinland. El Niño can have far-reaching effects



Tracking El Niño from Space

The colors in these images show sea-level height rel-ative to the average. When you focus on the images,remember that hills are warm colors and valleys arecool colors. The white and red areas indicate placesof higher-than-normal sea-surface heights. In thewhite areas, the sea surface is between 14 and 32centimeters above normal. In the red areas, sea levelis elevated by about 10 centimeters. Green areasindicate average conditions, whereas purple showszones that are at least 18 centimeters below aver-age sea level.

The images show the progression of the large warm-water mass from west to east across the equatorialPacific Ocean. At its peak in November 1997, thesurface area covered by the warm water mass wasabout one and one half times the size of the 48 con-tiguous United States. The amount of warm wateradded to the eastern Pacific with a temperaturebetween 21°C and 30°C was about 30 times thecombined volume of the water in all of the UnitedStates Great Lakes.

*Source: NASA’s Goddard Space Flight Center

The images in Figure 19 show the progression of the1997–98 El Niño. They were derived from data col-lected by the satellite TOPEX/Poseidon.* Thissatellite bounces radar signals off the ocean surfaceto precisely measure the distance between the satel-lite and the sea surface. When combined withhigh-precision data from the Global PositioningSystem (GPS) of satellites, maps of sea-surface

topography like these can be produced. These mapsshow the topography of the sea surface. The pres-ence of hills indicates warmer-than-average water,and the areas of low topography, or valleys, indicatecooler-than-normal water. Using water topography,scientists can determine the speed and direction ofsurface ocean currents.

April 25, 1997 July 25, 1997

November 10, 1997 March 14, 1998

Figure 19 Progression of the 1997–98 El Niño

Tracking El Niñofrom SpaceBackgroundThe image for April 25, 1997, showsthe Pacific Ocean on the eve of the1997–1998 El Niño event. By examiningthe images for July 25 and November10, you can see the dramatic buildup ofwarm water (white and red areas) in theeastern Pacific and the enlarging regionof cooler water (purple) in the westernPacific. By the time of the March 14,1998, image, the area of warm water inthe eastern Pacific was much smaller.

Teaching Tips• The directional terms eastern Pacific

and western Pacific may give somestudents trouble, since the easternPacific Ocean borders the westerncoast of the United States. Ask: Whatcontinents are bordered by theeastern Pacific Ocean? (NorthAmerica, South America, and CentralAmerica)

• As students examine each of the fourimages in Figure 19, ask: What colorsindicate warmer than average watertemperatures? (white and red) Whatcolors indicate cooler than averagewater temperatures? (purple) Whereis the warm water mass located onApril 25, 1997? (warm water mass notyet evident) Where is the warm watermass located on July 25 andNovember 10? (eastern Pacific) Whatdoes the map indicate about thewarm water mass on March 14?(It is dissipating.)

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