526

FOCUS om CONCEPTS

with each type?

tr How is wind measured?

<5: What is air pressure and how is it measured?=.'§:: What force creates wind, and what other factors influence wind?tiii What are the two types of pressure centers? What wind patterns and weather conditions are associated

f the various elements ofweather and climate, changes in air pressure are the least noticeable.. In listening to a weather report, generally we are interested in moisture conditions (humidity

and precipitation), temperature, and perhaps wind. It is the rare person, however, whowonders about air pressure. Although the hour-to-hour and day-to-day variations in air pressure arenot perceptible to human beings, they are very important in producing changes in our weather. For

l example, it is variations in air pressure from place to place that generate winds that in turn can bringI changes in temperature and humidity (1 . > Av »). Air pressure is one of the basic weather elements

I and is a significant factor in weather forecasting. As you will see, air pressure is closely tied to the otherelements ofweather in a cause-and-effect relationship.

To assist you in learning the important concepts in this chapter, focus on the following questions:

What is the idealized global circulation? How do continents complicate patterns of global circulation?ti: What are the names and causes of some local winds?

What is El Niiio and how is it different from La Nina?1 What factors control and influence the global distribution of precipitation?

Understanding Air PressureIn Chapter 16 we noted that air pressure is simply the pressureexerted by the weight of air above. Average air pressure at sea levelis about 1 kilogram per square centimeter, or 14.7 potmds per squareinch. This is roughly the same pressure that is produced by a col-umn ofwater 10 meters (33 feet) in height. With some simple arith-metic you can calculate that the air pressure exerted on the top of asmall (50 centimeter by 100 centimeter) school desk exceeds 5,000kilograms (11,000 pounds), or about the weight of a 50-passengerschool bus. Why doesn’t the desk collapse under the weight of theocean ofair above? Simply, air pressure is exerted in all directions-down, up, and sideways. Thus, the air pressure pushing down onthe desk exactly balances the air pressure pushing up on the desk.

You might be able to visualize this phenomenon better ifyouimagine a tall aquarium that has the same dimensions as thedesktop. When this aquarium is filled to a height of 10 meters (33feet), the water pressure at the bottom equals 1 atmosphere (14.7pounds per square inch). Now, imagine what will happen if thisaquarium is placed on top of our student desk so that all the forceis directed downward. Compare this to what results when the deskis placed inside the aquarium and allowed to sink to the bottom.In the latter situation the desk survives because the water pressureis exerted in all directions, not just downward as in our earlier

example. The desk, like your body, is “built” to withstand the pres-sure of 1 atmosphere. It is important to note that although we donot generally notice the pressure exerted by the ocean of airaround us, except when ascending or descending in an elevatoror airplane, it is nonetheless substantial. The pressurized suitsused by astronauts on space walks are designed to duplicate theatmospheric pressure experienced at Earth’s surface. Withoutthese protective suits to keep body fluids from boiling away, astro-nauts would perish in minutes.

The concept of air pressure can be better understood if weexamine the behavior of gases. Gas molecules, unlike those of theliquid and solid phases, are not “bound” to one another but arefreely moving about, filling all space available to them. “Then twogas molecules collide, which happens frequently under normalatmospheric conditions, they bounce off each other like very elas-tic balls. If a gas is confmed to a container, this motion is restrictedby its sides, much like the walls of a handball court redirect themotion of the handball. The continuous bombardment of gasmolecules against the sides of the container exerts an outwardpush that we call air pressure. Although the atmosphere is with-out walls, it is confined from below by Earth's surface and effec-tively from above because the force of gravity prevents its escape.Here we define air pressure as the force exerted against a surfaceby the continuous collision of gas molecules.

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FIGURE 18.1 Strong winds blowing snow during a blizzard. (Photo by AGRfoto/Alex Rowbotham/Alamy)

A CONCEPT CHECK 1 8 ' 1 t t A 1 FIGURE 18.2 Simple mercury barometer. The weight of the columnQ What is air pressure? of mercury is balanced by the pressure exerted on the dish ofQ Express air pressure in pounds per square inch and kilograms mercury by the air above. If the pressure decreases, the column of

per square cemimetel-_ mercury falls; if the pressure increases, the column rises

Measuring Air PressureFr:r;S.s;t:r;;rti::rm

When meteorologists measure atmospheric pressure, they employa unit called the millibar. Standard sea-level pressure is 1013.2millibars. Although the millibar has been the unit ofmeasure onall U.S. weather maps since Ianuary 1940, the media use “inchesof mercury” to describe atmospheric pressure. In the UnitedStates, the National Weather Service converts millibar values toinches of mercury for public and aviation use.

Inches ofmercury are easy to understand. The use ofmercuryfor measuring air pressure dates from 1643, when Torricelli, a stu-dent of the famous Italian scientist Galileo, invented the mercurybarometer (bar = pressure, me tron = measuring instrument).Torricelli correctly described the atmosphere as a vast ocean ofairthat exerts pressure on us and all objects about us. To measure thisforce, he filled a glass tube, which was closed at one end, with mer-cury. He then inverted the tube into a dish ofmercury (Figure 1.8.2).

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528 CHAPTER 18 Air Pressure and Wind

Torricelli found that the mercury flowed out of the tube rmtil theweight of the column was balanced by the pressure that theatmo-sphere exerted on the surface of the mercury in the dish. Inother words, the weight of mercury in the column equaled theweight of the same diameter column of air that extended from theground to the top of the atmosphere.

When air pressure increases, the mercury in the tube rises.Conversely, when air pressure decreases, so does the height ofthe mercury column. With some refinements the mercurialbarometer invented by Torricelli is still the standard pressure-measuring instrument used today. Standard atmospheric pres-sure at sea level equals 29.92 inches of mercury.

The need for a smaller and more portable instrument for mea-suring air pressure led to the development of the aneroid(an = without, ner = fluid) barometer Instead ofhaving a mercury column held up by air pressure, the aneroidbarometer uses a partially evacuated metal chamber. The cham-ber, being very sensitive to variations in air pressure, changesshape, compressing as the pressure increases and expanding asthe pressure decreases. A series oflevers transmits the movementsof the chamber to a pointer on a dial that is calibrated to read ininches of mercury and/or millibars.

As shown in Figure 18.3, the face of an aneroid barometerintended for home use is inscribed with words like fair, change,min, and stormy. Notice that “fair weather” corresponds withhigh-pressure readings, whereas “rain” is associated with lowpressures. Although barometric readings may indicate the pres-ent weather, this is not always the case. The dial may point to“fair” on a rainy day, or you may be experiencing “fair” weatherwhen the dial indicates “rainy.” If you want to “predict” theweather in a local area, the change in air pressure over the past fewhours is more important than the current pressure reading. Fallingpressure is often associated with increasing cloudiness and the

Aneroid Barometer. this instrument has a partiallyevacuated chamber that changes shape, compressing as atmosphericpressure increases, and expanding as pressure decreases.

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possibility of precipitation, whereas rising air pressure generallyindicates clearing conditions. It is useful to remember, however,that particular barometer readings or trends do not always cor-respond to specific types of weather.

One advantage of the aneroid barometer is that it can eas-ily be connected to a recording mechanism. The resultinginstrument is a barograph, which provides a continuous recordof pressure changes with the passage of time (fr,=‘irj;rrrrtr.;=": '_ur.»<f%).Another important adaptation of the aneroid barometer is itsuse to indicate altitude for aircraft, mountain climbers, andmapmakers.

CONCEPT cr-nszcx 1 8.2Q What is standard sea level pressure in millibars? In inches of

mercury‘?Q Describe the operating principles of a mercury barometer and

an aneroid barometer.

Students Sometimes Ask. .What is the lowest barometric pressure ever recorded?All of the lowest-recorded baro- Wilma in October 2005. Themetric pressures have been as- world record, 870 millibarssociated with strong hurricanes. (25.70 inches), occurred duringThe record for the United States Typhoon Tip (a Pacific hurri-is 882 millibars (26.12 inches) cane), in October 1979.measured during Hurricane

An aneroid barograph makes a continuous record ofpressure changes. (Photo courtesy of Qualimetrics, Inc., Sacramento, California)

I 4-

M

Factors Affecting Wind 529

Factors Affecting WindEarth's Dynamic AtmosphereP Air Pressure and Wind

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In Chapter 17 we examined the upward movement of air and itsrole in cloud formation. As important as vertical motion is, farmore air moves horizontally, the phenomenon we call wind. Vlfhatcauses wind‘?

Simply stated, wind is the result ofhorizontal differences in airpressure. Airflowsfi'om areas ofhigher pressure to areas of lowerpressure. You may have experienced this when opening a vac-uum-packed can of coffee. The noise you hear is caused by airrushing from the higher pressure outside the can to the lowerpressure inside. Wind is nature’s attempt to balance such inequal-ities in air pressure. Because unequal heating of Earth’s surfacegenerates these pressure differences, solar radiation is the ulti-mate energy sourcefor most wind.

If Earth did not rotate, and if there were no friction betweenmoving air and Earth’s surface, air would flow in a straight linefrom areas of higher pressure to areas of lower pressure. Butbecause both factors exist, wind is controlled by a combination offorces, including (1) the pressure-gradient force, (2) the Corioliseffect, and (3) friction. We now examine each of these factors.

Pressure-Gradient ForcePressure differences create wind, and the greater these differ-ences, the greater the wind speed. Over Earth’s surface, variationsin air pressure are determined from barometric readings taken athundreds ofweather stations. These pressure data are shown ona weather map using isobars, lines that connect places of equalair pressure The spacing of isobars indicates theamount of pressure change occurring over a given distance andis expressed as the pressure gradient (gradus I slope).

You might find it easier to visualize a pressure gradient ifyouthink of it as being similar to the slope of a hill. A steep pressuregradient, like a steep hill, causes greater acceleration of an airparcel than does a weak pressure gradient (a gentle hill). Thus,the relationship between wind speed and the pressure gradientis straightforward: Closely spaced isobars indicate a steep pres-sure gradient and high winds, whereas widely spaced isobars indi-cate a weak pressure gradient and light winds. Figure 18.5illustrates the relationship between the spacing of isobars andwind speed. Notice that wind speeds are greater in Ohio, Ken-tucky, Michigan, and Illinois, where isobars are more closelyspaced, than in the western states, where isobars are more widelyspaced.

The pressure gradient is the driving force of wind, and it hasboth magnitude and direction. Its magnitude is determined from

1='5~i '* it Isobars are lines connecting places of equal sea-level pressure. They are used to show the distribution of pressure ondaily weather maps. Isobars are seldom straight, but usually form broad curves. Concentric rings of isobars indicate cells of highand low pressure. The “wind flags" indicate the expected airflow surrounding pressure cells and are plotted as "flying" with thewind (i.e., the wind blows toward the station circle). Notice that where the isobars are more closely spaced, the wind speed isfaster. Closely spaced isobars indicate a strong pressure gradient and high wind speeds, whereas widely spaced isobars indicate aweak pressure gradient and low wind speeds.

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530 CHAPTER 18 Air Pressure and Wind

the spacing of isobars. The direction of force is always from areas target, Earth would have rotated 15 degrees to the east during itsofhigher pressure to areas of lower pressure and at right angles to flight. To someone standing on Earth itwould look as if the rocketthe isobars. Once the air starts to move, the Coriolis effect and veered off its path and hit Earth 15 degrees west of its target. Thefriction come into play, but then only to modify the movement, true path of the rocket is straight and would appear so to some-not to produce it. one out in space looking down at Earth. It was Earth turning under

the rocket that gave it its apparent deflection.Note that the rocket was deflected to the right of its path of

- - motion because of the counterclockwise rotation of the NorthernConohs Effect Hemisphere. In the Southern Hemisphere, the effect is reversed.The weather map in Figure 18.5 shows the typical air movements Clockwise rotation produces a similar deflection, but to the leftassociated with high- and low-pressure systems. As expected, the of the path ofmotion. The same deflection is experienced by windair moves out of the regions ofhigher pressure and into the regions regardless of the direction it is moving.of lower pressure. However, the wind does not cross the isobars at We attribute the apparent shift in wind direction to the Cori-right angles as the pressure-gradient force directs it. This deviation olis effect, This deflection (1) is always directed at right angles tois the result of Earth’s rotation and has been named the Coriolis the direction of airflow; (2) affects onlywind direction, not windeffect after the French scientist who first thoroughly described it. speed; (3) is affected by wind speed (the stronger the wind, the

All free-moving objects or fluids, including the wind, are greater the deflection); and (4) is strongest at the poles anddeflected t0 the Tight Of their path Of motion in the Northern weakens equatotwatd, becoming nonexistent at the equator,Hemisphere and to the lejtin the Southern Hemisphere. The rea- It is of interest to point out that any “free-moving” objectson for thiS deflection can be illustrated by imagining the path Of will experience a deflection caused by the Coriolis effect, Thisa rocket launched from the North Pole toward a target located on fact was dramatically discovered by the United States Navy inthe equator If the rocket tOOl( an hour to reach itS World War II, During target practice long-range guns on battle-

ships continually missed their targets by as.. - , . .- . . . . . . much as several hundred ards until ballis-The Coriolis effect illustrated using a one-hour flight of a rocket traveling from Ythe North Pole to a location on the equator. A. On a nonrotatirig Earth, the rocket would tic Corrections were made for the Changingtravel straight to its target. B. However, Earth rotates 15° each hour. Thus, although the Position of 3 Seemin-SIY Stationary target-rocket travels in a straight line, when we plot the path of the rocket on Earth’s surface, it Over a short distance, however, the Coriolisfollows a curved path that veers to the right of the target. effeet is relatively Smelt

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A- N°"l‘°’Ifl’Ii"9 Eflfih Target flow of air. Under these conditions, the pres-* sure-gradient force causes air to start moving

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B. Rotating Earth my “\ Target 1 by noting the wind flags, many of— ---- ~ - - ~ 1 1 l which indicate winds of50-100 miles per horn‘.

Factors Affecting Wind 531

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speed, thereby diminishing the ability of the moving air to carry snow. As a result, snowaccumulates as a drift. (Photo by Garry Black/Superstock)

The most prominent features of upper-level flow are the jetstreams. First encountered by high-flying bombers duringWorld War ll, these fast-moving rivers of air travel between 120and 240 kilometers ('7 5 and 150 miles) per hour in a west-to-eastdirection. One such stream is situated over the polar front, whichis the zone separating cool polar air from warm subtropical air.

Below 600 meters (2,000 feet), friction complicates the air-flow just described. Recall that the Coriolis effect is proportional

to wind speed. Friction lowers the windspeed, so it reduces the Coriolis effect.Because the pressure-gradient force is notaffected by wind speed, it wins the tug ofwar shown in The result is amovement of air at an angle across theisobars toward the area of lower pressure.

The roughness of the terrain deter-mines the angle of airflow across the iso-bars. Over the smooth ocean surface,friction is low and the angle is small. Overrugged terrain, where friction is higher, theangle that air makes as it flows across theisobars can be as great as 45 degrees.

In summary, upper airflow is nearlyparallel to the isobars, whereas the effectof friction causes the surface winds tomove more slowly and cross the isobarsat an angle.

CONCEPT cuscx 1 8.3Q What force is responsible for generating wind?Q Write a generalization relating the spacing of isobars to the

speed ofwind.Q How does the Coriolis effect modify air movement?Q Contrast surface winds and upper-air winds in terms of speed

and direction.

..--:-; The geostrophic wind. The only force acting on a stationary parcel of air is the pressure-gradient force. Onceair begins to accelerate, the Coriolis effect deflects it to the right in the Northern Hemisphere. Greater wind speeds result ina stronger Coriolis effect until the flow is parallel to the isobars. At this point the pressure-gradient force and Coriolis effectare in balance and the flow is called a geostrophic wind. In the “real” atmosphere, airflow is continually adjusting forvariations in the pressure field. As a result, the adjustment to geostrophic equilibrium is much more irregular than shown.

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flected when you are playing catch?Over very short distances theCoriolis deflection is too smallto be noticed. Nevertheless, in

F‘ gr _ ' effect is great enough to poten-,._ F. _ H _, . ,_;fi :;;§->_r1(§_1()r1_ ‘§w. ,-t. ' ' ' ' .-2- = -1.1- . .. ¢°"_°|!§"‘?fieCt tially affect the outcome of a

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(330 feet) in 4 seconds down theright field line will be deflected1.5 centimeters (more than 1/2inch) to the right by the Corioliseffect. This could be justenough to turn a potential homerun into a foul ball!

Highs and Lows 533

Highs and LowsEarth's Dynamic Atmosphere9 Air Pressure and Winds

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Among the most common features on any weather map areareas designated as pressure centers. Lows, or cyclones(kyklon = moving in a circle) are centers of low pressure, andhighs, or anticyclones, are high-pressure centers. As +":0;~::.:-i-Tillustrates, the pressure decreases from the outer isobars towardthe center in a low. In a high, just the opposite is the case—thevalues of the isobars increase from the outside toward the center.By knowing just a few basic facts about centers of high and lowpressure, you can greatly increase your understanding of currentand forthcoming weather.

Cyclonic and Anticyclonic WindsFrom the preceding section, you learned that the two most sig-nificant factors that affect wind are the pressure-gradient forceand the Coriolis effect. Winds move from higher pressure tolower pressure and are deflected to the right or left by Earth’srotation. When these controls of airflow are applied to pressurecenters in the Northern Hemisphere, the result is that windsblow inward and counterclockwise around a low

Around a high, they blow outward and clockwise(Figure 18.11).

In the Southern Hemisphere the Coriolis effect deflects thewinds to the left, and therefore winds around a low blow clockwise(Figure l8.l2B), and winds around a high move counterclock-wise. In either hemisphere, friction causes a net inflow (con-vergence) around a cyclone and a net outflow (divergence)around an anticyclone.

Weather Generalizationsabout Highs and Lows

Rising air is associated with cloud formation and precipitation,whereas subsidence produces clear skies. In this section you willlearn how the movement of air can itself create pressure changeand hence generate winds. In addition, you will examine the rela-tionship between horizontal and vertical flow and its effect onthe weather.

Let us first consider the situation around a surface low-pres-sure system where the air is spiraling inward. Here the net inwardtransport of air causes a shrinking of the area occupied by the airmass, a process that is termed horizontal convergence. Wheneverair converges horizontally, it must pile up, that is, increase inheight to allow for the decreased area it now occupies. This gen-erates a taller and therefore heavier air column. Yet a surface lowcan exist only as long as the column of air above exerts less pres-sure than that occurring in surrounding regions. We seem to haveencountered a paradox—a low-pressure center causes a net accu-mulation of air, which increases its pressure. Consequently, a sur-face cyclone should quickly eradicate itself in a manner not unlikewhat happens when a vacuum-packed can is opened.

You can see that for a surface low to exist for very long, com-pensation must occur aloft. For example, surface convergencecould be maintained if divergence (spreading out) aloft occurredat a rate equal to the inflow below. shows the rela-tionship between surface convergence (inflow) and divergence(outflow) aloft that is needed to maintain a low-pressure center.

Divergence aloft may even exceed surface convergence,thereby resulting in intensified surface inflow and acceleratedvertical motion. Thus, divergence aloft can intensify storm centersas well as maintain them. On the other hand, inadequate diver-gence aloft permits surface flow to “fill” and weaken the accom-panying cyclone.

J1 1-} Cyclonic and anticyclonic winds in the Northern Hemisphere. Arrows show that winds blow into andcounterclockwise around a low. By contrast, around a high, winds blow outward and clockwise.

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Note that surface convergence about a cyclone causes a netupward movement. The rate of this vertical movement is slow,generally less than 1 kilometer per day. Nevertheless, because ris-ing air often results in cloud formation and precipitation, a low-pressure center is generally related to unstable conditions andstormy weather

As often as not, it is divergence aloft that creates a surface low.Spreading out aloft initiates upflow in the atmosphere directly

1‘-:@f;Ir1i Airflow associated with surface cyclones and anticyclones. A low, or cyclone, hasconverging surface winds and rising air causing cloudy conditions. A high, or anticyclone, hasdiverging surface winds and descending air, which lead to clear skies and fair weather.

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below, eventually working its way to the surface, where inflow isencouraged.

Like their cyclonic counterparts, anticyclones must be main-tained from above. Outflow near the surface is accompanied byconvergence aloft and general subsidence of the air column(Figure 18.13). Because descending air is compressed andwarmed, cloud formation and precipitation are unlikely in ananticyclone. Thus, “fair” weather can usually be expected with

the approach of a high-pressure cen-ter (Figure 18.14B).

For reasons that should now beobvious, it has been common prac-tice to print on household barometersthe words “stormy” at the low-pres-sure end and “fair” on the high-pres-sure end. By noting whether thepressure is rising, falling, or steady, wehave a good indication of what theforthcoming weather will be. Such adetermination, called the pressure,or barometric tendency, is a very

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weather programs is always the low-

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"4. These two photographs illustrate the basic weather generalizations associated with pressure centers. A. A rainy day in NewYork City. Centers of low pressure are frequently associated with cloudy conditions and precipitation. (Photo by David Grossma/Alamy) B. By contrast,clear skies and "fair" weather may be expected when an area is under the influence of high pressure. This scene is in New York City's CentralPark. (Photo by Kevin C. Downs/Photolibrary)

pressure center, which produces “bad” weather in any season.Lows move in roughly a west-to-east direction across the UnitedStates and require a few days to more than a week for the jour-ney. Because their paths can be somewhat erratic, accurate pre-diction of their migration is difficult, although essential, forshort-range forecasting.

Meteorologists must also determine if the flow aloft will inten-sify an embryo storm or act to suppress its development. Becauseof the close tie between conditions at the surface and those aloft,a great deal of emphasis has been placed on the importance andunderstanding of the total atmospheric circulation, particularly inthe mid-latitudes. We now examine the workings of Earth’s gen-eral atmospheric circulation, and then again consider the struc-ture of the cyclone in light of this knowledge.

CONCEPT CHECK 1 8 .4Q Describe the weather that usually accompanies a drop in

barometric pressure and a rise in barometric pressure.Q Sketch a simple diagram (including isobars and wind arrows)

showing the winds associated with surface cyclones and anti-cyclones in both the Northern and Southern hemispheres.

General Circulationof the AtmosphereAs noted, the underlying cause of wind is unequal heating ofEarth’s surface (see Box 18.1). In tropical regions, more solar radi-ation is received than is radiated back to space. In polar regionsthe opposite is true: less solar energy is received than is lost.Attempting to balance these differences, the atmosphere acts asa giant heat-transfer system, moving warm air poleward and coolair equatorward. On a smaller scale, but for the same reason,

ocean currents also contribute to this global heat transfer. Thegeneral circulation is very complex. We can, however, develop ageneral understanding by first considering the circulation thatwould occur on a nonrotating Earth having a uniform surface. Wethen modify this system to fit observed patterns.

Circulation on a Nonrotating EarthOn a hypothetical nonrotating planet with a smooth surface ofeither all land or all water, two large thermally produced cellswould form 5).The heated equatorial air would riseuntil it reached the tropopause, which, acting like a lid, would

E.i‘ii§T;t;i it.'§ii_% Global circulation on a nonrotating Earth. A simpleconvection system is produced by unequal heating of the atmosphere.

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Wind Energy——AnAlternative with

PotentialAir has mass, and when it moves (i.e. whenthe wind blows), it contains the energy ofthat motion—kinetic energy. A portion ofthat energy can be converted into other

forms-—mechanical force or electricity-—thatwe can use to perform work (Figure 18.A).

Mechanical energy from wind is com-monly used for pumping water in rural or re-mote places. The "farm windmill," still afamiliar sight in many rural areas, is an ex-ample. Mechanical energy converted fromwind can also be used for other purposes,such as sawing logs, grinding grain, and pro-pelling sailboats. By contrast, wind-poweredelectric turbines generate electricity forhomes, businesses, and for sale to utilities.

Today, modem wind turbines are beinginstalled at break-neck speed. In fact, world-wide, in 2010 the installed wind power ca-pacity was expected to exceed 203,000megawatts, an increase of 28 percent over

2009.* This is equivalent to the total electri-cal demand of Italy—2 percent of global elec-tricity production. Worldwide, wind energyinstallations have doubled every 3 yearssince 2000. The United States is the world‘sleading producer (22.3%), followed by China(16.3%), Germany (16.2%), and Spain(11.5%). Within the next decade, China is ex-pected to produce the most wind-generatedelectricity.

Wind speed is a crucial element in deter-mining whether a place is a suitable site forinstalling a wind-energy facility. Generally a

*One megawatt is enough electricity to supply 250-300average American households.

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Students Sometimes Ask...What is the highest wind speed ever recorded?The highest wind speedrecorded at a surface station is3'72 kilometers (231 miles) perhour, measured April 12, 1934,at Mount Washington, NewHampshire. Located at an eleva-tion of 1,879 meters (6,262 feet),the observatory atop Mount

Washington has an averagewind speed of 56 kilometers (35miles) per hour. Faster windspeeds have undoubtedly oc-curred on mountain peaks, butno instruments were in place torecord them.

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FIGURE 18.A Farm windmills such as the oneon the left are still familiar sights in someareas. Mechanical energy from wind iscommonly used to pump water. (Photo byMehmet Dilsiz/Shutterstock) The wind turbineson the right are operating near Palm Springs,California. Although California is the statewhere significant wind-power developmentgot its start, by 2009 it had been surpassed byTBXHS and IOWE1. (Photo by John Mead/Science PhotoLibrary/Photo Researchers, Inc.)

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deflect the air poleward. Eventually, this upper-level airflowwouldreach the poles, sink, spread out in all directions at the surface,and move back toward the equator. Once there, it would bereheated and start its journey over again. This hypothetical cir-culation system has upper-level air flowing poleward and surfaceair flowing equatorward.

Ifwe add the effect of rotation, this simple convection systemwill break down into smaller cells. Fil{_§tlR'@E*? i.s.'ii-*5 illustrates the threepairs of cells proposed to carry on the task of heat redistributionon a rotating planet. The polar and tropical cells retain the char-acteristics of the thermally generated convection described ear-lier. The nature of the mid-latitude circulation is more complexand is discussed in more detail in a later section.

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52minimum average wind speed of 21 kilome-ters (13 miles) per hour is necessary for alarge-scale wind-power plant to be prof-itable. A small difference in wind speedresults in a large difference in energy pro-duction, and therefore a large difference inthe cost of the electricity generated. For ex-ample, a turbine operating on a site with anaverage wind speed of 12-mph would gener-ate about 33 percent more electricity thanone operating at 11-mph. Also, there is littleenergy to be harvested at low windspeeds—6-mph winds contain less than oneeighth the energy of 12-mph winds.

The United States has tremendous windenergy resources (Figure 18.3). In 2010, 36

states had commercial facilities that pro-duced electricity from wind power. The lead-ing states in installed capacity were Texas,number one, followed by Iowa, California,and Minnesota. Despite the fact that Cali-fornia gave birth to the modem U.S. windindustry, 16 states have greater wind poten-tial. As shown in Table ’l8.A, the top fivestates for wind energy potential includeNorth Dakota, Texas, Kansas, South Dakota,and Montana. Although only a small fractionof U.S. electrical generation currently comesfrom wind energy, it has been estimatedthat wind energy potential equals morethan twice the total electricity currentlyconsumed.

‘The total amount of electricity that could potentially begenerated each year, measured in billions of kilowatt hours.A typical American home would use several hundredkilowatt hours per month.Source: U.S. Department of Energy

The U.S. Department of Energy has an-nounced a goal of obtaining 5 percent of U.S.electricity from wind by the year 2020-—agoal that seems consistent with the currentgrowth rate of wind energy nationwide.Thus wind-generated electricity appears tobe shifting from being an “alternative” to a"mainstream" energy source.

At the surface, airflow is outward from the center of the sub-Idealized Global CirculationNear the equator, the rising air is associated with the pressurezone known as the equatorial low—a region marked by abun-dant precipitation. As the upper-level flow from the equatoriallow reaches 20-30 degrees latitude, north or south, it sinks backtoward the surface. This subsidence and associated adiabaticheating produce hot, arid conditions. The center of this zone ofsubsiding dry air is the subtropical high, which encircles theglobe near 30 degrees latitude, north and south (Figure 18.16).The great deserts of Australia, Arabia, and North Africa existbecause of the stable dry conditions associated with the sub-tropical highs.

tropical high. Some of the air travels equatorward and is deflectedby the Coriolis effect, producing the reliable trade winds. Theremainder travels poleward and is also deflected, generating theprevailing westerlies of the mid-latitudes. As the westerlies movepoleward, they encounter the cool polar easterlies in the regionof the subpolar low. The interaction of these warm and coolwinds produces the stormy belt known as the polar front. Thesource region for the variable polar easterlies is the polar high.Here, cold polar air is subsiding and spreading equatorward.

In summary, this simplified global circulation is dominated byfour pressure zones. The subtropical and polar highs are areas ofdry subsiding air that flows outward at the surface, producing the

538 CHAPTER 18 Air Pressure and Wind

Subpolar Polar highlow

asterlies-

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circulation over the oceans is dominated by semi-permanent cells of high pressure in the subtrop-ics and cells of low pressure over the subpolarregions. The subtropical highs are responsible forthe trade winds and westerlies, as mentionedearlier.

The large landmasses, on the other hand,particularly Asia, become cold in the winter anddevelop a seasonal high-pressure system from

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prevailing winds. The low-pressure zones of the equatorial andsubpolar regions are associated with inward and upward airflowaccompanied by clouds and precipitation.

Influence of ContinentsUp to this point, we have described the surface pressure and asso-ciated winds as continuous belts around Earth. However, the onlytruly continuous pressure belt is the subpolar low in the SouthernHemisphere. Here the ocean is uninterrupted by landmasses. Atother latitudes, particularly in the Northern Hemisphere wherelandmasses break up the ocean surface, large seasonal tempera-ture differences disrupt the pattern. shows the result-ing pressure and wind patterns for lanuary and Iuly. The

Students Sometimes Ask. ..Does monsoon mean “rainy season"?No. Regions that experience winter monsoon. By contrast, inmonsoons typically have both a summer, warm, moisture-ladenwet and a dry season. Monsoonrefers to a wind system that ex-hibits a pronounced seasonalreversal in direction. In general,winter is associated with windsthat blow predominantly off thecontinents and produce a dry

air blows from the sea towardthe land. Thus, the summermonsoon, which is usually asso-ciated with abundant precipita-tion, is the source of themisconception.

(Figure l8.l7A). In the summer, the oppositeoccurs; the landmasses are heated and developlow-pressure cells, which permit air to flow ontothe land (Figure l8.17B). These seasonal changesin wind direction are known as the monsoons.During warm months, areas such as India expe-rience a flow of warm, water-laden air from theIndian Ocean, which produces the rainy summermonsoon. The winter monsoon is dominated bydry continental air. A similar situation exists, butto a lesser extent, over North America.

In summary, the general circulation is pro-duced by semipermanent cells of high and lowpressure over the oceans and is complicated byseasonal pressure changes overland.

The WesterliesCirculation in the mid-latitudes, the zone of the westerlies, is com-plex and does not fit the convection system proposed for the trop-ics. Between about 30 and 60 degrees latitude, the generalwest-to-east flow is interrupted by migrating cyclones and anti-cyclones. In the Northern Hemisphere these cells move from westto east around the globe, creating an anticyclonic (clockwise) flowor a cyclonic (counterclockwise) flow in their area of influence. Aclose correlation exists between the paths taken by these surfacepressure systems and the position of the upper-level airflow, indi-cating that the upper air strongly influences the movement ofcyclonic and anticyclonic systems.

Among the most obvious features of the flow aloft are the sea-sonal changes. The steep temperature gradient across the mid-dle latitudes in the winter months corresponds to a stronger flowaloft. In addition, the polar jet stream fluctuates seasonally suchthat its average position migrates southward with the approachof winter and northward as summer nears. By midwlnter, the jetcore may penetrate as far south as central Florida.

Because the paths of low-pressure centers are guided by theflow aloft, we can expect the southern tier of states to experiencemore of their stormy weather in the winter season. During the hotsummer months, the storm track is across the northern states,and some cyclones never leave Canada. The northerly storm trackassociated with summer applies also to Pacific storms, whichmove toward Alaska during the warm months, thus producing anextended dry season for much of the West Coast. The number ofcyclones generated is seasonal as well, with the largest numberoccurring in the cooler months when the temperature gradientsare greatest. This fact is in agreement with the role of cyclonicstorms in the distribution of heat across the mid-latitudes.

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CONCEPT cnrzcx 1 8. 50 In which belt ofprevailing winds is most of the United States

situated?Q The trade winds diverge from which pressure belt?Q Which prevailing wind belts converge in the stormy region

known as the polar front?Q Which pressure belt is associated with the equator?Q Describe the monsoon circulation of India.

Local WindsHaving examined Earth’s large-scale circulation, let us turn brieflyto winds that influence much smaller areas. Remember that allwinds are produced for the same reason: pressure differences thatarise because of temperature differences that are caused byunequal heating of Earth’s surface. Local winds are simply small-scale winds produced by a locally generated pressure gradient.Those described here are caused either by topographic effects orvariations in surface composition in the immediate area.

540 CHAPTER 18 Air Pressure and Wind

Land and Sea BreezesIn coastal areas during the warm summer months, the land sur-face is heated more intensely during the daylight hours than isthe adjacent body ofwater (see the section “Land and Water” inChapter 16). As a result, the air above the land surface heats,expands, and rises, creating an area of lower pressure. A seabreeze then develops, because cooler air over the water (higherpressure) moves toward the warmer land (lower pressure)(F.lgr.u;‘e fttilsrr). The sea breeze begins to develop shortly beforenoon and generally reaches its greatest intensity during the mid-to late afternoon. These relatively cool winds can be a significantmoderating influence on afternoon temperatures in coastal areas.

At night, the reverse may take place. The land cools morerapidly than the sea, and the land breeze develops (Figure 18.18B).Small-scale sea breezes can also develop along the shores of largelakes. People who live in a city near the Great Lakes, such asChicago, recognize this lake effect, especially in the summer. Theyare reminded daily by weather reports of the cool temperaturesnear the lake as compared to warmer outlying areas.

Mountain and Valley BreezesA daily wind similar to land and sea breezes occurs in manymountainous regions. During daylight hours, the air along theslopes of the mountains is heated more intensely than the air atthe same elevation over the valley floor. Because this warmer airis less dense, it glides up along the slope and generates a valleybreeze (Figure 1is.19A). The occurrence of these daytime upslopebreezes can often be identified by the cumulus clouds thatdevelop on adjacent mountain peaks.

After sunset, the pattern may reverse. Rapid radiation cool-ing along the mountain slopes produces a layer of cooler air nextto the ground. Because cool air is denser than warm air, it drainsdownslope into the valley. Such a movement of air is called amountain breeze (Figure 18.19B). The same type of cool airdrainage can occur in places that have very modest slopes. The

result is that the coldest pockets of air are usually found in thelowest spots. Like many other winds, mountain and valley breezeshave seasonal preferences. Although valley breezes are most com-mon during the warm season when solar heating is most intense,mountain breezes tend to be more dominant in the cold season.

Chinook and Santa Ana WindsWarm, drywinds are common on the eastern slopes of the Rock-ies, where they are called chinooks. Such winds are created whenair descends the leeward (sheltered) side of a mountain andwarms by compression. Because condensation may have oc-curred as the air ascended the windward side, releasing latentheat, the air descending the leeward slope will be warmer anddrier than it was at a similar elevation on the windward side.Although the temperature of these winds is generally less than10° C (50° F), which is not particularlywarm, they occur mostly inthe winter and spring when the affected areas may be experienc-ing below-freezing temperatures. Thus, by comparison, these dry,warm winds often bring a drastic change. When the ground hasa snow cover, these winds are known to melt it in short order.

A chinooklike wind that occurs in southern California is theSanta Ana. These hot, desiccating winds greatly increase thethreat of fire in this already dry area (Figure rs.;§:0).

Country BreezeOne type of local wind, called a country breeze, is associatedwith large urban areas. As the name implies, this circulation pat-tern is characterized by a light wind blowing into the city from thesurrounding countryside. The countrybreeze is best developed onrelatively clear, calm nights. Under these conditions, cities,because they contain massive buildings and surfaces composedof rocklike materials, tend to retain the heat accumulated duringthe day more than the less built-up outlying areas. The result isthat the warm, less dense air over the city rises, which in turn ini-tiates the country-to-city flow.

Fliiétiitfi it 8-3.1% Illustration of a sea breeze and a land breeze. A. During the daylight hours the air above the land heats andexpands, creating an area of lower pressure. Cooler and denser air over the water moves onto the land, generating a seabreeze. B. At night the land cools more rapidly than the sea, generating an offshore flow called a land breeze.

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Measuring Wind 541

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FIGURE 18.19 Valley and mountain breezes. A. Heating during the daylight hours warms the airalong the mountain slopes. This warm air rises, generating a valley breeze. In the photo, theoccurrence of a daytime valley breeze is identified by cloud development on mountain peaks,sometimes leading to an afternoon rain shower. (Photo by James E. Patterson/James Patterson Collection)B. After sunset, cooling of the air near the mountain can result in cool air drainage into the valley,producing the mountain breeze.

One investigation in Toronto showed that heat accumulatedwithin this city created a rural-city pressure difference that wassufficient to cause an inward and counterclockwise circulationcentered on the downtown area. One of the unfortunate conse-quences of the country breeze is that pollutants emitted near theurban perimeter tend to drift in and concentrate near the city'scenter.

Students Sometimes Ask...A friend who lives in Colorado talks about “snoweaters." What are they?“ Snow eaters " is a local term for day. A chinook that movedchinooks, the warm, dry winds through Granville, Norththat descend the eastern slopes Dakota, on February 21, 1918,of the Rockies. These winds caused the temperature to risehave been known to melt more from —3 3 ° F to 50°F, an in-than a foot of snow in a single crease of 83°F!

concsprour-:01: 1 8.6Q What is a local wind?Q Describe the formation ofa sea

breeze.Q Does a land breeze blow toward

or away from the shore?Q During what time of day would

you expect to experience a well-developed valley breeze—mid-night, late morning, or lateafternoon‘?

Measuring WindTwo basic wind measurements—direction and speed-—are importantto the weather observer. One simpledevice for determining both mea-surements is the simple windsock thatis a common sight at small airportsand landing strips (Figure i8.21A). Thecone-shaped bag is open at both endsand is free to change position withshifts inwind direction. The degree towhich the sock is inflated is an indica-tion ofwind speed.

Winds are always labeled by thedirection from which they blow. Anorth wind blows from the northtoward the south, an east windfiomthe east toward the west. The instru-ment most commonly used to deter-mine wind direction is the windvane

(Figure 18.21B, upper right). This instrument, a common sight onmany buildings, always points into the Wind. Often the wind

FIGURE 18.20 This satellite image shows strong Santa Ana windsfanning the flames of several large wildfires in southern California onOctober 27, 2003. These fires scorched more than 740,000 acres anddestroyed more than 3,000 homes. (NASA)

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FIGURE 18.21 A. A wind sock is a common device used for determining wind direction and estimating wind speed. They are common sights atsmall airports and landing strips. (Photo by Lourens Smak/Alamy Images) B. Wind vane (right) and cup anemometer (left). The wind vane shows winddirection and the anemometer measures wind speed. (Photo by Belfort Instrument Company)

direction is shown on a dial that is connected to the wind vane.The dial indicates wind direction, either by points ofthe compass(N, NE, E, SE, etc.) or by a scale of 0° to 360°. On the latter scale,0° or 360° are both north, 90° is east, 180° is south, and 270° is west.

When the wind consistently blows more often from one direc-tion than from any other, it is called a prevailing wind. You maybe familiar with the prevailing westerlies that dominate the cir-culation in the mid-latitudes. In the United States, for example,these winds consistently move the “weather” from west to eastacross the continent. Embedded within this gen-eral eastward flow are cells ofhigh and low pres-sure with the characteristic clockwise andcounterclockwise flow. As a result, the windsassociated with the westerlies, as measured at the

changes in temperature and moisture conditions, the ability topredict the winds can be very useful. In the Midwest, for example,a north wind may bring cool, dry air from Canada, whereas asouth wind may bring warm, humid air from the Gulf of Mexico.

Recall that about 70 percent ofEarth’s surface is covered by theocean, where conventional methods of gathering wind data areseldom possible. Ocean buoys and ships at sea provide very lim-ited coverage. However, since the 1990s, weather forecasts haveimproved significantly due to the availability of satellite-derived

FIGURE 18.22 Wind roses showing the percentage of time airflow is coming fromvarious directions. A. Wind frequency for the winter in the eastern United States. B. Windfrequency for the winter in northern Australia. Note the reliability of the southeasttrades in Australia as compared to the westerlies in the eastern United States. (Data from

surface, often vary considerably from day to dayand from place to place. By contrast, the direc-tion of airflow associated with the belt of tradewinds is much more consistent, as can be seenin Figure 18.22.

Wind speed is commonly measured using acup anemometer (anemo = wind, metron = mea-suring instrument) (Figure 18.21B, upper left).The wind speed is read from a dial much like thespeedometer of an automobile. Places wherewinds are steady and speeds are relatively highare potential sites for tapping wind energy.

By knowing the locations of cyclones andanticyclones in relation to where you are, you canpredict the changes in wind direction that will beexperienced as a pressure center moves past.Because changes in wind direction often bring542

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El Nifio and La Niiia 543

wind data. One way that wind speed and direction can be estab-lished is by using satellite images to track cloud movements.

CONCEPT cnrzcx 18.70 Vllhat are the two basic wind measurements? “That instru-

ments are used to make these measurements?Q From what direction does a northeast wind blow? Toward

what direction does a south wind blow?

El Niiio and La NiiiaAs can be seen in so the cold Peruvian current flowsequatorward along the coast of Ecuador and Peru. This flowencourages upwelling of cold nutrient-filled waters that serve asthe primary food source for millions of fish, particularly anchovies.Near the end of each year, however, a warm current that flowssouthward along the coasts ofEcuador and Peru replaces the coldPeruvian current. During the 19th century the local residentsnamed this warm countercurrent El Nifio (“the child“) after theChrist child because it usually appeared during the Christmas

'=-Ii -I I-.“-“T: The relationship between the Southern Oscillation and El Nino is illustrated on these simplified maps.A. Normally, the trade winds and strong equatorial currents flow toward the west. At the same time, the strong Peruviancurrent causes upwelling of cold water along the west coast of South America. B. When the Southern Oscillation occurs, thepressure over the eastern and western Pacific flip-flops. This causes the trade winds to diminish, leading to an eastwardmovement of warm water along the equator. As a result, the surface waters of the central and eastern Pacific warm, withfar-reaching consequences to weather patterns.

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544 CHAPTER 18 Air Pressure and Wind

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.1 " I §' _._

Sally Benson:Climate and Energy

ScientistSally Benson has spent her career research-ing solutions to the most pressing environ-mental problems of our time. In themid-1970s, as a young scientist withLawrence Berkeley National Laboratory, shetackled the first oil shortage by investigat-ing ways to harness the power of geother-mal energy.

was the most significant issue facing theworld." Benson organized research pro-grams that developed regional models of cli-mate change to help residents plan fordroughts and temperature shifts, and stud-ied the carbon cycle of the oceans, amongother projects.

In 2007, Benson was appointed directorof the Global Climate and Energy Project atStanford University, which seeks to developenergy sources that release fewer green-house gases. “Energy efficiency in lighting,heating, and cooling systems, and autos,makes all the sense in the world. But at theend of the day, we need to do a lot morethan that. If our current understanding iscorrect, we need to out overall emissions by80 percent of today’s levels," Bensonstates.

Benson sees many promising ways toachieve that goal. One is renewable energy.

“I get to do important work . . . and have funwhile I do it."

Ten years later, she was elbow-deep inthe mud of Kesterson Reservoir in Califor-nia's Central Valley. Irrigation runoff hadcaused selenium from local soil to accumu-late in the water, causing local birds to hatchchicks with horrifying deformities. Benson'sstudies made her realize microbes could beenvironmentally friendly tools to clean upother sites with toxic metal contamination.

“ The experience was so positivebecause I was doing this cutting-edge sci-ence at the same time regulators had tomake a decision about how to clean up thesite. I got an idea of the impact that sciencecan make, and how research could getresults," Benson says.

By the mid-1990’s, Benson was directingall Earth science research at the laboratory.“I talked with many people, read a lot, andcame to the conclusion that climate change

“ Today's biofuels don’t provide much advan-tage in terms of carbon dioxide emissions.But alternatives such as cellulosic ethanol(producing ethanol with plant fibers), wherethere are low emissions in the process ofgrowing and making them, are incrediblyimportant, " Benson opines.

Another is to continue using some fossilfuels to run power plants and ships, but tocapture the greenhouse gases they emit.“ More than half of the electricity worldwideis produced by burning coal, a plentifulresource in many places. We need to find away to make it carbon neutral, " Bensonsays. One way to remove such emissionsfrom the atmosphere altogether would be toinject them in aquifers deep within Earth.Benson herself has studied how to use tech-nology developed by the oil and gas indus-try to select sites where the rock offers a

good seal, inject the gas, and monitor thearea for leaks.

“I get to do important work solving criti-cal problems, get to be outdoors, do experi-ments at scale, and have fun while I do it.Interacting with the huge number of peopleimpacted by these issues makes the Earthsciences very rewarding."

—Kathleen Wong

Professor Sally Benson is Executive Directorof Stanford University's Global Climate andEnergy Project, a $225 million effort todevelop energy resources that are not harm-ful to the environment, especially energysources that release fewer greenhouse gasesDr. Benson has a multidisciplinary back-ground with degrees in geology, materialsscience, and minerals engineering, and withapplied research in hydrology and reservoirengineering. (Courtesy Dr. Sally Benson)

season. Normally, these warm countercurrents last for at most afew weeks when they again give way to the cold Peruvian flow.However, at irregular intervals of 3 to 7 years, these countercur-rents become unusually strong and replace normally cold off-shore waters with warm equatorial waters (Figure 18.23B). Today,scientists use the term El Nifio for these episodes of ocean warm-ing that affect the eastern tropical Pacific.

The onset of El Niiio is marked by abnormal weather patternsthat drastically affect the economies of Ecuador and Peru. Asshown in Figure 18.238, these unusually strong undercurrentsamass large quantities ofwarm water that block the upwelling ofcolder, nutrient-filled water. As a result, the anchovies starve, dev-astating the fishing industry. At the same time, some inland areasthat are normally arid receive an abnormal amount of rain. Here,

pastures and cotton fields have yields far above the average. Theseclimatic fluctuations have been known for years, but they wereoriginally considered local phenomena. Today, we know that ElNifio is part of the global circulation and affects the weather atgreat distances from Peru and Ecuador.

Two of the strongest El Nifio events on record occurredbetween 1982-83 and 1997-98 and were responsible for weatherextremes of a variety of types in many parts of the world. The1997-98 El Nifio brought ferocious storms that struck the Cali-fornia coast, causing unprecedented beach erosion, landslides,and floods. In the southern United States, heavy rains alsobrought floods to Texas and the Gulf states. The same energizedjet stream that produced storms in the South, upon reaching theAtlantic, sheared off the northern portions ofhurricanes, destroy-

g El Nino and La Nina 545

ing the storms. It was one of the quietest Atlantic hurricane sea-sons in years.

Major El Nino events, such as the one in 1997-98, are inti-mately related to the large-scale atmospheric circulation. Eachtime an El Nino occurs, the barometric pressure drops over largeportions of the southeastern Pacific, whereas in the westernPacific, near Indonesia and northern Australia, the pressure rises(-*' A 1 1 1). Then, as a major El Nino event comes to an end, thepressure difference between these two regions swings back in theopposite direction. This seesaw pattern of atmospheric pressurebetween the eastern and western Pacific is called the SouthernOscillation. It is an inseparable part of the El Nino warmings thatoccur in the central and eastern Pacific every 3 to 7 years. There-fore, this phenomenon is often termed El Nino/Southern Oscil-lation, or ENSO for short.

Winds in the lower atmosphere are the link between the pres-sure change associated with the Southern Oscillation and theextensive ocean warming associated with El Nino. During a typ-ical year, the trade winds converge near the equator and flow west-ward toward Indonesia (Figure 18.24A). This steady westwardflow creates a warm surface current that moves from east to westalong the equator. The result is a “piling up" of a thick layer ofwarm surface water that produces higher sea levels (by 30 cen-timeters) in the western Pacific. Meanwhile, the eastern Pacific ischaracterized by a strong Peruvian current, upwelling of coldwater, and lower sea levels.

Then when the Southern Oscillation occurs, the normal situa-tion just described changes dramatically. Barometric pressure rises

=ii‘ - 1 Simplified illustration of the see-saw pattern of atmospheric pressurebetween the eastern and western pacific, called the Southern Oscillation. A. Duringaverage years, high pressure over the eastern pacific causes surface winds and warmequatorial waters to flow westward. The result is a pileup of warm water in the westernPacific, which promotes the lowering of pressure. B. An el Nino event begins as surfacepressure increases in the western Pacific and decreases in the eastern Pacific. This airpressure reversal weakens, or may even reverse the trade winds, and results in aneastward movement of the warm waters that had accumulated in the western Pacific.

V.

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in the Indonesian region, causing the pressure gradient along theequator to weaken or even to reverse. As a consequence, the once-steady trade winds diminish and may even change direction. Thisreversal creates a major change in the equatorial current system,with warm water flowing eastward (Figure 18.2/11B). \/Vlllfl time, watertemperatures in the central and eastern Pacific increase and sealevel in the region rises. This eastward shift of the warmest surfacewater marks the onset of El Nino and sets up changes in atmo-spheric circulation that affect areas far outside the tropical Pacific.

When an El Nino began in the summer of 1997, forecasterspredicted that the pool of warm water over the Pacific would dis-place the paths of both the subtropical and mid-latitude jetstreams, which steer weather systems across North America (seeFigure I 8.23). As predicted, the subtropical jet brought rain to theGulf Coast, where Tampa, Florida, received more than three timesits normal winter precipitation. Furthermore, the mid-latitude jetpumped warm air far north into the continent. As a result, wintertemperatures west of the Rockies were significantly above normal.

The effects ofEl Nino are somewhat variable depending in parton the temperatures and size of the warm pools. Nevertheless,some locales appear to be affected more consistently. In particu-lar, during most El Ninos, warmer-than-normal winters occur inthe northern United States and Canada. In addition, normally aridportions of Peru and Ecuador, as well as the eastern United States,experience wet conditions. By contrast, drought conditions aregenerally observed in Indonesia, Australia, and the Philippines.One major benefit of the circulation associated with El Nino is asuppression of the number ofAtlantic hurricanes.

The opposite of El Nino is an atmosphericphenomenon known as La Nina. Once thoughtto be the normal conditions that occur betweentwo El Nino events, meteorologists now con-sider La Nina an important atmospheric phe-nomenon in its own right. Researchers havecome to recognize that when surface tempera-tures in the eastern Pacific are colder than aver-age, a La Nina event is triggered that has adistinctive set ofweather patterns. A typical LaNina winter blows colder than normal air overthe Pacific Northwest and the northern Great

-. . Plains while warming much of the rest of theUnited States. Furthermore, greater precipita-tion is expected in the Northwest. During theLa Nina winter of I998-99, a world-record

1 snowfall for one season occurred in Washing-ton State. Another La Nina impact is greaterhurricane activity. A recent study concludedthat the cost of hurricane damages in the UnitedStates is 20 times greater in La Nina years ascompared to El Nino years.

.1‘

.T'

C éuouth CONCEPT cnscx 1 8.8" - '* ' 'f- A rrcaLow ‘ . me . ... . ...H('é:3g0?$Z§;'ie Warm Countemurrent pressure 3'-,}_ 0 How 1s La Nina different from ElN1no?

- (warm water) ;-r,;" Q Describe the relationship between theAustralia _ _ ,,,j-13-if Southern Oscillation and a major El Nino

B. El Nino yearsevent.

546 CHAPTER 18 Air Pressure and Wind

Global Distributionof PrecipitationA casual glanceat shows a relatively complex patternfor the distribution of precipitation. Although the map appearsto be complicated, the general features of the map can beexplained by applying our knowledge of global winds and pres-sure systems.

In general, regions influenced by high pressure, with its asso-ciated subsidence and diverging winds, experience relatively dryconditions. On the other hand, regions under the influence of lowpressure and its converging winds and ascending air receiveample precipitation. This pattern is illustrated by noting that thetropical region dominated by the equatorial low is the rainiestregion on Earth (£1 It includes the rain forests of theAmazon basin in South America and the Congo basin in Africa.Here the warm, humid trade winds converge to yield abundantrainfall throughout the year. By way of contrast, areas dominatedby the subtropical high-pressure cells clearly receive much smalleramounts of precipitation. These are regions of extensive deserts.In the Northern Hemisphere the largest is the Sahara. Examplesin the Southern Hemisphere include the Kalahari in southernAfrica and the dry lands ofAustralia.

If Earth’s pressure and wind belts were the only factors con-trolling precipitation distribution, the pattern shown inFigure 18.25 would be simpler. The inherent nature of the air isalso an important factor in determining precipitation potential.Because cold air has a low capacity for moisture compared with

warm air, we would expect a latitudinal variation in precipita-tion, with low latitudes receiving the greatest amounts of precip-itation and high latitudes receiving the smallest amounts.Figure 18.25 indeed reveals heavy rainfall in equatorial regionsand meager precipitation in high-latitude areas. Recall that thedry region in the warm subtropics is explained by the presence ofthe subtropical high.

In addition to latitudinal variations in precipitation, the dis-tribution of land and water complicates the precipitation pattern.Large landmasses in the middle latitudes commonly experiencedecreased precipitation toward their interiors. For example, cen-tral North America and central Eurasia receive considerably lessprecipitation than do coastal regions at the same latitude. Fur-thermore, the effects ofmountain barriers alter the idealized pre-cipitation patterns we would expect solely from global wind andpressure systems. Windward mountain slopes receive abundantrainfall resulting from orographic lifting, whereas leeward slopesand adjacent lowlands are usually deficient in moisture.

CONCEPT cnacx 1 8.9Q With which global pressure belt are the rain forests of

Africa’s Congo Basin associated? Which pressure system islinked to the Sahara Desert?

Q Other than Earth’s pressure and wind belts, list two other fac-tors that exert a significant influence on the global distribu-tion ofprecipitation.

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Global Distribution of Precipitation 547

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Mercury is 13.5 times denser (heavier) than water. lfyou built a barometerusing water rather than mercury, how tall (in inches) would it have to be torecord standard sea-level pressure?The accompanying map shows the distribution of air pressure at 4:00 P.M.CDT on May 24,2010. Within which one of these states should wind speedsbe greatest—New York, Texas, Illinois, Kansas, Idaho, or California? Whatguided your selection?Ifyou wanted to erect wind turbines to generate electricity, would you searchfor a location that typically experiences a strong pressure gradient or a weakpressure gradient? Explain.IfEarth did not rotate on its axis and was completely covered with water, inwhat direction would a sailboat move if it started its journey in the mid-latitudes of the Northern Hemisphere? How did you figure this out?Refer to the accompanying satellite image of a hurricane. Is the storm locatedin the Northern Hemisphere or the Southern Hemisphere? How were you ableto determine your answer?If divergence in the jet stream above a surface low-pressure center exceedsconvergence at the surface, will surface winds likely get stronger or weaker?Explain.You and a friend are watching TV on a rainy day when the weather reporterstates that, “The barometric pressure is 28.8 inches and rising." Hearing this,you say, “It looks like fair weather is on its way.” Your friend responds with thefollowing questions. “I thought air pressure had something to do with theweight of air. How does inches relate to weight? And, why do you think theweather is going to improve?” How would you respond to your friend"squeries?Ifyou live in the Northern Hemisphere and are directly west of the center of acyclone, what is the probable wind direction? What ifyou were west of an anti-cyclone?It is late afternoon on a warm summer day and you are enjoying some time at

-4.i-?’ir;§UR}E; '18.;-in The Intertropical Convergence Zone(ITCZ) is associated with the pressure zone knownas the equatorial low. In this satellite image,produced with data from the Tropical RainfallMeasuring Mission (TRMM), the ITCZ is seen as aband of heavy rainfall shown in reds and yellows,which extends east-west just north of the equator.(Courtesy of NOAA)

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the beach. Until the last hour or two, winds were calm. Then a breeze began to develop. Is it morelikely a cool breeze from the water or a warm breeze from the adjacent land area? Explain.

g 548 CHAPTER 18 Air Pressure and Wind

10. When designing an airport it is important to have the planes take off into N5the wind. Refer to the accompanying wind rose and discuss the orientationof the runway and the direction planes would travel when they took off.Bonus: Where on Earth would youfind a wind rose like this? N

1 1. The accompanying maps ofAfrica show the distribution of precipitationfor Iuly and Ianuary. Which map represents Iuly and which represents Ian-

NWuary? How were you able to figure this out?

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In Review Chapter 18 Air Pressure and Wind0 Air has weight. At sea level it exerts a pressure of I kilogram

per square centimeter (14.7 pounds per square inch). Airpressure is the force exerted by the weight of air above. Withincreasing altitude there is less air above to exert a force, andthus air pressure decreases with altitude, rapidly at first, thenmuch more slowly. The unit used by meteorologists to mea-sure atmospheric pressure is the millibar. Standard sea-levelpressure is expressed as 1013.2 millibars. Isobars are lines on aweather map that connect places of equal air pressure.

O A mercury barometer measures air pressure using a column ofmercury in a glass tube that is sealed at one end and invertedin a dish of mercury. As air pressure increases, the mercury inthe tube rises; conversely, when air pressure decreases, sodoes the height of the column of mercury. A mercury barom-eter measures atmospheric pressure in inches ofmercury, theheight of the column of mercury in the barometer. Standardatmospheric pressure at sea level equals 29.92 inches of mer-cury. Aneroid (without liquid) barometers consist of partiallyevacuated metal chambers that compress as air pressure

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than surface winds because friction is greatly reduced aloft.Friction slows surface winds, which in turn reduces the Cori-olis effect. The result is air movement at an angle across theisobars toward the area of lower pressure.The two types of pressure centers are (1) cyclones, or lows(centers of low pressure), and (2) anticyclones, or highs (high-pressure centers). In the Northern Hemisphere, windsaround a low (cyclone) are counterclockwise and inward.Around a high (anticyclone), winds are clockwise and out-ward. In the Southern Hemisphere, the Coriolis effect causeswinds to move clockwise around a low and counterclockwisearound a high. Because air rises and cools adiabatically inlow-pressure centers, cloudy conditions and precipitation areoften associated with their passage. In high-pressure centers,descending air is compressed and warmed; therefore, cloudformation and precipitation are unlikely, and “fair” weather isusually expected.Earth’s global pressure zones include the equatorial low,subtropical high, subpolar low, and polar high. The global sur-

increases and expand as pressure deal-eaSeS_ face winds associated with these pressure zones are the trade0 Wind is the horizontal flow of air from areas of higher pres-

sure toward areas of lower pressure. Winds are controlled bythe following combination of forces: (1) the pressure-gradientforce (amount of pressure change over a given distance);(2) the Coriolis eflect (deflective effect of Earth’s rotation tothe right in the Northern Hemisphere and to the left in theSouthern Hemisphere); and (3)fi'iction with Earth’s surface(slows the movement of air and alters wind direction).

0 Upper-air winds, called geostrophic winds, blow parallel tothe isobars and reflect a balance between the pressure-gradi-ent force and the Coriolis effect. Upper-air winds are faster

winds, westerlies, and polar easterlies.Particularly in the Northern Hemisphere, large seasonal tem-perature differences over continents disrupt the idealized, orzonal, global patterns of pressure and wind. In winter, large,cold landmasses develop a seasonal high-pressure systemfrom which surface airflow is directed off the land. In sum-mer, landmasses are heated and low pressure develops overthem, which permits air to flow onto the land. The seasonalchanges in wind direction are known as monsoons.In the middle latitudes, between 30 and 60 degrees latitude,the general west-to-east flow of the westerlies is interrupted

g Mastering Geology 549

by migrating cyclones and anticyclones. The paths taken by .these pressure systems are closely related to upper-level air-flow and the polar jet stream. The average position of thepolar jet stream, and hence the paths followed by cyclones,migrates equatorward with the approach ofwinter and pole- .ward as summer nears.

0 Local winds are small-scale winds produced by a locally gen-erated pressure gradient. Local winds include sea and landbreezes (formed along a coast because of daily pressure differ-ences caused by the differential heating of land and water);valley and mountain. breezes (daily wind similar to sea andland breezes except in a mountainous area where the airalong slopes heats differently from the air at the same eleva-tion over the valley floor); and chinoolc and Santa Ana winds ,(warm, dry winds created when air descends the leeward sideof a mountain and warms by compression).

Key Termsair pressure (p. 526)aneroid barometer (p. 528)anticyclone (p. 533)barograph (p. 528)barometric tendency (p. 534)chinook (p. 540)convergence (p. 533)Coriolis effect (p. 530)country breeze (p. 540)cup anemometer (p. 542)cyclone (p. 533)divergence (p. 533)ElNif1o (p. 544)equatorial low (p. 537)

geostrophic wind (p. 530)high (p. 533)isobar (p. 529)jet stream (p. 531)land breeze (p. 540)La Nina (p. 545)low (p. 533)

monsoon (p. 538)mountain breeze (p. 540)polar easterlies (p. 537)polar front (p. 537)polar high (p. 537)

Examining the Earth Systeml. Examine the image of Africa in Figure 1.9B (p. 13) and pick

out the region dominated by the equatorial low and the areasinfluenced by the subtropical highs in each hemisphere.What clue(s) did you use? Speculate on the differences in thebiosphere between the regions dominated by high pressureand the zone influenced by low pressure.

2. How are global winds related to surface ocean currents?(Try comparing Figure 18.17, p. 539, with Figure 15.2,

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Looking for additional review and test prep materials? Visit the ,Self Study area in www.masteringgeology.com to find practicequizzes, study tools, and multimedia that will aid in your under- ,standing of this chapter’s content. In MasteringGeology“' you ,will find: ,

O GEODe: Earth Science: An interactive visual wallcthrough of 'key concepts '

mercury barometer (p. 527)

The two basic wind measurements are direction and speed.Winds are always labeled by the directionfrom which theyblow. Wind direction is measured with a wind vane, and windspeed is measured using a cup anemometer:El Nino is the name given to the periodic warming of the oceanthat occurs in the central and eastern Pacific. It is associatedwith periods when a weakened pressure gradient causes thetrade winds to diminish. A major El Nifio event triggers extremeweather in many parts of the world. Vvhen surface temperaturesin the eastern Pacific are colder than average, a La Nina event istriggered. A typical La Nifia winter blows colder-than-normalair over the Pacific Northwest and the northern Great Plainswhile warming much of the rest of the United States.The global distribution of precipitation is strongly influencedby the global pattern of air pressure and wind, latitude, anddistribution of land and water.

pressure gradient (p. 529)pressure tendency (p. 534)prevailing wind (p. 542)Santa Ana (p. 540)sea breeze (p. 540)Southern Oscillation (p. 545)subpolar low (p. 537)subtropical high (p. 537)trade winds (p. 537)valley breeze (p. 540)westerlies (p. 537)wind (p. 529)wind vane (p. 541)

p. 428.) What is the ultimate source of energy that drivesboth of these circulations?

3. Winds and ocean currents change in the tropical Pacific dur-ing an El Nino event. How might this impact the biosphereand geosphere in Peru and Ecuador? How about in Indone-sia? (One useful Web site that deals with El Nino is NOAA’s ElNino Theme Page at http://www.pmel.noaa.gov/tao/e1nino/nino-home.html).

Geoscience Animation Library: More than 100 animationsilluminating many difficult-to-understand Earth scienceconceptsIn The News RSS Feeds: Current Earth science events andnews articles are pulled into the site with assessmentPearson eTextOptional Self Study QuizzesWeb LinksGlossaryFlashcards