Chapter 18

526

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f the various elements of weather 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, who wonders about air pressure. Although the hour-to-hour and day-to-day variations in air pressure are not perceptible to human beings, they are very important in producing changes in our weather. For example, it is variations in air pressure from place to place that generate winds that in turn can bring changes in temperature and humidity (1 . > Av »). Air pressure is one of the basic weather elements and is a significant factor in weather forecasting. As you will see, air pressure is closely tied to the other elements of weather in a cause-and-effect relationship.

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FOCUS om CONCEPTS
To assist you in learning the important concepts in this chapter, focus on the following questions: <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 with each type? 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? tr How is wind measured? What is El Niiio and how is it different from La Nina? What factors control and influence the global distribution of precipitation?

1

Understanding Air Pressure
In Chapter 16 we noted that air pressure is simply the pressure exerted by the weight of air above. Average air pressure at sea level is about 1 kilogram per square centimeter, or 14.7 potmds per square
inch. This is roughly the same pressure that is produced by a col-

example. The desk, like your body, is “built” to withstand the pressure of 1 atmosphere. It is important to note that although we do
not generally notice the pressure exerted by the ocean of air around us, except when ascending or descending in an elevator or airplane, it is nonetheless substantial. The pressurized suits

umn ofwater 10 meters (33 feet) in height. With some simple arithmetic you can calculate that the air pressure exerted on the top of a small (50 centimeter by 100 centimeter) school desk exceeds 5,000 kilograms (11,000 pounds), or about the weight of a 50-passenger school bus. Why doesn’t the desk collapse under the weight of the ocean of air above? Simply, air pressure is exerted in all directionsdown, up, and sideways. Thus, the air pressure pushing down on the desk exactly balances the air pressure pushing up on the desk. You might be able to visualize this phenomenon better ifyou imagine a tall aquarium that has the same dimensions as the desktop. When this aquarium is filled to a height of 10 meters (33 feet), the water pressure at the bottom equals 1 atmosphere (14.7 pounds per square inch). Now, imagine what will happen if this
aquarium is placed on top of our student desk so that all the force

used by astronauts on space walks are designed to duplicate the atmospheric pressure experienced at Earth’s surface. Without these protective suits to keep body fluids from boiling away, astronauts would perish in minutes.
The concept of air pressure can be better understood if we

is directed downward. Compare this to what results when the desk is placed inside the aquarium and allowed to sink to the bottom.
In the latter situation the desk survives because the water pressure is exerted in all directions, not just downward as in our earlier

examine the behavior of gases. Gas molecules, unlike those of the liquid and solid phases, are not “bound” to one another but are freely moving about, filling all space available to them. “Then two gas molecules collide, which happens frequently under normal atmospheric conditions, they bounce off each other like very elastic balls. If a gas is confmed to a container, this motion is restricted by its sides, much like the walls of a handball court redirect the motion of the handball. The continuous bombardment of gas molecules against the sides of the container exerts an outward push that we call air pressure. Although the atmosphere is without walls, it is confined from below by Earth's surface and effectively from above because the force of gravity prevents its escape. Here we define air pressure as the force exerted against a surface
by 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 Q What is air pressure? Q Express air pressure in pounds per square inch and kilograms per square cemimetel-_

FIGURE 18.2 Simple mercury barometer. The weight of the column of mercury is balanced by the pressure exerted on the dish of mercury by the air above. If the pressure decreases, the column of mercury falls; if the pressure increases, the column rises

Measuring Air Pressure
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When meteorologists measure atmospheric pressure, they employ a unit called the millibar. Standard sea-level pressure is 1013.2 millibars. Although the millibar has been the unit of measure on all U.S. weather maps since Ianuary 1940, the media use “inches of mercury” to describe atmospheric pressure. In the United States, the National Weather Service converts millibar values to inches of mercury for public and aviation use. Inches ofmercury are easy to understand. The use of mercury for measuring air pressure dates from 1643, when Torricelli, a student ofthe famous Italian scientist Galileo, invented the mercury barometer (bar = pressure, me tron = measuring instrument). Torricelli correctly described the atmosphere as a vast ocean of air that exerts pressure on us and all objects about us. To measure this force, he filled a glass tube, which was closed at one end, with mercury. He then inverted the tube into a dish ofmercury (Figure 1.8.2).

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CHAPTER 18 Air Pressure and Wind

Torricelli found that the mercury flowed out of the tube rmtil the weight of the column was balanced by the pressure that the atmo-sphere exerted on the surface of the mercury in the dish. In other words, the weight of mercury in the column equaled the weight of the same diameter column of air that extended from the ground 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 of the mercury column. With some refinements the mercurial barometer invented by Torricelli is still the standard pressuremeasuring instrument used today. Standard atmospheric pressure at sea level equals 29.92 inches of mercury. The need for a smaller and more portable instrument for measuring air pressure led to the development of the aneroid (an = without, ner = fluid) barometer Instead of having a mercury column held up by air pressure, the aneroid barometer uses a partially evacuated metal chamber. The chamber, being very sensitive to variations in air pressure, changes shape, compressing as the pressure increases and expanding as the pressure decreases. A series oflevers transmits the movements of the chamber to a pointer on a dial that is calibrated to read in inches of mercury and/or millibars. As shown in Figure 18.3, the face of an aneroid barometer intended for home use is inscribed with words like fair, change, min, and stormy. Notice that “fair weather” corresponds with high-pressure readings, whereas “rain” is associated with low pressures. Although barometric readings may indicate the present weather, this is not always the case. The dial may point to “fair” on a rainy day, or you may be experiencing “fair” weather when the dial indicates “rainy.” If you want to “predict” the weather in a local area, the change in air pressure over the past few hours is more important than the current pressure reading. Falling pressure is often associated with increasing cloudiness and the

possibility of precipitation, whereas rising air pressure generally indicates clearing conditions. It is useful to remember, however, that particular barometer readings or trends do not always correspond to specific types of weather. One advantage of the aneroid barometer is that it can easily be connected to a recording mechanism. The resulting instrument is a barograph, which provides a continuous record of pressure changes with the passage of time (fr,=‘irj;rrrrtr.;=": '_ur.»<f%). Another important adaptation of the aneroid barometer is its use to indicate altitude for aircraft, mountain climbers, and mapmakers.

CONCEPT cr-nszcx 1 8.2
Q 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 barometric pressures have been associated with strong hurricanes. The record for the United States is 882 millibars (26.12 inches) measured during Hurricane Wilma in October 2005. The world record, 870 millibars (25.70 inches), occurred during Typhoon Tip (a Pacific hurricane), in October 1979.

Aneroid Barometer. this instrument has a partially evacuated chamber that changes shape, compressing as atmospheric pressure increases, and expanding as pressure decreases.

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

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Factors Affecting Wind
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Pressure-Gradient Force
Pressure differences create wind, and the greater these differences, the greater the wind speed. Over Earth’s surface, variations in air pressure are determined from barometric readings taken at
hundreds of weather stations. These pressure data are shown on

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Earth's Dynamic Atmosphere P Air Pressure and Wind

In Chapter 17 we examined the upward movement of air and its role in cloud formation. As important as vertical motion is, far
more air moves horizontally, the phenomenon we call wind. Vlfhat

a weather map using isobars, lines that connect places of equal
air pressure The spacing of isobars indicates the

amount of pressure change occurring over a given distance and
is expressed as the pressure gradient (gradus I slope). You might find it easier to visualize a pressure gradient if you think of it as being similar to the slope of a hill. A steep pressure

causes wind‘? Simply stated, wind is the result of horizontal differences in air pressure. Airflowsfi'om areas ofhigher pressure to areas oflower pressure. You may have experienced this when opening a vacuum-packed can of coffee. The noise you hear is caused by air rushing from the higher pressure outside the can to the lower pressure inside. Wind is nature’s attempt to balance such inequalities in air pressure. Because unequal heating of Earth’s surface

gradient, like a steep hill, causes greater acceleration of an air
parcel than does a weak pressure gradient (a gentle hill). Thus, the relationship between wind speed and the pressure gradient

generates these pressure differences, solar radiation is the ultimate energy sourcefor most wind. If Earth did not rotate, and if there were no friction between moving air and Earth’s surface, air would flow in a straight line from areas of higher pressure to areas of lower pressure. But because both factors exist, wind is controlled by a combination of

is straightforward: Closely spaced isobars indicate a steep pressure gradient and high winds, whereas widely spaced isobars indicate a weak pressure gradient and light winds. Figure 18.5
illustrates the relationship between the spacing of isobars and wind speed. Notice that wind speeds are greater in Ohio, Kentucky, Michigan, and Illinois, where isobars are more closely

forces, including (1) the pressure-gradient force, (2) the Coriolis effect, and (3) friction. We now examine each of these factors.

spaced, than in the western states, where isobars are more widely spaced. The pressure gradient is the driving force of wind, and it has both 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 on daily weather maps. Isobars are seldom straight, but usually form broad curves. Concentric rings of isobars indicate cells of high and low pressure. The “wind flags" indicate the expected airflow surrounding pressure cells and are plotted as "flying" with the wind (i.e., the wind blows toward the station circle). Notice that where the isobars are more closely spaced, the wind speed is faster. Closely spaced isobars indicate a strong pressure gradient and high wind speeds, whereas widely spaced isobars indicate a weak pressure gradient and low wind speeds. I 01°”
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CHAPTER 18 Air Pressure and Wind
target, Earth would have rotated 15 degrees to the east during its flight. To someone standing on Earth it would look as if the rocket veered off its path and hit Earth 15 degrees west of its target. The true path of the rocket is straight and would appear so to someone 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 ofthe Northern Hemisphere. In the Southern Hemisphere, the effect is reversed. Clockwise rotation produces a similar deflection, but to the left of the path of motion. The same deflection is experienced by wind regardless of the direction it is moving. We attribute the apparent shift in wind direction to the Coriolis effect, This deflection (1) is always directed at right angles to the direction of airflow; (2) affects only wind direction, not wind speed; (3) is affected by wind speed (the stronger the wind, the greater the deflection); and (4) is strongest at the poles and
weakens equatotwatd, becoming nonexistent at the equator,

the spacing of isobars. The direction of force is always from areas of higher pressure to areas of lower pressure and at right angles to the isobars. Once the air starts to move, the Coriolis effect and friction come into play, but then only to modify the movement, not to produce it.

- Conohs Effect The weather map in Figure 18.5 shows the typical air movements associated with high- and low-pressure systems. As expected, the air moves out ofthe regions ofhigher pressure and into the regions of lower pressure. However, the wind does not cross the isobars at right angles as the pressure-gradient force directs it. This deviation is the result of Earth’s rotation and has been named the Coriolis effect after the French scientist who first thoroughly described it. All free-moving objects or fluids, including the wind, are
deflected t0 the Tight Of their path Of motion in the Northern

Hemisphere and to the lejtin the Southern Hemisphere. The reason for thiS deflection can be illustrated by imagining the path Of

It is of interest to point out that any “free-moving” object
will experience a deflection caused by the Coriolis effect, This

a rocket launched from the North Pole toward a target located on
the equator If the rocket tOOl( an hour to reach itS

fact was dramatically discovered by the United States Navy in
World War II, During target practice long-range guns on battle-

The Coriolis effect illustrated using a one-hour flight of a rocket traveling from the North Pole to a location on the equator. A. On a nonrotatirig Earth, the rocket would travel straight to its target. B. However, Earth rotates 15° each hour. Thus, although the rocket travels in a straight line, when we plot the path of the rocket on Earth’s surface, it follows a curved path that veers to the right of the target.
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The effect of friction on wind is important only within a few kilometers ofEarth’s surface. We know that friction acts to slow the movement of air '.'-i;§;i~. ‘;.-1"). As a consequence, wind direction is also affected. To illustrate friction’s effect on wind direction, let us look at a situation in which it has no role. Above the friction layer, the pressure-gradient force and Coriolis effect work together to direct the flow of air. Under these conditions, the pressure-gradient force causes air to start moving across the isobars. As soon as the air starts to move, the Coriolis effect acts at right angles to this motion. The faster the wind speed, the greater the deflection. Eventually, the Coriolis effect will balance the pressure-gradient force, and the wind will blow parallel to generally the isobars Upper-air winds take this path and are called geostrophic winds. Because of the lack of friction with Earth’s surface, geostrophic winds travel at higher speeds than do surface winds. This can be observed in by noting the wind flags, many of which indicate winds of50-100 miles per horn‘.

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parallel to the isobars, whereas the effect of friction causes the surface winds to move more slowly and cross the isobars
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-- .1 The effect of friction is to slow the wind. In this image, a snow fence reduces wind speed, thereby diminishing the ability of the moving air to carry snow. As a result, snow
accumulates as a drift. (Photo by Garry Black/Superstock)

The most prominent features of upper-level flow are the jet streams. First encountered by high-flying bombers during World War ll, these fast-moving rivers of air travel between 120 and 240 kilometers ('75 and 150 miles) per hour in a west-to-east direction. One such stream is situated over the polar front, which is the zone separating cool polar air from warm subtropical air. Below 600 meters (2,000 feet), friction complicates the airflow just described. Recall that the Coriolis effect is proportional

CONCEPT cuscx 1 8.3
Q What force is responsible for generating wind? Q Write a generalization relating the spacing of isobars to the speed of wind. 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. Once air begins to accelerate, the Coriolis effect deflects it to the right in the Northern Hemisphere. Greater wind speeds result in a stronger Coriolis effect until the flow is parallel to the isobars. At this point the pressure-gradient force and Coriolis effect are in balance and the flow is called a geostrophic wind. In the “real” atmosphere, airflow is continually adjusting for variations in the pressure field. As a result, the adjustment to geostrophic equilibrium is much more irregular than shown.
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EV ':% 1§_ ii . Comparison between upper-level winds and surface winds showing the effects of friction on airflow. Friction slows surface wind speed, which weakens the Coriolis effect, causing the winds to cross the isobars and move toward the lower pressure.

Low F Pressure gradient force Wind Coriolis effect High _

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Students Sometimes Ask ..
Why doesn’t the Coriolis effect cause a baseball to be deflected when you are playing catch?
(330 feet) in 4 seconds down the right field line will be deflected 1.5 centimeters (more than 1/2 inch) to the right by the Coriolis effect is great enough to poten- effect. This could be just . tially affect the outcome of a enough to turn a potential home baseball game. Aballhitahori- run into a foul ball! zontal distance of 100 meters Over very short distances the Coriolis deflection is too small to be noticed. Nevertheless, in the middle latitudes the Coriolis

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Highs and Lows
Earth's Dynamic Atmosphere 9 Air Pressure and Winds
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Weather Generalizations about Highs and Lows
Rising air is associated with cloud formation and precipitation,

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

whereas subsidence produces clear skies. In this section you will learn how the movement of air can itself create pressure change
and hence generate winds. In addition, you will examine the relationship between horizontal and vertical flow and its effect on

the weather.
Let us first consider the situation around a surface low-pressure system where the air is spiraling inward. Here the net inward

transport of air causes a shrinking of the area occupied by the air mass, a process that is termed horizontal convergence. Whenever air converges horizontally, it must pile up, that is, increase in
height to allow for the decreased area it now occupies. This gen-

erates a taller and therefore heavier air column. Yet a surface low can exist only as long as the column of air above exerts less pres-

Cyclonic and Anticyclonic Winds
From the preceding section, you learned that the two most sig-

sure than that occurring in surrounding regions. We seem to have

nificant factors that affect wind are the pressure- gradient force
and the Coriolis effect. Winds move from higher pressure to

lower pressure and are deflected to the right or left by Earth’s rotation. When these controls of airflow are applied to pressure centers in the Northern Hemisphere, the result is that winds blow 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 the winds to the left, and therefore winds around a low blow clockwise (Figure l8.l2B), and winds around a high move counterclockwise. In either hemisphere, friction causes a net inflow (convergence) around a cyclone and a net outflow (divergence) around an anticyclone.

encountered a paradox—a low-pressure center causes a net accumulation of air, which increases its pressure. Consequently, a surface cyclone should quickly eradicate itself in a manner not unlike what happens when a vacuum-packed can is opened. You can see that for a surface low to exist for very long, compensation must occur aloft. For example, surface convergence could be maintained if divergence (spreading out) aloft occurred at a rate equal to the inflow below. shows the relationship 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 accelerated vertical motion. Thus, divergence aloft can intensify storm centers as well as maintain them. On the other hand, inadequate divergence aloft permits surface flow to “fill” and weaken the accompanying cyclone.

J1 1-} Cyclonic and anticyclonic winds in the Northern Hemisphere. Arrows show that winds blow into and counterclockwise 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 net upward movement. The rate of this vertical movement is slow, generally less than 1 kilometer per day. Nevertheless, because rising air often results in cloud formation and precipitation, a lowpressure center is generally related to unstable conditions and stormy weather As often as not, it is divergence aloft that creates a surface low. Spreading out aloft initiates upflow in the atmosphere directly

below, eventually working its way to the surface, where inflow is

encouraged. Like their cyclonic counterparts, anticyclones must be maintained from above. Outflow near the surface is accompanied by
convergence aloft and general subsidence of the air column

1‘-:@f;Ir1i converging surface winds and rising air causing cloudy conditions. A high, or anticyclone, has diverging surface winds and descending air, which lead to clear skies and fair weather.
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(Figure 18.13). Because descending air is compressed and warmed, cloud formation and precipitation are unlikely in an anticyclone. Thus, “fair” weather can usually be expected with the approach of a high-pressure center (Figure 18.14B). For reasons that should now be Airflow associated with surface cyclones and anticyclones. A low, or cyclone, has
obvious, it has been common prac-

tice to print on household barometers the words “stormy” at the low-pressure end and “fair” on the high-pressure end. By noting whether the pressure is rising, falling, or steady, we
have a good indication of what the forthcoming weather will be. Such a

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Park. (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 United States and require a few days to more than a week for the journey. Because their paths can be somewhat erratic, accurate prediction of their migration is difficult, although essential, for short-range forecasting. Meteorologists must also determine if the flow aloft will intensify an embryo storm or act to suppress its development. Because of the close tie between conditions at the surface and those aloft, a great deal of emphasis has been placed on the importance and understanding of the total atmospheric circulation, particularly in the mid-latitudes. We now examine the workings of Earth’s general atmospheric circulation, and then again consider the structure of the cyclone in light of this knowledge.

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

Circulation on a Nonrotating Earth
On a hypothetical nonrotating planet with a smooth surface of either all land or all water, two large thermally produced cells would form 5). The heated equatorial air would rise until it reached the tropopause, which, acting like a lid, would

CONCEPT CHECK 1 8 .4
Q 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 anticyclones in both the Northern and Southern hemispheres.

E.i‘ii§T;t;i it.'§ii_% Global circulation on a nonrotating Earth. A simple convection system is produced by unequal heating of the atmosphere.
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General Circulation of the Atmosphere
As noted, the underlying cause of wind is unequal heating of Earth’s surface (see Box 18.1). In tropical regions, more solar radiation is received than is radiated back to space. In polar regions the opposite is true: less solar energy is received than is lost. Attempting to balance these differences, the atmosphere acts as a giant heat-transfer system, moving warm air poleward and cool air equatorward. On a smaller scale, but for the same reason,

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Wind Energy——An Alternative with Potential
Air has mass, and when it moves (i.e. when the wind blows), it contains the energy of that motion—kinetic energy. A portion of that energy can be converted into other

forms-—mechanical force or electricity-—that we can use to perform work (Figure 18.A). Mechanical energy from wind is commonly used for pumping water in rural or remote places. The "farm windmill," still a familiar sight in many rural areas, is an example. Mechanical energy converted from wind can also be used for other purposes, such as sawing logs, grinding grain, and propelling sailboats. By contrast, wind-powered electric turbines generate electricity for homes, businesses, and for sale to utilities. Today, modem wind turbines are being installed at break-neck speed. In fact, worldwide, in 2010 the installed wind power capacity was expected to exceed 203,000 megawatts, an increase of 28 percent over

2009.* This is equivalent to the total electrical demand of Italy—2 percent of global electricity production. Worldwide, wind energy installations have doubled every 3 years since 2000. The United States is the world‘s leading producer (22.3%), followed by China (16.3%), Germany (16.2%), and Spain (11.5%). Within the next decade, China is expected to produce the most wind-generated electricity. Wind speed is a crucial element in determining whether a place is a suitable site for installing a wind-energy facility. Generally a

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

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Mehmet Dilsiz/Shutterstock) The wind turbines
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Students Sometimes Ask...
What is the highest wind speed ever recorded?
The highest wind speed recorded at a surface station is 3'72 kilometers (231 miles) per hour, measured April 12, 1934, at Mount Washington, New Hampshire. Located at an elevation of 1,879 meters (6,262 feet), the observatory atop Mount Washington has an average wind speed of 56 kilometers (35 miles) per hour. Faster wind speeds have undoubtedly occurred on mountain peaks, but no instruments were in place to record them.

deflect the air poleward. Eventually, this upper-level airflow would reach the poles, sink, spread out in all directions at the surface, and move back toward the equator. Once there, it would be reheated and start its journey over again. This hypothetical circulation system has upper-level air flowing poleward and surface air flowing equatorward. Ifwe add the effect of rotation, this simple convection system will break down into smaller cells. Fil{_§tlR'@E*? i.s.'ii-*5 illustrates the three pairs of cells proposed to carry on the task of heat redistribution on a rotating planet. The polar and tropical cells retain the characteristics of the thermally generated convection described earlier. The nature of the mid-latitude circulation is more complex and is discussed in more detail in a later section.

General Circulation of the Atmosphere

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minimum average wind speed of 21 kilometers (13 miles) per hour is necessary for a large-scale wind-power plant to be profitable. A small difference in wind speed results in a large difference in energy production, and therefore a large difference in the cost of the electricity generated. For example, a turbine operating on a site with an average wind speed of 12-mph would generate about 33 percent more electricity than one operating at 11-mph. Also, there is little energy to be harvested at low wind speeds—6-mph winds contain less than one eighth the energy of 12-mph winds. The United States has tremendous wind energy resources (Figure 18.3). In 2010, 36

states had commercial facilities that produced electricity from wind power. The leading states in installed capacity were Texas, number one, followed by Iowa, California, and Minnesota. Despite the fact that California gave birth to the modem U.S. wind industry, 16 states have greater wind potential. As shown in Table ’l8.A, the top five states for wind energy potential include North Dakota, Texas, Kansas, South Dakota, and Montana. Although only a small fraction of U.S. electrical generation currently comes from wind energy, it has been estimated that wind energy potential equals more than twice the total electricity currently consumed.

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

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

Idealized Global Circulation
Near the equator, the rising air is associated with the pressure zone known as the equatorial low—a region marked by abundant precipitation. As the upper-level flow from the equatorial low reaches 20-30 degrees latitude, north or south, it sinks back toward the surface. This subsidence and associated adiabatic heating produce hot, arid conditions. The center of this zone of subsiding dry air is the subtropical high, which encircles the globe near 30 degrees latitude, north and south (Figure 18.16). The great deserts of Australia, Arabia, and North Africa exist because of the stable dry conditions associated with the subtropical highs.

At the surface, airflow is outward from the center of the subtropical high. Some of the air travels equatorward and is deflected by the Coriolis effect, producing the reliable trade winds. The remainder travels poleward and is also deflected, generating the prevailing westerlies of the mid-latitudes. As the westerlies move poleward, they encounter the cool polar easterlies in the region of the subpolar low. The interaction of these warm and cool winds produces the stormy belt known as the polar front. The source 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 by four pressure zones. The subtropical and polar highs are areas of dry subsiding air that flows outward at the surface, producing the

538

CHAPTER 18 Air Pressure and Wind Subpolar Polar high circulation over the oceans is dominated by semipermanent cells of high pressure in the subtrop-

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the land (Figure l8.17B). These seasonal changes in wind direction are known as the monsoons. During warm months, areas such as India experience a flow of warm, water-laden air from the Indian Ocean, which produces the rainy summer monsoon. The winter monsoon is dominated by dry continental air. A similar situation exists, but
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Idealized global circulation proposed for the three-cell circulation model of a rotating Earth.

The Westerlies
Circulation in the mid-latitudes, the zone of the westerlies, is complex and does not fit the convection system proposed for the tropics. Between about 30 and 60 degrees latitude, the general west-to-east flow is interrupted by migrating cyclones and anticyclones. In the Northern Hemisphere these cells move from west to east around the globe, creating an anticyclonic (clockwise) flow or a cyclonic (counterclockwise) flow in their area of influence. A close correlation exists between the paths taken by these surface pressure systems and the position of the upper-level airflow, indicating that the upper air strongly influences the movement of cyclonic and anticyclonic systems. Among the most obvious features of the flow aloft are the seasonal changes. The steep temperature gradient across the middle latitudes in the winter months corresponds to a stronger flow aloft. In addition, the polar jet stream fluctuates seasonally such that its average position migrates southward with the approach of winter and northward as summer nears. By midwlnter, the jet core may penetrate as far south as central Florida. Because the paths of low-pressure centers are guided by the flow aloft, we can expect the southern tier of states to experience more of their stormy weather in the winter season. During the hot summer months, the storm track is across the northern states, and some cyclones never leave Canada. The northerly storm track associated with summer applies also to Pacific storms, which move toward Alaska during the warm months, thus producing an extended dry season for much of the West Coast. The number of cyclones generated is seasonal as well, with the largest number occurring in the cooler months when the temperature gradients are greatest. This fact is in agreement with the role of cyclonic storms in the distribution of heat across the mid-latitudes.

prevailing winds. The low-pressure zones of the equatorial and subpolar regions are associated with inward and upward airflow

accompanied by clouds and precipitation.

Influence of Continents
Up to this point, we have described the surface pressure and associated winds as continuous belts around Earth. However, the only truly continuous pressure belt is the subpolar low in the Southern Hemisphere. Here the ocean is uninterrupted by landmasses. At other latitudes, particularly in the Northern Hemisphere where landmasses break up the ocean surface, large seasonal temperature 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 monsoons typically have both a wet and a dry season. Monsoon refers to a wind system that exhibits a pronounced seasonal reversal in direction. In general, winter is associated with winds that blow predominantly off the continents and produce a dry winter monsoon. By contrast, in summer, warm, moisture-laden air blows from the sea toward the land. Thus, the summer monsoon, which is usually associated with abundant precipitation, is the source of the misconception.

Local Winds

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CONCEPT cnrzcx 1 8. 5
0 In which belt of prevailing 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 Winds
Having examined Earth’s large-scale circulation, let us turn briefly to winds that influence much smaller areas. Remember that all winds are produced for the same reason: pressure differences that arise because of temperature differences that are caused by unequal heating of Earth’s surface. Local winds are simply smallscale winds produced by a locally generated pressure gradient. Those described here are caused either by topographic effects or variations in surface composition in the immediate area.

540

CHAPTER 18 Air Pressure and Wind result is that the coldest pockets of air are usually found in the

Land and Sea Breezes
In coastal areas during the warm summer months, the land sur-

face is heated more intensely during the daylight hours than is the adjacent body of water (see the section “Land and Water” in Chapter 16). As a result, the air above the land surface heats,
expands, and rises, creating an area of lower pressure. A sea breeze then develops, because cooler air over the water (higher

lowest spots. Like many other winds, mountain and valley breezes have seasonal preferences. Although valley breezes are most common 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 Winds
Warm, dry winds are common on the eastern slopes of the Rockies, where they are called chinooks. Such winds are created when air descends the leeward (sheltered) side of a mountain and warms by compression. Because condensation may have occurred as the air ascended the windward side, releasing latent heat, the air descending the leeward slope will be warmer and drier than it was at a similar elevation on the windward side. Although the temperature of these winds is generally less than 10° C (50° F), which is not particularly warm, they occur mostly in the winter and spring when the affected areas may be experiencing below-freezing temperatures. Thus, by comparison, these dry, warm winds often bring a drastic change. When the ground has a snow cover, these winds are known to melt it in short order. A chinooklike wind that occurs in southern California is the Santa Ana. These hot, desiccating winds greatly increase the threat of fire in this already dry area (Figure rs.;§:0).

pressure) moves toward the warmer land (lower pressure) (F.lgr.u;‘e fttilsrr). The sea breeze begins to develop shortly before
noon and generally reaches its greatest intensity during the midto late afternoon. These relatively cool winds can be a significant moderating influence on afternoon temperatures in coastal areas.

At night, the reverse may take place. The land cools more
rapidly than the sea, and the land breeze develops (Figure 18.18B).

Small-scale sea breezes can also develop along the shores oflarge
lakes. People who live in a city near the Great Lakes, such as Chicago, recognize this lake effect, especially in the summer. They

are reminded daily by weather reports of the cool temperatures near the lake as compared to warmer outlying areas.

Mountain and Valley Breezes
A daily wind similar to land and sea breezes occurs in many mountainous regions. During daylight hours, the air along the slopes of the mountains is heated more intensely than the air at the same elevation over the valley floor. Because this warmer air is less dense, it glides up along the slope and generates a valley breeze (Figure 1is.19A). The occurrence of these daytime upslope breezes can often be identified by the cumulus clouds that develop on adjacent mountain peaks. After sunset, the pattern may reverse. Rapid radiation cooling along the mountain slopes produces a layer of cooler air next to the ground. Because cool air is denser than warm air, it drains downslope into the valley. Such a movement of air is called a mountain breeze (Figure 18.19B). The same type of cool air drainage can occur in places that have very modest slopes. The

Country Breeze
One type of local wind, called a country breeze, is associated with large urban areas. As the name implies, this circulation pattern is characterized by a light wind blowing into the city from the surrounding countryside. The country breeze is best developed on relatively clear, calm nights. Under these conditions, cities, because they contain massive buildings and surfaces composed of rocklike materials, tend to retain the heat accumulated during the day more than the less built-up outlying areas. The result is that the warm, less dense air over the city rises, which in turn initiates 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 and expands, creating an area of lower pressure. Cooler and denser air over the water moves onto the land, generating a sea breeze. B. At night the land cools more rapidly than the sea, generating an offshore flow called a land breeze.
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Q What is a local wind? Q Describe the formation of a 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 welldeveloped valley breeze—midnight, late morning, or late afternoon‘?

A. Valley breeze

Measuring Wind

Two basic wind measurements— direction and speed-—are important to the weather observer. One simple device for determining both measurements is the simple wind sock that is a common sight at small airports and landing strips (Figure i8.21A). The cone-shaped bag is open at both ends and is free to change position with shifts in wind direction. The degree to which the sock is inflated is an indicaB. Mountain breeze tion ofwind speed. Winds are always labeled by the FIGURE 18.19 Valley and mountain breezes. A. Heating during the daylight hours warms the air along the mountain slopes. This warm air rises, generating a valley breeze. In the photo, the direction from which they blow. A occurrence of a daytime valley breeze is identified by cloud development on mountain peaks, north wind blows from the north sometimes leading to an afternoon rain shower. (Photo by James E. Patterson/James Patterson Collection) toward the south, an east windfiom B. After sunset, cooling of the air near the mountain can result in cool air drainage into the valley, the east toward the west. The instruproducing the mountain breeze. ment most commonly used to determine wind direction is the wind vane (Figure 18.21B, upper right). This instrument, a common sight on One investigation in Toronto showed that heat accumulated many buildings, always points into the Wind. Often the wind within this city created a rural-city pressure difference that was sufficient to cause an inward and counterclockwise circulation centered on the downtown area. One of the unfortunate conseFIGURE 18.20 This satellite image shows strong Santa Ana winds quences of the country breeze is that pollutants emitted near the fanning the flames of several large wildfires in southern California on October 27, 2003. These fires scorched more than 740,000 acres and urban perimeter tend to drift in and concentrate near the city's destroyed more than 3,000 homes. (NASA) center.

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

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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 at small airports and landing strips. (Photo by Lourens Smak/Alamy Images) B. Wind vane (right) and cup anemometer (left). The wind vane shows wind direction and the anemometer measures wind speed. (Photo by Belfort Instrument Company)

changes in temperature and moisture conditions, the ability to direction is shown on a dial that is connected to the wind vane. predict the winds can be very useful. In the Midwest, for example, The dial indicates wind direction, either by points ofthe compass a north wind may bring cool, dry air from Canada, whereas a (N, NE, E, SE, etc.) or by a scale of 0° to 360°. On the latter scale, south wind may bring warm, humid air from the Gulf of Mexico. 0° or 360° are both north, 90° is east, 180° is south, and 270° is west. Recall that about 70 percent ofEarth’s surface is covered by the When the wind consistently blows more often from one direcocean, where conventional methods of gathering wind data are tion than from any other, it is called a prevailing wind. You may seldom possible. Ocean buoys and ships at sea provide very limbe familiar with the prevailing westerlies that dominate the cirited coverage. However, since the 1990s, weather forecasts have culation in the mid-latitudes. In the United States, for example, improved significantly due to the availability of satellite-derived these winds consistently move the “weather” from west to east across the continent. Embedded within this general eastward flow are cells ofhigh and low pressure with the characteristic clockwise and FIGURE 18.22 Wind roses showing the percentage of time airflow is coming from counterclockwise flow. As a result, the winds various directions. A. Wind frequency for the winter in the eastern United States. B. Wind associated with the westerlies, as measured at the frequency for the winter in northern Australia. Note the reliability of the southeast trades in Australia as compared to the westerlies in the eastern United States. (Data from surface, often vary considerably from day to day G. T. Trewartha) and from place to place. By contrast, the direc- ' ‘. - 1', IL-I .'_-r - I. I '“‘ I ‘? ¢_.:_:_-.;_1 ‘\___1‘-‘-t¢»__£,'€--_:_j- ~ .- .i;J_ y ‘:1 -. ,,_ _ \_ I tion of airflow associated with the belt of trade I;'>-.-; ' *‘-1"’-"7-""—“:’ -Is . 7?. ‘ : ' i ' § *t ‘ I : ' ; 7 ‘ * =\ } ~: ‘ < "1‘ E" It M * -. I; I~ winds is much more consistent, as can be seen
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Wind speed is commonly measured using a cup anemometer (anemo = wind, metron = measuring instrument) (Figure 18.21B, upper left). The wind speed is read from a dial much like the speedometer of an automobile. Places where winds are steady and speeds are relatively high are potential sites for tapping wind energy. By knowing the locations of cyclones and anticyclones in relation to where you are, you can predict the changes in wind direction that will be experienced as a pressure center moves past. Because changes in wind direction often bring 542

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El Nifio and La Niiia

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

El Niiio and La Niiia
As can be seen in so the cold Peruvian current flows equatorward along the coast of Ecuador and Peru. This flow encourages upwelling of cold nutrient-filled waters that serve as the primary food source for millions offish, particularly anchovies.
Near the end of each year, however, a warm current that flows

CONCEPT cnrzcx 18.7
0 Vllhat are the two basic wind measurements? “That instruments are used to make these measurements? Q From what direction does a northeast wind blow? Toward what direction does a south wind blow?

southward along the coasts of Ecuador and Peru replaces the cold Peruvian current. During the 19th century the local residents named this warm countercurrent El Nifio (“the child“) after the
Christ 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 Peruvian current causes upwelling of cold water along the west coast of South America. B. When the Southern Oscillation occurs, the pressure over the eastern and western Pacific flip-flops. This causes the trade winds to diminish, leading to an eastward movement of warm water along the equator. As a result, the surface waters of the central and eastern Pacific warm, with far-reaching consequences to weather patterns.
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CHAPTER 18 Air Pressure and Wind

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Sally Benson: Climate and Energy Scientist
Sally Benson has spent her career researching solutions to the most pressing environmental problems of our time. In the mid-1970s, as a young scientist with Lawrence Berkeley National Laboratory, she tackled the first oil shortage by investigating ways to harness the power of geothermal energy.

was the most significant issue facing the world." Benson organized research programs that developed regional models of climate change to help residents plan for droughts and temperature shifts, and studied the carbon cycle of the oceans, among other projects. In 2007, Benson was appointed director of the Global Climate and Energy Project at Stanford University, which seeks to develop energy sources that release fewer greenhouse gases. “Energy efficiency in lighting, heating, and cooling systems, and autos, makes all the sense in the world. But at the end of the day, we need to do a lot more than that. If our current understanding is correct, we need to out overall emissions by 80 percent of today’s levels," Benson states. Benson sees many promising ways to achieve that goal. One is renewable energy.

good seal, inject the gas, and monitor the area for leaks. “I get to do important work solving critical problems, get to be outdoors, do experiments at scale, and have fun while I do it. Interacting with the huge number of people impacted by these issues makes the Earth sciences very rewarding." —Kathleen Wong

“I get to do important work . . . and have fun while I do it."
Ten years later, she was elbow-deep in the mud of Kesterson Reservoir in California's Central Valley. Irrigation runoff had caused selenium from local soil to accumulate in the water, causing local birds to hatch chicks with horrifying deformities. Benson's studies made her realize microbes could be environmentally friendly tools to clean up other sites with toxic metal contamination. “ The experience was so positive because I was doing this cutting-edge science at the same time regulators had to make a decision about how to clean up the site. I got an idea of the impact that science can make, and how research could get results," Benson says. By the mid-1990’s, Benson was directing all Earth science research at the laboratory. “I talked with many people, read a lot, and came to the conclusion that climate change “ Today's biofuels don’t provide much advantage in terms of carbon dioxide emissions. But alternatives such as cellulosic ethanol (producing ethanol with plant fibers), where there are low emissions in the process of growing and making them, are incredibly important, " Benson opines. Another is to continue using some fossil fuels to run power plants and ships, but to capture the greenhouse gases they emit. “ More than half of the electricity worldwide is produced by burning coal, a plentiful resource in many places. We need to find a way to make it carbon neutral, " Benson says. One way to remove such emissions from the atmosphere altogether would be to inject them in aquifers deep within Earth. Benson herself has studied how to use technology developed by the oil and gas industry to select sites where the rock offers a

Professor Sally Benson is Executive Director of Stanford University's Global Climate and Energy Project, a $225 million effort to develop energy resources that are not harmful to the environment, especially energy sources that release fewer greenhouse gases Dr. Benson has a multidisciplinary background with degrees in geology, materials science, and minerals engineering, and with applied research in hydrology and reservoir
engineering. (Courtesy Dr. Sally Benson)

season. Normally, these warm countercurrents last for at most a few weeks when they again give way to the cold Peruvian flow. However, at irregular intervals of 3 to 7 years, these countercurrents become unusually strong and replace normally cold offshore waters with warm equatorial waters (Figure 18.23B). Today, scientists use the term El Nifio for these episodes of ocean warming that affect the eastern tropical Pacific. The onset of El Niiio is marked by abnormal weather patterns that drastically affect the economies of Ecuador and Peru. As shown in Figure 18.238, these unusually strong undercurrents amass large quantities ofwarm water that block the upwelling of colder, nutrient-filled water. As a result, the anchovies starve, devastating the fishing industry. At the same time, some inland areas that are normally arid receive an abnormal amount of rain. Here,

pastures and cotton fields have yields far above the average. These climatic fluctuations have been known for years, but they were
originally considered local phenomena. Today, we know that El

Nifio is part of the global circulation and affects the weather at
great distances from Peru and Ecuador. Two of the strongest El Nifio events on record occurred

between 1982-83 and 1997-98 and were responsible for weather extremes of a variety of types in many parts of the world. The 1997-98 El Nifio brought ferocious storms that struck the California coast, causing unprecedented beach erosion, landslides, and floods. In the southern United States, heavy rains also brought floods to Texas and the Gulf states. The same energized
jet stream that produced storms in the South, upon reaching the Atlantic, sheared off the northern portions of hurricanes, destroy-

g

El Nino and La Nina

545

ing the storms. It was one of the quietest Atlantic hurricane seasons in years.
Major El Nino events, such as the one in 1997-98, are inti-

in the Indonesian region, causing the pressure gradient along the equator to weaken or even to reverse. As a consequence, the oncesteady trade winds diminish and may even change direction. This

mately related to the large-scale atmospheric circulation. Each time an El Nino occurs, the barometric pressure drops over large portions of the southeastern Pacific, whereas in the western
Pacific, near Indonesia and northern Australia, the pressure rises

(-*' A 1 1 1). Then, as a major El Nino event comes to an end, the pressure difference between these two regions swings back in the
opposite direction. This seesaw pattern of atmospheric pressure

reversal creates a major change in the equatorial current system, with warm water flowing eastward (Figure 18.2/11B). \/Vlllfl time, water temperatures in the central and eastern Pacific increase and sea level in the region rises. This eastward shift of the warmest surface water marks the onset of El Nino and sets up changes in atmospheric circulation that affect areas far outside the tropical Pacific. When an El Nino began in the summer of 1997, forecasters predicted that the pool of warm water over the Pacific would displace the paths of both the subtropical and mid-latitude jet streams, which steer weather systems across North America (see Figure I 8.23). As predicted, the subtropical jet brought rain to the Gulf Coast, where Tampa, Florida, received more than three times

between the eastern and western Pacific is called the Southern
Oscillation. It is an inseparable part of the El Nino warmings that occur in the central and eastern Pacific every 3 to 7 years. Therefore, this phenomenon is often termed El Nino/Southern Oscillation, or ENSO for short.

Winds in the lower atmosphere are the link between the pressure change associated with the Southern Oscillation and the
extensive ocean warming associated with El Nino. During a typical year, the trade winds converge near the equator and flow westward toward Indonesia (Figure 18.24A). This steady westward flow creates a warm surface current that moves from east to west along the equator. The result is a “piling up" of a thick layer of warm surface water that produces higher sea levels (by 30 centimeters) in the western Pacific. Meanwhile, the eastern Pacific is

its normal winter precipitation. Furthermore, the mid-latitude jet
pumped warm air far north into the continent. As a result, winter temperatures west of the Ro ckies were significantly above normal. The effects of El Nino are somewhat variable depending in part on the temperatures and size of the warm pools. Nevertheless,

some locales appear to be affected more consistently. In particular, during most El Ninos, warmer-than-normal winters occur in the northern United States and Canada. In addition, normally arid

characterized by a strong Peruvian current, upwelling of cold
water, and lower sea levels.

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

portions of Peru and Ecuador, as well as the eastern United States, experience wet conditions. By contrast, drought conditions are generally observed in Indonesia, Australia, and the Philippines. One major benefit of the circulation associated with El Nino is a suppression of the number of Atlantic hurricanes.
The opposite of El Nino is an atmospheric phenomenon known as La Nina. Once thought

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

to be the normal conditions that occur between two El Nino events, meteorologists now consider La Nina an important atmospheric phenomenon in its own right. Researchers have

come to recognize that when surface temperatures in the eastern Pacific are colder than average, a La Nina event is triggered that has a distinctive set of weather patterns. A typical La
Nina winter blows colder than normal air over

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. ... . ... 0 How 1s La Nina different from ElN1no? Q Describe the relationship between the Southern Oscillation and a major El Nino event.

CONCEPT cnscx 1 8.8

546

CHAPTER 18 Air Pressure and Wind
warm air, we would expect a latitudinal variation in precipitation, with low latitudes receiving the greatest amounts of precipitation and high latitudes receiving the smallest amounts. Figure 18.25 indeed reveals heavy rainfall in equatorial regions and meager precipitation in high-latitude areas. Recall that the dry region in the warm subtropics is explained by the presence of the subtropical high. In addition to latitudinal variations in precipitation, the distribution of land and water complicates the precipitation pattern. Large landmasses in the middle latitudes commonly experience decreased precipitation toward their interiors. For example, central North America and central Eurasia receive considerably less precipitation than do coastal regions at the same latitude. Furthermore, the effects of mountain barriers alter the idealized precipitation patterns we would expect solely from global wind and pressure systems. Windward mountain slopes receive abundant rainfall resulting from orographic lifting, whereas leeward slopes and adjacent lowlands are usually deficient in moisture.

Global Distribution of Precipitation
A casual glance at shows a relatively complex pattern for the distribution of precipitation. Although the map appears to be complicated, the general features of the map can be explained by applying our knowledge of global winds and pressure systems. In general, regions influenced by high pressure, with its associated subsidence and diverging winds, experience relatively dry conditions. On the other hand, regions under the influence oflow pressure and its converging winds and ascending air receive ample precipitation. This pattern is illustrated by noting that the tropical region dominated by the equatorial low is the rainiest region on Earth (£1 It includes the rain forests of the Amazon basin in South America and the Congo basin in Africa. Here the warm, humid trade winds converge to yield abundant rainfall throughout the year. By way of contrast, areas dominated by the subtropical high-pressure cells clearly receive much smaller amounts of precipitation. These are regions of extensive deserts. In the Northern Hemisphere the largest is the Sahara. Examples
in the Southern Hemisphere include the Kalahari in southern

CONCEPT cnacx 1 8.9
Q With which global pressure belt are the rain forests of Africa’s Congo Basin associated? Which pressure system is linked to the Sahara Desert? Q Other than Earth’s pressure and wind belts, list two other factors that exert a significant influence on the global distribution of precipitation.

Africa and the dry lands of Australia. If Earth’s pressure and wind belts were the only factors controlling precipitation distribution, the pattern shown in
Figure 18.25 would be simpler. The inherent nature of the air is

also an important factor in determining precipitation potential. Because cold air has a low capacity for moisture compared with

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

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states 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 the following questions. “I thought air pressure had something to do with the weight of air. How does inches relate to weight? And, why do you think the

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weather is going to improve?” How would you respond to your friend"s queries? If you live in the Northern Hemisphere and are directly west of the center of a
cyclone, what is the probable wind direction? What if you were west of an anti-

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cyclone? (Photo from NASA) It is late afternoon on a warm summer day and you are enjoying some time at the beach. Until the last hour or two, winds were calm. Then a breeze began to develop. Is it more likely a cool breeze from the water or a warm breeze from the adjacent land area? Explain.

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548

CHAPTER 18

Air Pressure and Wind

10. When designing an airport it is important to have the planes take off into the wind. Refer to the accompanying wind rose and discuss the orientation of the runway and the direction planes would travel when they took off. Bonus: Where on Earth would you find a wind rose like this? 1 1. The accompanying maps of Africa show the distribution of precipitation for Iuly and Ianuary. Which map represents Iuly and which represents Ianuary? How were you able to figure this out?
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In Review Chapter 18 Air Pressure and Wind
0 Air has weight. At sea level it exerts a pressure of I kilogram per square centimeter (14.7 pounds per square inch). Air pressure is the force exerted by the weight of air above. With increasing altitude there is less air above to exert a force, and thus air pressure decreases with altitude, rapidly at first, then much more slowly. The unit used by meteorologists to measure atmospheric pressure is the millibar. Standard sea-level pressure is expressed as 1013.2 millibars. Isobars are lines on a weather map that connect places of equal air pressure. A mercury barometer measures air pressure using a column of mercury in a glass tube that is sealed at one end and inverted in a dish of mercury. As air pressure increases, the mercury in the tube rises; conversely, when air pressure decreases, so does the height of the column of mercury. A mercury barometer measures atmospheric pressure in inches of mercury, the height of the column of mercury in the barometer. Standard atmospheric pressure at sea level equals 29.92 inches of mercury. Aneroid (without liquid) barometers consist of partially evacuated metal chambers that compress as air pressure increases and expand as pressure deal-eaSeS_ than surface winds because friction is greatly reduced aloft. Friction slows surface winds, which in turn reduces the Coriolis effect. The result is air movement at an angle across the isobars 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 (highpressure centers). In the Northern Hemisphere, winds around a low (cyclone) are counterclockwise and inward. Around a high (anticyclone), winds are clockwise and outward. In the Southern Hemisphere, the Coriolis effect causes winds to move clockwise around a low and counterclockwise around a high. Because air rises and cools adiabatically in low-pressure centers, cloudy conditions and precipitation are often associated with their passage. In high-pressure centers, descending air is compressed and warmed; therefore, cloud formation and precipitation are unlikely, and “fair” weather is usually expected. Earth’s global pressure zones include the equatorial low, subtropical high, subpolar low, and polar high. The global surface winds associated with these pressure zones are the trade winds, westerlies, and polar easterlies. Particularly in the Northern Hemisphere, large seasonal temperature differences over continents disrupt the idealized, or zonal, global patterns of pressure and wind. In winter, large, cold landmasses develop a seasonal high-pressure system from which surface airflow is directed off the land. In summer, landmasses are heated and low pressure develops over them, which permits air to flow onto the land. The seasonal changes 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

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Wind is the horizontal flow of air from areas of higher pressure toward areas of lower pressure. Winds are controlled by the following combination of forces: (1) the pressure-gradient force (amount of pressure change over a given distance); (2) the Coriolis eflect (deflective effect of Earth’s rotation to the right in the Northern Hemisphere and to the left in the Southern Hemisphere); and (3)fi'iction with Earth’s surface (slows the movement of air and alters wind direction). 0 Upp er-air winds, called geostrophic winds, blow parallel to the isobars and reflect a balance between the pressure-gradient force and the Coriolis effect. Upper- air winds are faster

0

g by migrating cyclones and anticyclones. The paths taken by these pressure systems are closely related to upper-level airflow and the polar jet stream. The average position of the .

Mastering Geology

549

The two basic wind measurements are direction and speed. Winds are always labeled by the direction from which they blow. Wind direction is measured with a wind vane, and wind

polar jet stream, and hence the paths followed by cyclones,
migrates equatorward with the approach of winter and poleward as summer nears. Local winds are small-scale winds produced by a locally generated pressure gradient. Local winds include sea and land breezes (formed along a coast because of daily pressure differences caused by the differential heating of land and water); valley and mountain. breezes (daily wind similar to sea and land breezes except in a mountainous area where the air along slopes heats differently from the air at the same elevation over the valley floor); and chinoolc and Santa Ana winds (warm, dry winds created when air descends the leeward side of a mountain and warms by compression). .

speed is measured using a cup anemometer:
El Nino is the name given to the periodic warming of the ocean that occurs in the central and eastern Pacific. It is associated with periods when a weakened pressure gradient causes the trade winds to diminish. A major El Nifio event triggers extreme weather in many parts of the world. Vvhen surface temperatures in the eastern Pacific are colder than average, a La Nina event is triggered. A typical La Nifia winter blows colder-than-normal air over the Pacific Northwest and the northern Great Plains while warming much of the rest of the United States. The global distribution of precipitation is strongly influenced by the global pattern of air pressure and wind, latitude, and distribution of land and water.

0

,

Key Terms
air 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) 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) mercury barometer (p. 527) monsoon (p. 538) mountain breeze (p. 540) polar easterlies (p. 537) polar front (p. 537) polar high (p. 537) 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)

ElNif1o (p. 544)
equatorial low (p. 537)

Examining the Earth System
l. Examine the image of Africa in Figure 1.9B (p. 13) and pick out the region dominated by the equatorial low and the areas influenced by the subtropical highs in each hemisphere. What clue(s) did you use? Speculate on the differences in the biosphere between the regions dominated by high pressure and 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, p. 428.) What is the ultimate source of energy that drives both of these circulations? 3. Winds and ocean currents change in the tropical Pacific during an El Nino event. How might this impact the biosphere and geosphere in Peru and Ecuador? How about in Indonesia? (One useful Web site that deals with El Nino is NOAA’s El

Nino Theme Page at http://www.pmel.noaa.gov/tao/e1nino/
nino-home.html).

Mastering Geology
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quizzes, study tools, and multimedia that will aid in your understanding of this chapter’s content. In MasteringGeology“' you will find: O GEODe: Earth Science: An interactive visual wallcthrough of key concepts

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