Ab

Figure 5.1. Maps of (a) an anticyclone and (b) a cyclone, both in the Northern Hemisphere near the earth's surface. The closed loops are isobars. The winds (arrows) blow outward and clockwise from the center of the anticyclone; they blow inward and counterclockwise toward the center of a cyclone. Compare figure 4.7.

total, only about 2 W m-2 is consumed in driving the winds, with more than half of it energizing the jet streams.4 Low-level winds, however, are nearly as important as those aloft in transferring solar heat from the tropics to colder climes, and we now consider how friction with the surface (drag) affects the global wind pattern close to the surface.

Figures 4.5 and 4.7 showed how, at elevations above the friction layer, the winds blow parallel to the isobars. Their direction, at that level, is wholly determined by the pressure gradient and the Coriolis effect; in a word, upper-level winds are geostrophic. At lower elevations, where frictional drag is appreciable, the wind is governed by three factors: the pressure gradient, the Coriolis effect, and drag. Drag reduces the Coriolis effect, with the result that the wind is deflected through an angle of less than 90° from the direction of the pressure gradient.5 The amount by which the Coriolis deflection is diminished depends on the nature of the surface over which the wind blows.6 Over a calm sea, for example, the deflection is likely to be between 75° and 80°; over rough, hilly ground, between 50° and 55°.

The way this diminution of the Coriolis effect alters the winds blowing around anticyclones and cyclones (highs and lows, respectively) is shown in figure 5.1; compare it with figure 4.7. As figure 5.1 shows, the winds around an anticyclone blow outward down the pressure gradient as well as clockwise;

Figure 5.2. Idealized pattern of global wind circulation near the earth's surface, assuming there are no localized cyclones and anticyclones. Compare figure 4.5.

Westerlies

Westerlies

Southeast trades

Northeast trades

Polar easterlies

Polar easterlies

Figure 5.2. Idealized pattern of global wind circulation near the earth's surface, assuming there are no localized cyclones and anticyclones. Compare figure 4.5.

conversely, the winds around a cyclone blow inward down the pressure gradient as well as counterclockwise; that is, they are not parallel with the isobars but are deflected to the right in both cases.

Now consider the effect on the global average wind pattern at the surface as it would be in the absence of the temporary, localized hot spots and cold spots that cause anticyclones and cyclones to form; this idealized pattern is shown in figure 5.2. Compare it with figure 4.5 and note how, in place of high-elevation westerlies and easterlies blowing along the parallels of latitude, the winds at the surface tend to blow across the latitudes. In this way air, and its thermal energy, is shifted from one latitude belt to another. The process is called advection, defined as the horizontal displacement of air and all its attributes. Note, too, that figure 5.2 shows zones of polar easterlies in both hemispheres that were not present in figure 4.5. The polar easterlies are caused by high pressures at a low elevation over the poles, where the air near the surface is cold and dense. The names "westerlies" and "easterlies" for the zones so labeled are inexact but traditional.

Figure 5.2 reveals a seeming paradox. The argument that cross-latitude winds transfer heat from warm regions to cold doesn't apply to the trade winds, which blow toward the equator in both hemispheres: they go the wrong way to equalize worldwide temperatures. In fact there is no paradox. Above every prevailing wind at the surface is a "return wind" higher up— there has to be, to prevent an impossible pileup wherever surface winds converge—and these high-level winds, especially the meandering jet streams, play a large part in transferring atmospheric heat poleward.

To say that low-level winds "return" at high elevations or, which comes to the same thing, that high-level winds "return" at low elevations (see fig. 4.3) amounts to saying that the air circulates vertically. Indeed it does, carrying its thermal energy with it.

Vertical Movements of the Air

Look at figure 5.1 again. The winds blowing in toward the center of a cyclone converge; then, because the air must go somewhere, it rises. Likewise, the surface winds blowing outward from an anticyclone diverge, and air descends to fill the void. These upward and downward movements—up in a cyclone and down in an anticyclone—are too slow to be called winds. Vertically moving air usually moves at only one or two centimeters a second.7

To repeat: the air drifts downward into a surface anticyclone. This may seem like a paradox to anybody familiar with thermals, the warm air currents that rise from ground on a hot, sunny day when the barometer reads "high" and an anticyclone obviously prevails. The presence of thermals is revealed by the small, puffy cumulus clouds that often cap them, and also by soaring hawks and eagles using them for lift. Can their presence be squared with the general downward movement of the air in an anticyclone? It can: the thermals are scattered; their arrangement is like "the holes in the top of a pepper pot."8 The rising thermals flow up as separate small airstreams through a mass of slowly descending air.

These various up and down movements show that the different layers of the atmosphere do not act independently; on the contrary, air and its thermal energy are continually exchanged between one layer and another, as well as shifting great distances horizontally.

Water Vapor and Energy Transfers

A northerner feeling the warmth of a south wind is experiencing sensible heat. This surprising term simply means perceptible heat. The adjective is to differentiate it from latent heat, which is imperceptible. Two air masses can have the same temperature and feel the same, but if the concentration of water vapor in them is not the same, the moister air contains more latent heat per unit volume than does the drier air.

Latent heat is the heat liberated when water vapor condenses to liquid water or when liquid water turns to ice. Consider what happens when moist air cools and the water vapor in it condenses. At the outset, the water molecules are thoroughly mixed with all the other molecules of the air (mostly nitrogen and oxygen) and share in their constant motion. As the temperature drops, all the molecules slow down. Once the water molecules have slowed to a critical speed, they cling to any tiny particle they chance to bump into, which may be a dust mote, a smoke particle, a floating bacterium, a fragment of fly ash or the like, or any one of these that has picked up a few water molecules already; this is the way cloud droplets and fog droplets come into existence.9 In uniting to form liquid water, the molecules slow down, abruptly losing energy. The energy cannot vanish: what the water loses is passed on to the air molecules, whose motion speeds up. That is, the temperature of the air rises: the heat that was latent in the water vapor manifests itself as sensible heat— a rise in temperature.

Similarly, heat is released when cloud droplets freeze or when water vapor condenses directly into ice crystals without going through a liquid stage. Conversely, when water evaporates, ice or snow melts, or ice evaporates directly to vapor (sublimates), heat is absorbed; the air or ground that supplies the heat is cooled, and the heat itself becomes latent in the water vapor.

Condensation is extremely important in conveying heat from the surface to the atmosphere, which then carries it from one latitude to another. The hot, moist air of an equatorial rain forest, for example, carries both sensible heat and latent heat to cooler latitudes; the amount of latent heat it carries is about twice as great as the amount of sensible heat; it gains the latter by conduction and convection from the surface. In rainy, temperate latitudes, too, the air gains more latent heat than sensible heat; the opposite is true only in the subtropical deserts north and south of the equator. Over the oceans, the air acquires latent heat at a greater rate than sensible heat at all latitudes.10 Notice that we are now considering the rate at which heat flows from land or sea into the air, called the heat flux and measured in watts per square meter.

Everybody experiences the release of latent heat from time to time, often without realizing it. Think of a clear summer evening when the temperature drops quickly after sunset because the ground radiates its heat out into space, promising a chilly dawn. If clouds gather, however, the temperature stops falling—it may even rise somewhat—and the night stays comfortably warm. This familiar scenario is usually attributed to the fact that the long wave radiation that had streamed from the earth into space when the sky was clear is absorbed by the clouds and radiated back to earth, keeping it warm. That is part of the explanation but not all of it: an appreciable fraction of the warmth is the latent heat of the water vapor in the atmosphere, released and made sensible as it condenses into clouds.

Concentrated Energy: Storms

Strong winds, heavy rain, lightning and thunder, and huge waves at sea all show that at times atmospheric energy becomes concentrated in confined areas. Why? The simple answer is that in a confined area something has happened to upset the normal equilibrium of calm weather. The next question— What sort of something?—has no single answer; it varies from one storm to another, depending on the kind of storm.

Most storms can be classed as cyclones of one sort or another: they are accompanied by winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. They are one of two strongly contrasted kinds of cyclones, however, depending on the latitude: tropical cyclones differ radically from midlatitude cyclones. The most intense tropical cyclones occur over the ocean: those in the western Atlantic are called hurricanes, and those in the western Pacific are typhoons.

Tropical cyclones and midlatitude cyclones derive their energy from entirely different sources.11 Water vapor is abundant in the air over warm ocean currents, and the latent heat of the vapor, released when it condenses, is the energy source for tropical cyclones. The strongest of them also differ from mid-latitude cyclones in having an "eye" at the center, where a current of warm air flows downward; the eye shows as a dark spot at the center of the swirl of white clouds in many satellite photos of hurricanes. But the central current of air in midlatitude cyclones flows upward.

The energy source for midlatitude cyclones develops when two air masses at different temperatures come to be side by side; a horizontal temperature contrast results, which is intrinsically unstable. The separation of warm, light air from heavy, cold air by a vertical surface (a front) creates potential energy (PE). The greater the temperature contrast, the greater the energy. Some of this energy is released, that is, converted to kinetic energy (KE), by a re-

.••;. Cold Warm

•.'.■:':>;. Warm v':':'; Cold

Cold .^V-iC^i

a

Figure 5.3. Side view of two adjacent air masses. (a) A cold air mass has drifted into contact with a warm air mass, and they are separated by a vertical front; the arrangement is unstable (possesses high available PE) because the warm air is less dense than the cold. (b) The cold air has started flowing under the warm air. (c) The final arrangement, with the cold air wholly below the warm air; the arrangement still possesses PE relative to the ground surface, but none of it is available.

Figure 5.3. Side view of two adjacent air masses. (a) A cold air mass has drifted into contact with a warm air mass, and they are separated by a vertical front; the arrangement is unstable (possesses high available PE) because the warm air is less dense than the cold. (b) The cold air has started flowing under the warm air. (c) The final arrangement, with the cold air wholly below the warm air; the arrangement still possesses PE relative to the ground surface, but none of it is available.

arrangement of the air masses. The cold air, because of its greater density, flows under the warm air (see fig. 5.3); the flow, down a pressure gradient, undergoes Coriolis deflection, and the result is a cyclone.

After the repositioning of the air masses, the surface separating them is horizontal. The upper layer still has PE relative to the ground, in the same way that a boulder sitting in the middle of a level plateau has PE relative to sea level (see chapter 2), but it is unavailable—there is no way for it to be converted to KE. Because of this, the efficiency of the conversion of PE to KE is low, only about 1 percent.12

The energy released, from start to finish, by a typical midlatitude cyclone averages about 2 x 1013 MJ (megajoules), and that of a tropical cyclone about 2 x 1011 MJ, only one-hundredth as much. The average area of a midlatitude cyclone is about two hundred times that of a tropical cyclone, however, so that the energy per unit area is actually about twice as great in the tropical cyclone. Tropical cyclones are the most energetic of all storms. If we assign a rating of 100 percent to the energy per unit area of an average tropical cyclone, the average energy per unit area of lesser storms is approximately as follows: a tornado, 80 percent; a midlatitude cyclone, 50 percent; a dust devil, 8 percent; and a severe thunderstorm, 6 percent.13

Solar energy is transported from the tropics to the polar regions by ocean currents and winds acting in combination. The rate of transport is greatest at about latitudes 30° N and 30° S, where the two modes of transport are approximately equal. Between these latitudes, in the tropics and warmer sub-tropics, more energy is transported by ocean currents than by the atmosphere, whereas poleward of them atmospheric transport dominates.14

Violent winds—sometimes more than 500 km/h—are not the only displays of nature's energy that storms provide. Lightning and thunder from electrical storms are equally awe inspiring. We consider them in chapter 16.

How Atmospheric Energy Is Dissipated

The atmosphere is gaining solar energy all the time, at a rate of almost 220 W m-2. The earth as a whole receives solar energy at a rate of 340 W m-2. As figure 4.1 showed, 30 percent of this total is reflected back into space by the atmosphere and 6 percent by the ground (land and sea); the remaining 64 percent, nearly 220 W m-2, provides the energy that is held, temporarily, in the atmosphere. It does not accumulate; if it did, the temperature would go on rising indefinitely. Therefore it must somehow be dissipated.

The atmosphere holds energy in four forms.15 First in importance is its internal (thermal) energy, the energy of its molecules' never-ending random motion. Second is the potential energy resulting from the temperature differences within the atmosphere (see the preceding section). Third is the latent energy of the water vapor in the atmosphere, ready to liberate heat when it condenses. Fourth, and least in quantity at any one moment, is the kinetic energy of moving masses of air—winds.

All this energy is constantly being augmented by incoming solar radiation and must be continuously dissipated at the same rate if equilibrium is to be maintained. The absorption of solar radiation increases the total internal energy of the atmosphere and also, because of unequal heating, its potential energy. Most of this energy is dissipated as fast as it is absorbed, by reradiation into space in the form of long wave (infrared) radiation.

About 30 percent of the absorbed solar energy evaporates water, both from the sea and from freshwater lakes and rivers. The latent energy in the vapor turns back into heat when the vapor condenses to raindrops; the raindrops warm the air and the ground, and the energy is ultimately lost to space as long wave radiation.

The wind does much of its work on the sea. A large proportion of its energy is used in driving ocean currents and raising waves. Some of this energy is transferred to the water as kinetic energy while some, as always, is lost as "waste" heat—that is, entropy (see chapter 3).

The wind also does the work of wind erosion—the natural sandblasting that shapes rocks and cliffs in regions with dry climates; erosion entails friction, hence more waste heat. Indeed, whenever the wind shifts anything—when it builds a sand dune, knocks down a tree, or blows your hat off—the entropy of the universe is increased by a tiny amount. Much wind energy is dissipated as the heat produced by viscous drag because of shearing within the wind itself.

A very small fraction of the wind's work consists in turning humanity's windmills. The output is sometimes milled flour, plus of course the inevitable waste heat; or it may be electrical energy from a generator driven by windmills. For more on the latter topic, see chapter 19.

The Energy in a Rainstorm

Back to the topic of rain. Rain provides an opportunity to demonstrate with actual numbers what becomes of the energy in a common meteorological event.

A heavy rainstorm obviously holds considerable energy. So, what becomes of the energy? It is easy to calculate the work done when a measured depth of rain falls on a known area of land from a cloud at known height. (At least it's easy provided you can assume that the air was saturated with moisture so that none of the rain evaporated as it fell, as it always does in relatively dry air.) Suppose, for example, that 10 mm of rain falls on one hectare (10,000 m2) of land from a cloud 200 m up. Take the density of the rainwater to be 1,000 kg m-3. The volume of the rainfall is depth x area = 0.01 m x 10,000 m2 = 100 m3, and its mass is volume x density = 100 m3 x 1,000 kg m-3 = 100,000 kg.

It follows (see chapter 2) that the work done, or energy used up, as the rain falls to the ground from a height of 200 m is mass x acceleration due to gravity (9.81 m s-2) x distance fallen = 100,000 kg x 9.81 m s-2 x 200 m = 1.962 x 108 J.

The computation is straightforward, but where have the 1.962 x 108 J gone once the rain has reached the ground? They cannot have vanished, so what has become of them? They have been dissipated, partly in the air and partly at the point of impact with the surface (land or sea).

Consider the details of what happens: as the raindrops fall, they are slowed by the drag between each drop and the air. Indeed, a falling raindrop cannot fall faster than its terminal velocity, the velocity at which the pull of gravity is exactly canceled out by the drag. For a raindrop 2 mm in diameter (a typical size), the terminal velocity is 6.5 meters per second;16 it reaches this speed after falling a mere 2 m. Once a falling drop reaches its terminal velocity it cannot speed up any more. If it were not for drag—if the drops were falling in a vacuum—their velocity after a fall of 200 m would be 62.64 m s-1, almost ten times as great.17 The kinetic energy (KE) of the falling rain when it hits the ground is (see chapter three)

12 x mass x velocity2 = 1/2 x 100,000 kg x (6.5 m s-1)2 J = 2.1125 x 106 J.

This is a mere one-hundredth of the KE it would have possessed had there been no drag. In the no-drag case, the KE would have been

1/2 x 100,000 kg x (62.64 m s-1)2 J = 1.962 x 108 J, as we found above in computing the work done by the falling rain.

The rain has indeed done this much work; all but 1.94 x 108 J, however, has been dissipated as heat on the way down. In practice, the rise in temperature (it would be 2°C at most) is masked because the air is simultaneously losing heat to the cold raindrops, falling from cold air above. The 2.1125 x 106 J that the rain still has left on reaching the ground remains to be disposed of. It no more vanishes than the rain itself vanishes when it soaks into the ground or becomes one with the water in the sea or a lake: it only seems to vanish. Consider a single raindrop. If it strikes water, or a rock, it creates a small splash, and the drop itself is deformed. If it strikes soil (or soft clay, or sand) it makes a tiny dent, and both the raindrop and the surface struck are deformed. These events—splash, deformation—consume each raindrop's remaining energy, and the account is precisely balanced at last.

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