ATMOSPHERE ■__THE WEATHER NEAR THE GROUND
Wherever the wind blows, some of its energy is dissipated— converted to entropy—by the shearing of air against air; this happens at all elevations. The losses are far greater in the lowermost layer, however, because of friction with the surface—the drag of moving air as it passes across land or water. Drag also affects the direction of the wind, making atmospheric circulation far more complicated than it is aloft.
A number of other factors, too, have a stronger effect at low elevations than at high ones: the air temperature often varies greatly over small distances in response to the temperature at the surface, which may be a sunbaked desert, a cool forest, a cold lake, or the sea. Hills and mountains deflect the wind, funneling it through narrow valleys or forcing it up over high ridges. And there is more water vapor in the air at low elevations; most of the water vapor in the atmosphere is in the lowermost five kilometers.1
Daily fluctuations in air temperature are most pronounced in the layer heated directly by the surface. The air in the first few centimeters above the ground is heated by conduction, that is, by energy exchanges brought about by collisions between molecules of the air and of the ground. The warmed air becomes less dense and rises, conveying heat upward by convection. The layer warmed by conduction plus convection is seldom more than a few hundred meters thick. Air temperature at a greater height is not influenced by local surface temperature; it is controlled by more distant causes—the movement of air masses, and radiation from the sun and the ground.
The temperature of the air beyond the reach of surface effects is slow to react to changes in incoming radiation: it has high thermal inertia. The inertia is such that air temperature is scarcely affected by the daily variation of incoming sunlight caused by the earth's rotation.2 Unless a new air mass invades an area, heralded by the arrival of a warm or cold front (the boundary between two air masses), daytime and nighttime air temperatures at a site do not differ significantly.
At first thought this is surprising: think of the chill of early morning in summer, just before sunrise while the grass is still drenched with dew. The chill affects only the air nearest the ground, however; you can perceive the marked temperature difference simply by noting that your face is warm while your feet feel frozen. The air temperature a mere hundred meters up may be the same as it was at sunset the previous evening.
In sum, conditions near the surface are much more variable, both spatially and temporally, than those higher up. Temperature, pressure, and humidity often change abruptly over short distances—in a word, energy becomes concentrated in confined areas—and changeable weather is the result.
The amount of heat stored in the atmosphere at any one time is about 1.3 x 1024 J (joules).3 Were it not for the constant movement of the air, this vast quantity of energy would be much less evenly distributed than it is. The energy itself makes the air move; that is, it causes the winds. As we saw in chapter 4, the total power of incoming solar radiation averages 340 W m-2; of this
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