Subdivisions of the homosphere. All of the component gases of the homosphere are perfectly diffused among one another, so as to give the pure, dry air a definite set of physical properties, just as if it were a single gas.
However, the homosphere can be subdivided into layers according to temperatures and zones of temperature change. Starting at the earth's surface, air-temperature falls steadily with increasing altitude at the fairly uniform average rate of 3 1/2 F° per 1000 ft (6.4 C° per km). This rate of temperature drop is known as the environmental temperature lapse rate. Departures from this rate will be observed, depending upon geographical location an d season of year. The layer in which the environmental lapse rate is approximately uniform is known as the troposphere.
The uniform lapse rate gives way rather abruptly at a height of 8 to mi (12.5 to 15 km) to a layer, known as the stratosphere, in which temperature at first holds essentially constant with increasing height. The level at which the troposphere gives way to the stratosphere is termed the tropopause. The altitude of the tropopause is least at the poles, 5 to 6 mi (8 to 10 km), whereas at the equator, the tropopause is encountered at 10 mi (17 km). If the troposphere is thought of as a complete surface in three dimensions, it resembles an oblate ellipsoid with a polar flattening and an equatorial bulge.
Seasonal changes in the elevation of the tropopause are marked in middle and high latitudes. For example, at 45° latitude the average altitude in January is 8 mi (12.5km), but rises to 9 mi (15 km) in July. Temperatures at the tropopause are markedly lower at the equator than at the poles. At first glance, this relationship may seem strange, accustomed as we are to considering the equatorial region to be hot and the poles cold. However, with a constant temperature lapse rate assumed, the higher the tropopause, the colder will be the air.
Upward through the stratosphere there sets in a slow rise in temperature until a value of about 32°F (0°C) is reached at about 30 mi (50 km). Here, at the stratopause, a reversal to falling temperature sets in. Temperature decreases through the overlying mesosphere, a layer extending upward to about 50 mi (80 km), where a low point of -120°F (-83°C) is reached, This level of temperature minimum and reversal is termed die mesopause. With further increasing altitude, a steep climb in temperature is observed within the thermosphere.
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