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8.6 The Atmosphere

8.6.2 Describe the structure of the troposphere and lower stratosphere.

 

Structure of the Atmosphere

The atmosphere is divided into layers based on temperature: the troposphere, the stratosphere, the mesosphere and the thermosphere. Most flying occurs in the troposphere, although high-flying jets may cruise in the stratosphere. The boundary between these two regions is known as the tropopause.

Subdivisions of the atmosphere based on temperature.

Troposphere

Temperature usually falls as height increases within the troposphere until the tropopause is reached. The rate at which it falls is called the environmental lapse rate (or temperature lapse rate). Above the tropopause, the temperature tends to remain at a constant value.

As explained below, and shown in Figure 1-1, the height of the tropopause varies with latitude as does the temperature. At the equator, the tropopause can be as high as 60,000 ft and temperatures as low as -80°C are not uncommon. At the poles, on the other hand, the tropopause can be as low as 20,000 ft and the temperature a relatively mild -40°C.

The earth spins on its axis, carrying the atmosphere with it and tending to throw the air in the lower part of the atmosphere to the outside. This, plus strong heating in tropical areas, causes the troposphere to extend further into space above the equator than above the poles. The tropopause is not, in fact, continuous as shown in Figure 1-1, but consists of a polar tropopause at about 20,000 ft and a tropical tropopause at approximately 60,000 ft. Between these two regions, there can be multiple overlapping mid-latitude tropopauses separated by jetstreams. The tropopause is higher in each region in summer than in winter.

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