Temperature Distribution in the Atmosphere, Vertical

the distribution of air temperature with respect to height. This distribution is characterized by the vertical temperature gradient γ, which is expressed in degrees per 100 m.

In the troposphere, the temperature decreases with height at an average rate of 0.6° per 100 m—that is, γ = 0.6°/100 m. At each individual point, however, γ may deviate from this average value. The deviations may be different over each point on the earth and in each layer of the troposphere. Moreover, the deviations may be considerable. In the surface layer on a hot summer day, for example, the air above the ground is heated, and γ increases markedly. At night, the ground cools off owing to radiation, and the air temperature falls. Sometimes the air temperature drops to such an extent that γ changes sign—that is, the temperature increases with height instead of decreasing. This phenomenon is known as a ground, or surface, temperature inversion.

In the free atmosphere, different values of γ are again observed. The values for individual layers may range from more than 1°/100 m to negative values in pronounced inversions. In the stratosphere, the values of γ are small or negative.

Stability with respect to vertical air motions depends on the temperature distribution in the atmosphere—that is, on what may be called the stratification of the atmosphere. When air rises, it cools in accordance with a definite law. Dry or unsaturated air cools at the greatest rate: almost 1° per 100 m of ascent. Saturated air cools at a lower rate: a few tenths of a degree per 100 m. The reason for this lower rate is that latent heat is released when the water vapor in the air condenses. Descending air is warmed in an analogous manner.

In accordance with Archimedes’ principle, ascending air continues rising as long as it is warmer than the surrounding atmosphere; it stops rising when it reaches a warmer atmospheric layer. The downward motion of descending air ceases when the increasing air temperature becomes equal to the temperature of the surrounding atmosphere. Thus, the greater the drop in temperature in the surrounding atmosphere (that is, the greater the value of γ), the more intense the convection, turbulence, and gliding of the warm air at fronts. Whether the air is moving upward or downward, the temperature difference between it and the surrounding atmosphere remains. This temperature difference maintains or intensifies the vertical motion. The stratification of the atmosphere in this case is said to be unstable. On the other hand, when the vertical temperature gradient is small or a temperature inversion is present, the temperature of the vertically moving air quickly becomes equal to that of the surrounding atmosphere, and the vertical motions cease. In this case, the atmosphere is said to be stable.

An unstable stratification of the atmosphere is a necessary condition for the development of convective clouds (cumulus and cumulonimbus clouds) and for the intensification of frontal cloudiness. A stable stratification of the atmosphere results in the predominance of clear skies or the development of layer clouds under inversion layers. In the stratosphere, the stratification of the atmosphere is always very stable where the temperature does not vary with height or where inversions are present. For this reason, convection is absent here, and turbulence is low.