The movements of air and daily temperature changes are the two important factors that bring about a change in atmospheric stability. It may be pointed out that the steep lapse rate renders the air more unstable, while a smaller lapse rate promotes stability.
It is, therefore, natural that any factor that causes the warming of air near the surface of earth promotes instability. Contrary to it, the chilling of surface air makes the air more stable. The cooling of the earth’s surface after sunset by nocturnal radiation is one such factor that increases the stability of air.
Horizontally moving wind streams play an important role in determining the stability of air. When warm air from lower latitudes moves northward over the colder land surfaces in higher latitudes, the air masses are cooled from below and become more stable.
The cooling of lower layers of the atmosphere reduces the prevailing lapse rate which in turn leads to stability. On the other hand, advection of cold air over warm surfaces leads to warming of the lower layers. It steepens the lapse rate. Thus, the air becomes unstable.
Besides, vertical motions produced in the atmosphere influence its stability. Subsidence of an air column increases stability, while its upward movement usually enhances instability.
While discussing convective instability, we have seen that if a deep and extensive layer of air is rising with higher moisture content in its lower portion, then the ultimate result of this lifting is an enhancement in atmospheric instability.
It has also been discussed earlier that conditional instability refers to the air that becomes unstable provided it is lifted to a certain level. Similarly, upward movement of air associated with general convergence enhances instability.
That is why the convergence and associated uplift aided by the intense daytime heating of the surface produce mid-afternoon thunderstorms in a certain favoured locality.
At times, radiation cooling from cloud tops at about sunset enhances their instability. This is so because it steepens the lapse rate, so that warm air from below starts flowing upward adding to the growth of clouds. Nocturnal thunderstorms are produced in this way.