The atmosphere has got a layered structure. Layers of the atmosphere differ from one another in respect of temperature, density and humidity of the air. Vertical motions are produced due to these different physical properties of the atmospheric layers.
Undoubtedly these motions are far more important meteorologically than the horizontal movements of air. This is so because various weather phenomena, such as the cumulus or cumulonimbus clouds, different forms of precipitation, thunderstorms, hail-storms, etc. are directly associated with the upward movement of air masses.
A moving air mass undergoes continuous change in respect of its temperature and humidity. But it is worthwhile to remember that the temperature and humidity changes occurring in horizontally moving air are not as rapid as those in ascending or descending air currents.
Vertical motions in the atmosphere lead to vertical mixing, a process through which heat, moisture, and similar properties become redistributed through deep layers.
It has been noted earlier that clouds, fogs, precipitation and similar other weather phenomena occur as a result of condensation; and condensation on a large scale is caused by the cooling of an air mass.
However, there are several ways in which the atmosphere is substantially cooled so that condensation may occur. Different processes of the cooling of the atmosphere consist of radiation, conduction, advection and adiabatic cooling. Of these cooling processes, the most important is the adiabatic process.
The only method of adiabatic cooling is by way of the uplift of air masses. Large-scale condensation and the cloud formation which results in heavy amount of precipitation over an extensive area are caused by the expansion brought about by ascending air currants. This is what is termed as adiabatic cooling.
From the above discussion it is evident that of all the factors of precipitation, vertical motions in the atmosphere play the most dominant role.
Therefore the atmospheric conditions which are favourable or antagonistic to such vertical motions in the atmosphere merit special attention. In fact, stability or instability refers to these atmospheric conditions, and these properties of the atmosphere determine different forms of equilibrium.