Like solid objects the concept of equilibrium is equally applicable to the atmosphere as well. But the following characteristics render the discussion of atmospheric equilibrium complicated:
(1) The atmosphere being compressible, the density of air undergoes progressive change as it descends or ascends.
(2) When a saturated air mass rises, the latent heat of evaporation is released which warms up the air. The warming of rising air in this way affects its density. The changes brought about in the density of an air mass materially affect the atmospheric equilibrium.
To test the stability or instability of the atmosphere, it is to be seen as to what happens to any parcel of air which is displaced through a small vertical distance from its initial height. If in its new position it is subjected to forces which tend to restore it to its original position, the atmosphere is considered to be in stable equilibrium.
Contrary to it, if in its new position it is subjected to no forces tending either to restore it to its original level, or to displace it still further from its original position, the atmosphere is said to be in neutral equilibrium. However, the equilibrium is said to be unstable when in its new position, it is subjected to forces tending it to displace still further from its original position.
It is true that a moving parcel of air will at all stages of its ascent take up automatically the pressure of its immediate environment. Therefore the relative densities of the moving parcel of air and of its surroundings will be largely determined by the absolute temperatures.
When the distribution of temperature and density is such as to resist vertical movements, the atmosphere is said to be in stable equilibrium. Under these conditions, an element of air, even if displaced, tends to return to its original position after some time.
Thus, when the atmosphere is in the state of stable equilibrium there is complete absence of convective activities and the weather remains clear. Such a condition is not conducive to precipitation.
On the contrary, when there exists a tendency in the air to move still further after being displaced, it is said to be in unstable equilibrium. Under these circumstances, the atmospheric conditions are favourable for convective activities which result in cloud formation and abundant precipitation.