A correct understanding of the concept of atmospheric equilibrium is the most important prerequisite to appreciate different atmospheric processes associated with various kinds of weather phenomena.

True, the concept of equilibrium is based on a complex mathematical calculation requiring some knowledge of higher mathematics. But in the present chapter care has been taken to bring home the fundamentals of equilibrium concept in simple and non-mathematical language.

It may be pointed out that in meteorology the term “stability’ is used to indicate a condition of equilibrium in the atmosphere.

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There are three types of equilibrium which must be made clear to a student of climatology; because it is only then that he can understand the implications of different forms of atmospheric equilibrium.

**(1) Stable equilibrium:**

If an object after being slightly disturbed or displaced from its original position tends to return to the same, it is said to be in stable equilibrium. This property of an object is also referred to as its stability.

For example, a book lying flat on a table or a cone resting on its base will be in stable equilibrium. Now, if the cone is tilted a little, it returns to its original position as soon as the tilting force is withdrawn. Similarly, one end of the book may be raised a little, but when the force is removed it tends to come back to its original position.

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**(2) Unstable equilibrium:**

An object is said to be in unstable equilibrium if it tends to continue in the direction of displacement. In this state of equilibrium the object after being displaced or pushed occupies an entirely new position.

A pencil or ruler standing on one end on a flat table is in unstable equilibrium. If the upper ends of these objects are disturbed slightly, they do not return to their original position, but fall down so that they lie on their sides in stable equilibrium.

Similarly, a cone would be unstable if it were made to stand on its apex instead of its base. A slight disturbance would cause it to fall, never to return to its original position.

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**(3) Neutral equilibrium:**

If an object tends to stay wherever it is pushed, it is said to be in neutral equilibrium. In this intermediary state of equilibrium, the displaced object neither returns to its original position, nor takes a new position.

Rather it tends to remain in whatever position it has been displaced. A bottle or a ball lying on the flat table is the best example of an object in neutral equilibrium.

It is interesting to note that the state of unstable equilibrium of any object does not last for long. A slight disturbance brings back an object from the state of unstable equilibrium to that of stable equilibrium.

The return of an object from unstable conditions to the state of stability would mean the diminution of its potential energy. However, every object tends to keep away from the state of instability because of its inherent tendency to have a certain minimum of potential energy.