One of the major constituents of the atmosphere is water. But, water is found in several different forms. It comes down as liquid drops of rain, or in a frozen state in the form of snow crystals. It exists in a gaseous form as water vapour and as water in liquid form in super cooled state well below its normal freezing point (0º C).
It has been estimated that during a typical monsoon day about 75,000 million tonnes of water vapour are transported across the west coast of India. When water vapour in the air reaches a saturation point, a very large number of liquid drops are produced. If the saturation point is reached at temperatures below the freezing point of water, the vapour changes directly into crystals of solid ice rather than water drops, but this mode of conversion from vapour to solid is comparatively rare in the atmosphere.
The saturation point of water vapour is one of the important parameters that determine the rate at which vapour is converted into liquid water. Why should there be a critical point at which vapour changes to water (or ice)? The answer lies in the fact that, at any specified temperature and pressure, there is an upper limit to the amount of vapour that a parcel of air can hold. Water vapour is only a collection of water molecules in gaseous form. Consequently, if we confine the space available to a parcel of air by specifying its temperature and pressure, it follows that there will be only room for a certain number of water molecules and no more. If we try and force more water vapour into the atmosphere than it can possibly hold, it gets rid of the surplus vapour by making it condense in the form of liquid drops. It is possible to decrease the vapour retaining capacity of the atmosphere by lowering its temperature.
In the atmosphere, the condensation of water vapour gives rise to clouds. A convenient way of describing a cloud would be to picture it as a population of liquid water drops held in animated suspension. Indeed, the size of water droplets often determines the shape and growth of an individual cloud