2. Water vapour

Water is present in the atmosphere in three forms namely – gaseous, liquid and solid.. The water vapour constitute about 2 per cent of the total composition of the atmosphere. This percentage varies from zero per cent in cold dry air of the Arctic regions during the winter season to as much as 5 per cent of the volume in warm humid equatorial regions.

The temperature of the atmosphere is the most important factor, as the capacity of the warm air to hold water vapour is more than that of the cold air. About half of the total moisture present in the atmosphere is concentrated in the lower layer of the atmosphere up to a height of about 2 kilometres.