Water vapour varies from zero to four per cent by volume of the atmosphere. Water is present in the atmosphere in three forms namely – gaseous, liquid and solid. The moisture in the atmosphere is derived from water bodies through evaporation and from plants through transpiration. Thus, there is a continuous exchange of water between the atmosphere, the oceans and the continents through the processes of evaporation, transpiration, condensation and precipitation. 

Further Reading:

• General Circulation Of The Atmosphere
• Composition of the atmosphere