How much percent of water vapour is in the atmosphere?

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