Water vapour is the biggest contributor to the 'natural greehouse effect'and varies the most in the atmosphere.
Atmospheric water vapour is essentially determined by the temperature, since atmosphere and oceans continuously exchange vapour, rain and snow.
Solar radiation that hits the Earth's surface bounces directly back into space. The remaining radiation heats up oceans and land, which in turn give off large amounts of heat. Part of the heat transfer from the surface takes the form of water vapour rising from oceans and land, especially in the tropics. This rises to a great height in the form of cumulo-nimbus clouds, which then releases, heat when the water condenses into precipitation.
However, most solar energy radiated back from the Earth's surface is stopped by clouds, by such greenhouse gases as water vapour, carbon dioxide, methane and nitrous oxide. This heat is returned to the Earth's surface as global warming.