Water Vapour is the biggest contributor to the 'natural greenhouse
effect' and varies the most in the atmosphere. Cold air can hold
little water and so the atmosphere over the polar regions contains very
little water vapour. In contrast, air over the tropics is very humid and
the atmosphere can contain up to 4% water vapour.
It
is this 'positive feedback' that makes water vapour important
in climate change as a small increase in global temperature would lead
to a rise in global water vapour levels thus further enhancing the greenhouse
effect.
Human activities have little impact on the level of water vapour in the atmosphere.
