Water vapour
Topic: Atmosphere: Composition, Structure and Elements of Weather · NCERT: Class 11, Ch 7 "Composition and Structure of Atmosphere"
Meaning
Water vapour is water in the form of an invisible gas in the air. It is a variable gas, which means its amount changes from place to place. It can be up to 4 per cent of the air in the warm, wet tropics but under 1 per cent in deserts and polar areas. It also falls with height and from the equator towards the poles. It absorbs part of the incoming sunlight and holds in the Earth's heat like a blanket. It decides whether air is stable (stays put) or unstable (rises), so it controls clouds and rain.
Example
In monsoon-time Kerala, the air is heavy with water vapour. Nights stay warm and clouds build quickly. In the dry Thar Desert of Rajasthan, there is little water vapour. Heat escapes fast at night, so nights can turn cold even after very hot days.
Don't confuse with
- Clouds and fog: these are tiny drops of liquid water or ice crystals that you can see. Water vapour is a gas and cannot be seen.