20.16 Water Vapour

20.16.2 Explain the sources of water vapour in the atmosphere.

Water vapour enters the atmosphere primarily through evaporation and transpiration, with smaller contributions from sublimation.

The main sources are:

Evaporation from oceans and seas — the largest source of atmospheric water vapour. Solar heating causes liquid water to change into water vapour, which enters the atmosphere.

Evaporation from lakes, rivers and soil — water evaporates from inland water bodies, wet ground and other moist surfaces, adding water vapour to the atmosphere.

Transpiration from vegetation — plants absorb water through their roots and release it into the atmosphere as water vapour through small openings in their leaves.

Sublimation from snow and ice — snow and ice can change directly from a solid into water vapour without first becoming liquid.

Factors affecting water vapour supply

The amount of water vapour entering the atmosphere depends on:

• Temperature — higher temperatures generally increase evaporation rates.