Evaporation is a process by which a liquid is transformed into gas or vapour. The atmosphere receives its supply of moisture from the earth’s surface through evaporation from the oceans, lakes, rivers, etc. and through transpiration from plants.
It takes about 600 calories of energy to convert 1 gram of water to water vapour. During evaporation water molecules absorb energy which gives them the motion required to escape from the surface of liquid and become a gas.
This energy is subsequently released as heat when condensation takes place and water vapour changes back to the liquid state. The heat absorbed in the process of evaporation is referred to as the latent heat of vaporisation. Latent heat means hidden or stored heat.
Since a certain amount of energy is consumed in the process of evaporation, the remaining liquid is cooled by an equivalent amount. Evaporation is a cooling process and its has cooling effect on evaporating bodies.
When the rain drops fall through the dry layers of atmosphere the process of evaporation chills the air. This is so because most of the energy needed for evaporating water is provided by the air itself.
“The reservoir of energy required to maintain the earth’s surface temperature against evaporation cooling is supplied by part of the excess of solar radiation absorbed by the ground (mainly the oceans) over long-wave radiation emitted to space”.