At an interface with the atmosphere, water changes its state from a liquid to a vapour in response to an increase in temperature caused by an external heat source. This temperature change is normally the result of solar radiation. The transfer of moisture into the air is called evaporation. The process is also controlled by the relative humidity, or level of vapour saturation, of the air. The greater the relative humidity of the air, the less likely it is that evaporation will take place for a given temperature. In addition to the direct controls of temperature and humidity, the rate of evaporation is also influenced by wind velocity, since continuous wind currents will carry away saturated air from the water surface, allowing more water to evaporate from the surface.
Evaporation is variable with both time and place because the controlling factors themselves provide transient conditions. It will occur almost continuously from stretches of permanent open water and intermittently, but usually at a lower rate, from land surfaces.
Over land surfaces, the rate of evaporation varies with the extent to which the ground is saturated. If the soil saturation level (i.e. the level to which all the voids are filled with water) is low, water moves up to the surface by the effect of capillary action. This controls the rate at which water will evaporate. Evaporation takes place from a sandy soil saturated up to the surface as quickly as it will from a lake, but the evaporation rate from saturated loam and clay soils is only 75–90% of that from an open body of water.
As it begins to rain, a large proportion of the water droplets is intercepted by the leaves of trees and other vegetation before reaching the ground. By this process of interception, water held on leaves and branches is returned rapidly to the atmosphere by evaporation. In forested areas, as much as 40% of light rain may be intercepted by foliage, although overall the fraction is probably nearer to 10–25%.