(1) Amount of water available:
Rate of evaporation is greater over the oceans than over the continents.
It is also affected by temperature. As the temperature of air is increased, its capacity to hold moisture also increases.
Any increase in air temperature raises the temperature of water at the evaporation source which means that more energy is available to the water molecules for escaping from liquid to a gaseous state. Hence evaporation is directly proportional to the temperature of evaporating surface. Warmer the evaporating surface, higher the rate of evaporation.
(3) Relative humidity:
The rate of evaporation is closely related with the relative humidity of air. Since the moisture holding capacity of air at a given temperature is limited, drier air evaporates more water than moist air.
In other words, higher the vapour pressure, lower the rate of evaporation. It is a common experience that evaporation is greater in summer and at mid-day than in winter and at night.
Evaporation depends on the wind speed as well. When the winds are light, a thin layer of air just above the surface gets nearly saturated; under the circumstances the difference between the vapour pressure between ground and air is very small.
This results in very low evaporation. On the other hand, when the wind velocity is high, turbulence is set up in the air. Moisture evaporated from the ground is mixed upward and the vapour-pressure difference between the atmosphere and the surface remains large.
Thus, the rate of evaporation is accelerated. Wherever there is a combination of high temperature, very low relative humidity and strong wind, the rate of evaporation is exceptionally high. This leads to dehydration of soil to a depth of several inches.
(5) Area of the evaporating surface:
The rate of evaporation is determined by the area of the exposed surface of water. Larger areas of evaporating surface increase the rate of evaporation.
Evaporation is also affected by the atmospheric pressure exerted on the evaporating surface. Lower pressure on open surface of the liquid results in the higher rate of evaporation.
(7) Composition of water:
Evaporation is inversely proportional to the salinity of water. Rate of evaporation is always greater over fresh water than over salt water. Under similar conditions, the ocean water evaporates about 5 per cent more slowly than the fresh water.