Since temperature is the primary control of evaporation, potential evaporation (evaporating power) will naturally decrease from the equator towards the poles, given an unlimited supply of water. However, actual evaporation is different from potential evaporation.
Besides temperature wind speed and relative humidity, it also depends on the available water to be evaporated. On land surfaces, in fact, the evaporating power of air is many times greater than the available water supply.
The distribution of actual mean annual evaporation clearly illustrates two characteristic features:
(1) Generally actual evaporation is greater over the oceans than over continents. This is simply because of the unlimited supply of water at the ocean surface, while over land areas available water is scarce.
However, the equatorial regions are an exception to this general rule. Land areas in very low latitudes between 10°N and S of the equator lose more moisture through evapotranspiration than do the oceans and other water bodies.
(2) North-south distribution of actual evaporation is largely controlled by air temperature, since temperature decreases from the equator towards the poles. According to Trewartha, about 60 per cent of the earth’s evaporation occurs in the latitudinal belt extending from 20°N to 20°S and 80 per cent occurs in the zone extending from 35°N to 35°S latitude.