Evaporative cooling is cooling brought about when a liquid, such as water, changes phase and becomes a vapor or gas. The phase change is accompanied by the absorption of large amounts of heat from the air or the surface where evaporation takes place.

 

Direct evaporative cooling occurs when ventilated air passes over a wet surface (e.g. a pond or a fountain) and is cooled by evaporation. The decrease in the air's dry bulb temperature – the temperature of the air as measured by a standard thermometer – is accompanied by an increase in the air's moisture content.

 

Indirect evaporative cooling occurs the cooling of a surface from which evaporation is taking place. The air that comes in contact with the cool surface is subsequently cooled without increasing its moisture content.