In general terms evaporation is defined as the process in which the liquid state is convverted in to gas. This process requires heat energy. Evaporation causes cooling is general process. the principle involved here is the matter must either gain or lose energy to change its state. Here molecules move from liquid to gaseous state and so it needs potential energy and kinetic energy. Due to this energy transfer occurs in the surroundings as well and this is transferred from substance to surriundings or vice versa. The change in temperature till the evaporation process will lead to cooling. Hence evaporation causes cooling effect.