It's complicated and hard to explain, but I'll try my best.
Evaporation occurs when a liquid changes to a gas, but only on the surface of the liquid volume. The gas molecules transfer heat to the liquid molecules. An unequal distribution of heat causes some liquid molecules to become gas molecules. It is random and uneven. Energy from the sun also spawns evaporation. When evaporation does happen, the molecules that leave the liquid body takes a lot of heat with it, therefor the average temperature of the liquid decreases. Evaporation depends on heat, humidity, and air movement. Hope that helped :))
