Water evaporates when molecules at the interface of water and air have enough energy to escape the forces that hold them together in the liquid. Evaporation, along with boiling, is a type of vaporization.
Under normal conditions, a relatively small number of water molecules meet the parameters for evaporation. The kinetic energy of a molecule increases with temperature; this means evaporation rates are also higher with temperature. As temperature increases and more water molecules escape the surface of the water, the molecules beneath them lose energy and are less likely to contribute to further evaporation. This is evaporative cooling, and is the reason why sweating makes the body feel cooler. An increase in air movement also increases evaporation: A wet object dries faster on a windy day than on a still day.
The final contributing factor of evaporation is humidity. Just as some liquid water molecules transfer to the air, some water vapor molecules transfer back to the surface of the water. However, unless humidity is more than 100 percent, there are always more liquid molecules escaping to the air.
Evaporation is an important part of the water cycle. Water evaporating from Earth's water bodies enters the atmosphere where it condenses into clouds to later fall as rain. Overall, there is a balance between evaporation and precipitation.