Evaporation

Evaporation is a type of vaporization of a liquid that occurs from the surface of a liquid into a gaseous phase that is not saturated with the evaporating substance. The other type of vaporization is boiling, which, instead, occurs within the entire mass of the liquid and can also take place when the vapor phase is saturated, such as when steam is produced in a boiler. Evaporation that occurs from directly from the solid phase, as commonly observed with ice or moth crystals (naphthalene or paradichlorobenzine), is called sublimation.
On average, a fraction of the molecules in a glass of water have enough heat energy to escape from the liquid. Water molecules from the air enter the water in the glass, but as long as the relative humidity of the air in contact is less than 100% (saturation), the net transfer of water molecules will be to the air. The water in the glass will be cooled by the evaporation until an equilibrium is reached where the air supplies the amount of heat removed by the evaporating water. In an enclosed environment the water would evaporate until is saturates the air.
With sufficient temperature, the liquid would turn into vapor quickly (see boiling point). When the molecules collide, they transfer energy to each other in varying degrees, based on how they collide. Sometimes the transfer is so one-sided for a molecule near the surface that it ends up with enough energy to 'escape'.
Evaporation is an essential part of the water cycle. The sun (solar energy) drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water. In hydrology, evaporation and transpiration (which involves evaporation within plant stomata) are collectively termed evapotranspiration. Evaporation of water occurs when the surface of the liquid is exposed, allowing molecules to escape and form water vapor; this vapor can then rise up and form clouds.
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Theory
See also: Kinetic theory
For molecules of a liquid to evaporate, they...