The physical process by which a liquid or solid is transformed to the gaseous state; the opposite of condensation. Also called vaporization. </dd>
In meteorology, evaporation usually is restricted in use to the change of water from liquid to gas, while sublimation is used for the change from solid to gas as well as from gas to solid.
According to the kinetic theory of gases, evaporation occurs when liquid molecules escape into the vapor phase as a result of the chance acquisition of above average, outward-directed, translational velocities at a time when they happen to lie within about one mean free path below the effective liquid surface. It is conventionally stated that evaporation into a gas ceases when the gas reaches saturation. In reality, net evaporation does cease, but only because the numbers of molecules escaping from and returning to the liquid are equal, that is, evaporation is counteracted by condensation.
Energy is lost by an evaporating liquid; and, when no heat is added externally, the liquid always cools. The heat thus removed is termed the latent heat of vaporization. </dd>
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use