latent heat

The amount of heat that a unit mass of a body or substance releases or absorbs when it changes state. In the atmosphere, significant transfers of latent heat take place due to changes of phase of the water it contains. For every gram of water that condenses, about 540 calories (in round numbers) are released into the air, contributing to warming it. That same amount of heat is absorbed from the air when the evaporation of hydrometeors takes place. In cloud and precipitation formation processes, the amount of latent heat involved is very large, given the enormous quantities of water vapor transformed into droplets, drops, and ice crystals. Latent heat transfers during freezing (change from liquid water to ice) and melting (change from ice to liquid water) are considerably smaller (80 calories per gram).