Evaporation can be defined as the process where liquid water, or any other liquid, is transformed into a gaseous state. In terms of water, evaporation requires that the humidity of the atmosphere be less than the evaporating surface (at 100% relative humidity there is no more evaporation). The evaporation process requires an input of energy from the environment. For example, the evaporation of one gram of water requires 600 calories of heat energy. The reverse process, a transformation from gas to liquid, is referred to as condensation. Condensation releases energy to the environment.