Evaporation is an endothermic process that is the conversion of a liquid to the vapor state by the addition of latent heat. Condensation is a type of chemical reaction in which water or another simple substance is released by the combination of two or more molecules. The term is also used to describe the process by which a gas or vapor changes to a liquid. In weather, evaporation occurs when water on the earth changes to water vapor and enters the atmosphere. The opposite of evaporation, condensation transforms water vapor in the atmosphere into a liquid, often resulting in precipitation, most commonly in the form of rain or snow.