<P> On average, only a fraction of the molecules in a liquid have enough heat energy to escape from the liquid . The evaporation will continue until an equilibrium is reached when the evaporation of the liquid is the equal to its condensation . In an enclosed environment, a liquid will evaporate until the surrounding air is saturated . </P> <P> 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 . With sufficient energy, the liquid will turn into vapor . </P> <P> For molecules of a liquid to evaporate, they must be located near the surface, they have to be moving in the proper direction, and have sufficient kinetic energy to overcome liquid - phase intermolecular forces . When only a small proportion of the molecules meet these criteria, the rate of evaporation is low . Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures . As the faster - moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid decreases . This phenomenon is also called evaporative cooling . This is why evaporating sweat cools the human body . Evaporation also tends to proceed more quickly with higher flow rates between the gaseous and liquid phase and in liquids with higher vapor pressure . For example, laundry on a clothes line will dry (by evaporation) more rapidly on a windy day than on a still day . Three key parts to evaporation are heat, atmospheric pressure (determines the percent humidity), and air movement . </P> <P> On a molecular level, there is no strict boundary between the liquid state and the vapor state . Instead, there is a Knudsen layer, where the phase is undetermined . Because this layer is only a few molecules thick, at a macroscopic scale a clear phase transition interface cannot be seen . </P>

At what temp does water start to evaporate