Change of any liquid into vapour at any temperature below its boiling point. For example, water, when placed in a shallow open container exposed to air, gradually disappears, evaporating at a rate that depends on the exposed area, the air humidity, and the temperature. Evaporation occurs because among the molecules near the surface of the liquid there are always some with enough heat energy to overcome the cohesion of their neighbors and escape (see adhesion and cohesion; matter). At higher temperatures, the number of energetic molecules is higher, and evaporation is more rapid. Evaporation is also increased by increasing the surface area of the liquid or by speeding up the air circulation, thus carrying away the energetic molecules leaving the liquid before they can be slowed enough by collisions with air molecules to be reabsorbed into the liquid. If the air is humid some water molecules from the air will pass back into the liquid, thus reducing the rate of evaporation. An increase in atmospheric pressure also reduces evaporation. The process of evaporation is always accompanied by the cooling effect. For example, when a liquid evaporates from the skin, a cooling sensation results. The reason is, that only the most energetic molecules of liquid are lost by evaporation, so that the average energy of the remaining ones decreases; the surface temperature, which is a measure of this average energy, decreases also. Many refrigeration processes are based on this principle.