Up, Up in the Air = Evaporation

Warmth from the sun causes water from lakes, streams, ice, and soils to turn into water vapor in the air. Almost all of the precipitated water (80 percent) goes right back into the air because of evaporation. The rest runs off the land or soaks into the ground to become ground water.

The process by which a liquid is converted into a vapor. In the liquid phase, the substance is held together by intermolecular forces. As the temperature is raised, the molecules move more vigorously, and in increasingly high proportion have sufficient energy to escape from their neighbors. Evaporation is therefore slow at low temperatures but faster at higher temperatures. In an open vessel, the molecules escape from the vicinity of the liquid, and there is a net migration from the liquid to the atmosphere. In a closed vessel, net evaporation continues until the number of molecules in the vapor has risen to the stage at which the rate of return from the vapor to the liquid is equal to the rate of evaporation. At this stage there is a dynamic equilibrium between the liquid and its vapor, with evaporation and its reverse, condensation, occurring at the same rate. The pressure of the vapor in the closed vessel is called the vapor pressure of the substance; its value depends on the temperature. Boiling occurs in an open vessel (but not in a closed vessel) when the vapor pressure is equal to the ambient pressure. Evaporation is an endothermic (heat-absorbing) process because molecules must be supplied with energy to overcome the intermolecular forces. The enthalpy of vaporization, ΔvapH (formerly, the latent heat of vaporization) is the heat required at constant pressure per mole of substance for vaporization. The entropy of vaporization, ΔvapS, at the boiling point, Tb, is equal to ΔvapH/Tb. According to Trouton's rule, for many liquids the entropy of vaporization is close to 85 J/K · mol. This value reflects the similar change in disorder that occurs when a liquid is converted into a gas. However, certain liquids (water and mercury among them) are more structured than others, and have a bigger entropy of vaporization than Trouton's rule suggests.
Volatile liquids evaporate more rapidly than others at the same temperature. Such liquids have relatively weak intermolecular forces. In general, the rate of evaporation depends on the strengths of the intermolecular forces and the rate at which heat is supplied to the liquid.

Reference:"Evaporation." Answers.Com. 26 May 2008