The process of water molecules escaping the surface of the Earth and entering the atmosphere is known as evaporation. Evaporation takes place as molecules of water escape from a collective body of water. This can be a puddle, a lake, a stream, or just a droplet of water.
As water molecules evaporate, they take with them some of the heat from the object from which they evaporated. This heat is stored in the water molecule, and is referred to as latent heat. The result is that the object’s temperature is lowered slightly. Consider what happens to your body on a hot day. As the temperature rises, your body begins to produce sweat. As the sweat evaporates it carries with it some of the heat from your body, causing your body to cool down.
The process of cooling down an object via evaporation is known as evaporative cooling. Many air conditioners are actually evaporative coolers, and work by taking advantage of this process.
Some important factors affecting the speed of evaporation are temperature, the amount of water vapor already in the air, and the local wind speed.