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That is, water has a high heat of vaporization, the amount of energy needed to change one gram of a liquid substance to a gas at constant temperature. Waterâs heat of vaporization is around 540 cal/g at 100 Â°C, water's boiling point. Note that some molecules of water â ones that happen to have high kinetic energy â will escape from the surface of the water even at lower temperatures.
Evaporation is the change of water from a liquid to a gas. Water is continuously evaporating from the surface of the Earth, literally pumping more and more water vapor into the atmosphere. Averaged across the entire Earth, water evaporates from the surface at a rate of about 3 millimeters per day (about 1/8 of an inch). Over the tropical oceans, the value is much larger; over cold surfaces, it is much lower; and over deserts it is almost zero, since there is little or no water to evaporate.