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Water cycle illustration (DI01059)
Earth's water is always in circulation. It has been recycled in its different forms - as ice, liquid, or vapor -- for more than 3.5 billion years. The Sun's energy evaporates water from oceans and lakes into the atmosphere. Plants and animals also release water vapor as they respire. When atmospheric conditions are right, and cooling occurs, condensation of water vapor takes place and clouds form that can produce rain or snow.
copyright University Corporation for Atmospheric Research