The water cycle, frequently referred to as the hydrologic cycle, is a continuous movement of water on, above, and below the surface of the earth. It has no beginning or end. Water can be found in different states: water vapor, liquid, dew, ice, or cloud. The total amount of water on earth has remained constant over time. More than 97 percent of it is in the earth's oceans, more than 2 percent is stored in ice sheets and glaciers, and most of the remaining 1 percent is in lakes and rivers. The whole water cycle involves only a tiny fraction of earth's water, not more than 1 percent of the whole, yet the total volume of water in that 1 percent is huge: it has been estimated at more than 20 times the total amount of water in all the Great Lakes. A major factor in water's influence over weather and climate relates to the energy exchanges that occur when it changes state. When water changes to water vapor, for example, it extracts energy from its environment, making the environment cooler while water vapor gets warmer. When water vapor changes back to water, this energy, known as latent energy, is released back into the environment, making it warmer. Similar exchanges of latent energy occur in other transformations.
In fact, any change of state with water involves energy exchanges. Water can go from solid ice directly to water vapor and also from water vapor directly to ice or ice pellets or overnight frost. Latent heat is transferred in all of these. From solid to liquid, liquid to water vapor, or solid to water vapor, heat is withdrawn from the surrounding environment and stored as latent heat in the molecules of the new state. When changes go the other way, this heat is released. The water cycle is often seen as having two parts. It starts as the transfer of water from the earth into the atmosphere as water vapor. With its added warmth, this water vapor moves into the atmosphere and is carried globally by wind systems, thereby distributing water vapor around the world. The second part begins with cooling as the humid air reaches its dew point and precipitation begins. Rivers and underground flows of ground-water are the agencies of transportation to carry the water back to its main source, the oceans.
THE WATER CYCLE BEGINS
PRECIPITATION STORAGE AND RUNOFF
INTERFERENCE WITH THE WATER CYCLE
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