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Water Cycle

Water covers 70% of the Earth's surface. Almost all of this is stored in the oceans (97.5%) and in freshwater lakes, rivers and streams on land (2%). The atmosphere holds less than .001% in the form of water vapour. If all this water vapour was precipitated completely and evenly over the whole Earth, it would yield only about 25mm or 1 inch of rainfall. Nevertheless, water vapour in the atmosphere plays a very important role in the weather.

There is always water vapour present in the atmosphere. When the air becomes saturated, excess water vapour is released as condensation. This condensation is the source of all clouds and rain. Water vapour enters the atmosphere by evaporation from surface bodies of water. These include puddles, ponds, streams, rivers, lakes and oceans. Water also enters the atmosphere by evapotranspiration from plants and trees. The water vapour is returned to the Earth's surface as precipitation (rain, hail, sleet or snow), and is received by soil, vegetation, surface streams, rivers and lakes and ultimately the sea. This cycle of evaporation, condensation and precipitation is called the water cycle of the Earth and atmosphere. Annual precipitation for the Earth is more than 30 times the atmosphere's total capacity to hold water. This indicates the rapid recycling of water that occurs between the Earth's surface and the atmosphere.

Water cycle