The Water Cycle
WATER EVAPORATES from the land and ocean, then condenses and falls as precipitation; the water that falls over land either returns to the air by evaporation or plant transpiration, or it flows back to the sea. This ceaseless movement is called the water cycle.
Today we know approximately the amounts of water at each stage in the cycle and that, on average, a water molecule spends nine or ten days in the air between its evaporation and its return to the surface. Of all the water on earth, only about 0.35 per cent is in the air at any time, yet this small proportion brings us all our clouds, fog, rain, snow, and hail. If precipitation were distributed evenly, every place on earth would receive 33.8 in (85 7 mm) a year. In fact, about 7 7 percent of all precipitation falls over the sea. Edmund Halley (1656-1742), the English astronomer, calculated that evaporation from the Mediterranean Sea removes as much water as flows into it from rivers.
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