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How does the evaporation and precipitation cycle work?
Answered by Discovery Channel
  • Discovery Channel

    Discovery Channel

  1. Water is an abundant resource that exists on Earth in three forms. Water takes the form of a liquid in the oceans, seas, rivers, lakes and underground reservoirs. Water can also freeze into a solid crystal (ice); most of the world's ice is contained in glaciers. Its final form is as a gas; gaseous water exists as vapor in the air.

    The total amount of water on the Earth's surface remains nearly constant over the years -- only a tiny bit is lost from the upper atmosphere; an equally small amount is gained from volcanic activity. Yet water does not stay in the same place, and it is not always available to humans. Water moves around the Earth as it shifts among its various states. Liquid water flows from point A to point B according to gravity, and it eventually evaporates into the air. Once in gas form, the water rides around throughout the atmosphere with the prevailing wind currents.

    As the sun heats a body of water, its evaporation rate increases, and more of the water rises into the air as vapor. The water vapor eventually encounters cooler air, and condensation begins. When water vapor condenses, it becomes liquid again, forming into many tiny droplets. These droplets accumulate while suspended in the air to form clouds. When enough droplets accumulate in a single cloud, the cloud becomes too heavy. The heavy droplets begin to drop out of the cloud, through the air and back down to the ground as precipitation. Sometimes, precipitation is not liquid water but snow, sleet or hail. Precipitation collects on the Earth's surface, and eventually, it evaporates again. As glaciers melt, the solid-to-liquid water flows into larger bodies of water, providing even more fodder for evaporation and allowing the cycle to continue.

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