In meteorology, the term precipitation means water in either a liquid or solid state which falls from the atmosphere to the Earth.

Water vapor enters the atmosphere through evaporation from the oceans, inland water bodies, and the moisture in plants and soils. Droplets or ice crystals condense on condensation nuclei, giving rise to various cloud types. The droplets or ice crystals can agglomerate, thus becoming so heavy that they fall from the clouds and reach the Earth (falling precipitation in the form ofrain or snow, for instance). If condensation or sublimation of the water vapor occurs directly at or near to the surface of the Earth, it is the deposited precipitation such as dew or hoar frost.

Measurements, however, are generally confined to falling precipitation. They are given in units of volume per area (l m~2) or as depth of precipitation (mm): 11 m~2 is equal to 1 mm precipitation depth.

The gauge prescribed by the World Meteorological Organization consists of a cylindrical container with an area of 200 cm2. Additional methods of measurement are the use of radar and satellite technology. There is a relatively high level of error in measuring precipitation. Moreover, the great variability of precipitation causes problems in recording it in time and space.

It is estimated that 495 000 km3 of precipitation falls per annum, 385 000 km over the oceans, and 110 000 km over land. This amount of precipitation over land corresponds to a water column of about 700 mm, but this is very unevenly distributed. The reasons for this are the atmospheric and oceanic circulation, the distribution of land and water masses, the location of mountainous areas, vegetation, and the differing water vapor content of the atmosphere as a function of temperature. (Since there is no sufficient knowledge about the distribution ofprecipita-tion over the oceans, we do not attempt to describe it here.)

instability of the atmospheric layering, which in turn stimulates convection (vertical movement) and thus the formation of clouds. This process is clearly visible in the shifting of the rainy period over the year in tropical regions in conjunction with the Sun's peak position (see location of areas with greatest precipitation in the tropics, Figures 2 and 3).

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