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PrecipitationIn meteorology, precipitation is any kind of water that falls from the sky as part of the weather. This includes snow, rain, sleet, freezing rain and hail. Precipitation is a major part of the hydrologic cycle, and is responsible for depositing most of the fresh water on the planet. Precipitation is generated in clouds, which reach a point of saturation; at this point larger and larger droplets (or pieces of ice) form, which then fall to the earth under gravity. It is possible to 'seed' clouds to induce precipitation by releasing a fine dust or appropriate chemical (commonly silver nitrate) into a cloud, encouraging droplets to form, and increasing the probability of precipitation.
Orographic precipitationOrographic precipitation is precipitation generated by a forced upward movement of air upon encountering a physiographic upland. This upwards movement cools the air, resulting in cloud formation and rainfall. In parts of the world subjected to relatively consistent winds (for example the Tradewinds), a wetter climate prevails on the windward side of a mountain than on the leeward (downwind) side as moisture is removed by orographic precipitation, leaving drier air on the descending (generally warming), leeward side where a rain shadow is formed. Orographic precipitation is well known on oceanic islands, such as the Hawaiian Islands, where much of the rainfall received on an island is on the windward side, and the leeward side tends to be quite dry, almost desert-like, by compaison. This phenomenon results in substantial local gradients of average rainfall, with coastal areas receiving on the order of 20-30 inches per year, and interior uplands receiving over 100 inches per year. Leeward coastal areas are especially dry (<20 in/yr at Waikīkī), and the tops of moderately high uplands are especially wet (~475 in/yr at Wai'ale'ale on Kaua'i).
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Precipitation" and from http://www.white-on.com
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