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Definition:
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Precipitation caused or enhanced by one of the mechanisms of orographic lifting of moist air. Examples of precipitation caused by mountains include rainfall from orographic stratus produced by forced lifting and precipitation from orographic cumuli caused by daytime heating of mountain slopes. Many of the classic examples of locations having excessive annual precipitation are located on the windward slopes of mountains facing a steady wind from a warm ocean. As another example, wintertime orographic stratus (cap clouds) often produce the major water supply for populated semiarid regions such as the mountainous western United States, and as a result these cloud systems have been a target of precipitation enhancement, cloud-seeding projects intended to produce snowpack augmentation. Orographic precipitation is not always limited to the ascending ground, but may extend for some distance windward of the base of the barrier (upwind effect), and for a short distance to the lee of the barrier (spillover). The lee side with respect to prevailing moist flow is often characterized as the dry rain shadow.
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