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Encyclopedia > Orographic

Orography is the average height of land, measured in geopotential meters, over a certain domain. In geoscientific models, such as general circulation models, orography defines the lower boundary (except where there is ocean, of course). Because orography is spatially averaged, for example the height of the Himalaya mountains will depend on horizontal resolution. The higher the horizontal resolution, the better the orography will follow the actual terrain.

When a river's tributaries are listed in orographic sequence, they are in order from the highest (nearest the source of the river) to the lowest (nearest the mouth).

Orographic precipitation

Orographic precipitation, also known as relief precipitation, is precipitation generated by a forced upward movement of air upon encountering a physiographic upland (see anabatic wind). This upwards movement cools the air, resulting in a cloud formation and rainfall. In parts of the world subjected to relatively consistent winds (for example the trade winds), a wetter climate prevails on the windward side of a mountain than on the leeward (downwind) side as moisture is removed by orographic precipitation. Drier air (see katabatic wind) is left 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 comparison. This phenomenon results in substantial local gradients of average rainfall, with coastal areas receiving on the order of 20 to 30 inches (500 to 750 mm) per year, and interior uplands receiving over 100 inches (2,500 mm) per year. Leeward coastal areas are especially dry—less than 20 in (500 mm) per year at Waikiki—and the tops of moderately high uplands are especially wet—about 475 in (12,000 mm) per year at Wai'ale'ale on Kaua'i.

See Also

Orographic lift

  Results from FactBites:
orographic lift: Information from Answers.com (899 words)
This wave cloud pattern formed off of the Île Amsterdam in the far southern Indian Ocean, due to orographic lift of an airmass by the island, producing alternating bands of condensed and invisible humidity downwind of the island as the moist air moves in vertical waves and the moisture successively condenses and evaporates.
Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain.
A similar class of winds, the Sirocco, the Bora and Santa Ana winds, are examples where orographic lifting has limited effect since there is limited moisture to remove in the Saharan or other air masses; the Sirocco, Bora and Santa Ana are driven primarily by compression heating.
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