Orography
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'''Orography''' is the average height of land, measured in [[Geopotential|geopotential meters]], over a certain domain. In [[geoscientific model]]s, such as [[general circulation model]]s, 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 [[Image resolution|resolution]]. The higher the horizontal resolution, the better the orography will follow the actual terrain.<ref>[http://amsglossary.allenpress.com/glossary/search?p=1&query=Orography&submit=Search Orography] (from the [[American Meteorological Society]] website)</ref>
When a river's [[tributaries]] or settlements by the river are listed in 'orographic sequence', they are in order from the highest (nearest the source of the river) to the lowest or [[mainstem (hydrology)|mainstem]] (nearest the mouth). This method of listing tributaries is the opposite of the [[Strahler Stream Order]], where the headwater tributaries are listed as category = 1.
==Precipitation==
Orographic [[precipitation (meteorology)|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 lifting can be caused by two mechanisms: 1) The upward deflection of large scale horizontal flow by the Orography. 2) The anabatic or upward vertical propagation of moist air up an orographic slope caused by daytime heating of the mountain barrier surface. Upon ascent, the air that is being lifted will expand and cool. This [[adiabatic]] cooling of a rising moist air parcel may lower it's temperature to its [[dew point]], thus allowing for condensation of the water vapor contained within it, and hence the formation of a [[cloud]]. If enough water vapor condenses into cloud droplets, these droplets may become large enough to fall to the ground as precipitation. In parts of the world subjected to relatively consistent [[wind]]s (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.
[[Terrain]] induced precipitation is a major factor for [[meteorologists]] as they forecast the local weather. Orography can play a major role in the type, amount, intensity and duration of precipitation events. Researchers have discovered that barrier width, slope steepness and [[updraft]] speed are major contributors for the optimal amount and intensity of orographic precipitation. [[Computer model]] simulations for these factors showed that narrow barriers and steeper slopes produced stronger updraft speeds which, in turn, enhanced orographic precipitation.
Orographic precipitation is well known on oceanic [[island]]s, such as the [[Hawaiian Islands]] or [[New Zealand]], 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]].
Another well known area for Orographic precipitation is the [[Pennines]] in the north of [[England]] where the west side of the Pennines receives more rain than the east because the clouds (generally arriving from the west) are forced up and over the hills and cause the rain to fall preferentially on the western slopes. This is particularly noticeable between [[Manchester]] (West) and [[Leeds]] (East) where Leeds receives less rain due to a rain shadow of 12 miles from the Pennines.
==See also==
* [[Orographic lift]]
== References ==
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[[Category:Climate forcing agents]]
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