Orographic precipitation

Orography is the study of the topographic relief of mountains, and can more broadly include hills, and any part of a region's elevated terrain. Orography (also known as ''oreography'', ''orology'' or ''oreology'') falls within the broader discipline of geomorphology. The term orography comes from the Greek: , hill, , to write.

Uses

Mountain ranges and elevated land masses have a major impact on global climate. For instance, the elevated areas of East Africa substantially determine the strength of the Indian monsoon.Srinivasan, J., Nanjundiah, Ravi S. and Chakraborty, Arindam (2005
Impact of Orography on the Simulation of Monsoon Climate in a General Circulation Model
''Indian Institute of Science'' In scientific models, such as general circulation models, orography defines the lower boundary of the model over land.

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 (nearest the mouth). This method of listing tributaries is similar to the Strahler Stream Order, where the headwater tributaries are listed as category 1.

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 lifting can be caused by two mechanisms:

# The upward deflection of large scale horizontal flow by the orography.
# 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 adiabatically. This adiabatic cooling of a rising moist air parcel may lower its 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.

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 islands, 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 per year, and interior uplands receiving over per year. Leeward coastal areas are especially dry—less than per year at Waikiki—and the tops of moderately high uplands are especially wet—about 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 from the Pennines.

See also

* Coverage (telecommunication)
* Orographic lift

References

Sources

*
* {{cite book, author=Whiteman, C. David, title=Mountain Meteorology: Fundamentals and Applications , publisher=Oxford University Press, year=2000, isbn=0-19-513271-8

External links

Map of the Orography of Europe
from Euratlas.com

Geomorphology