Orography
Encyclopedia
Orography is the study of the formation and 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
.
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.
, 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:
Upon ascent, the air that is being lifted will expand and cool. 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. 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 in (508 to 762 mm) per year, and interior uplands receiving over 100 inches (2,540 mm) per year. Leeward coastal areas are especially dry—less than 20 in (508 mm) per year at Waikiki
—and the tops of moderately high uplands are especially wet—about 475 in (12,065 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.
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
.
Uses
Orography has a major impact on global climate, for instance the orography of East Africa substantially determines the strength of the Indian monsoon. In geoscientific 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
Mainstem (hydrology)
In relation to hydrology, a main stem is "the primary downstream segment of a river, as contrasted to its tributaries". Another common term for the main stem, the final large channel of a riverine system, is the trunk. Water enters the main stem from the river's drainage basin, the land area...
(nearest the mouth). This method of listing tributaries is similar to the Strahler Stream Order
Strahler Stream Order
In mathematics, the Strahler number or Horton–Strahler number of a mathematical tree is a numerical measure of its branching complexity....
, where the headwater tributaries are listed as category = 1.
Precipitation
Orographic precipitationPrecipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
, also known as relief precipitation, is precipitation generated by a forced upward movement of air upon encountering a physiographic upland (see anabatic wind
Anabatic wind
An anabatic wind, from the Greek anabatos, verbal of anabainein meaning moving upward, is a wind which blows up a steep slope or mountain side, driven by heating of the slope through insolation. It is also known as an upslope flow. These winds typically occur during the daytime in calm sunny weather...
). 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. This adiabatic cooling of a rising moist air parcel may lower its temperature to its dew point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
, thus allowing for condensation of the water vapor contained within it, and hence the formation of a cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
. 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
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
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
Katabatic wind
A katabatic wind, from the Greek word katabatikos meaning "going downhill", is the technical name for a drainage wind, a wind that carries high density air from a higher elevation down a slope under the force of gravity. Such winds are sometimes also called fall winds...
) is left on the descending, generally warming, leeward side where a rain shadow
Rain shadow
A rain shadow is a dry area on the lee side of a mountainous area. The mountains block the passage of rain-producing weather systems, casting a "shadow" of dryness behind them. As shown by the diagram to the right, the warm moist air is "pulled" by the prevailing winds over a mountain...
is formed.
Terrain
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...
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
Island
An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, cays or keys. An island in a river or lake may be called an eyot , or holm...
s, such as the Hawaiian Islands
Hawaiian Islands
The Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaii in the south to northernmost Kure Atoll...
or New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
, 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
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
-like, by comparison. This phenomenon results in substantial local gradients of average rainfall, with coastal areas receiving on the order of 20 to 30 in (508 to 762 mm) per year, and interior uplands receiving over 100 inches (2,540 mm) per year. Leeward coastal areas are especially dry—less than 20 in (508 mm) per year at Waikiki
Waikiki
Waikiki is a neighborhood of Honolulu, in the City and County of Honolulu, on the south shore of the island of Oahu, in Hawaii. Waikiki Beach is the shoreline fronting Waikīkī....
—and the tops of moderately high uplands are especially wet—about 475 in (12,065 mm) per year at Wai'ale'ale on Kaua'i.
Another well known area for orographic precipitation is the Pennines
Pennines
The Pennines are a low-rising mountain range, separating the North West of England from Yorkshire and the North East.Often described as the "backbone of England", they form a more-or-less continuous range stretching from the Peak District in Derbyshire, around the northern and eastern edges of...
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
Manchester
Manchester is a city and metropolitan borough in Greater Manchester, England. According to the Office for National Statistics, the 2010 mid-year population estimate for Manchester was 498,800. Manchester lies within one of the UK's largest metropolitan areas, the metropolitan county of Greater...
(West) and Leeds
Leeds
Leeds is a city and metropolitan borough in West Yorkshire, England. In 2001 Leeds' main urban subdivision had a population of 443,247, while the entire city has a population of 798,800 , making it the 30th-most populous city in the European Union.Leeds is the cultural, financial and commercial...
(East) where Leeds receives less rain due to a rain shadow of 12 miles from the Pennines.
External links
- Map of the Orography of Europe from Euratlas.com