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''For the television series see Rain Shadow (TV series).''
''For the Australian Gothic Metal band, see Rainshadow.''

A 'rain shadow' (or more accurately, 'precipitation shadow') or 'rainshadow' is a dry region on the surface of the Earth that is leeward of or behind a mountain with respect to the prevailing wind direction. A rain shadow area is dry because, as moist air masses rise to the top of a mountain range or large mountain, the air cools and the maximum moisture content decreases until it reaches the dew point, where the water vapor condenses as rain or snow and falls on the windward side or top of the mountain. This process is called orographic precipitation. The effect of the process is the creation, on the leeward side, of an area of descending dry and warming air (see Foehn wind), and a region that is quite arid^[1].

Contents

Description

Regions of notable rain shadow

Asia

South America

North America

Europe

Africa

Oceania

See also

External links and references

Description

A rain shadow is usually caused by a mountain range, but can be caused by other things as well, although to a much lesser degree.
Rainshadows can be observed in the western United States and also in southern central South America. These particular rainshadows are caused by mountain ranges, notably the Sierra Nevada and Cascades in the former, and the Andes in the latter. A rainshadow caused by mountain ranges works in a simple way. As a storm approaches the mountain, the air in the storm undergoes orographic lift, which causes all of the rain to be wrung out of the storm system as it crests the mountains, leaving no moisture to fall on the other side; this area has become a rainshadow.

Regions of notable rain shadow

There are regular patterns of prevailing winds found in bands around the Earth's equatorial region. The zone designated the trade winds is the zone between about 30° N. and 30° S., blowing predominantly from the northeast in the northern hemisphere and from the southeast in the southern hemisphere. The westerlies are the prevailing winds in the middle latitudes between 30 and 60 degrees latitude, blowing predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere. The strongest westerly winds in the middle latitudes can come in the Roaring Forties, between 40 and 50 degrees latitude.
When prevailing winds from the oceans blow onshore over continental regions of rising terrain, orographic lifting causes the moisture picked up over the ocean to condense and fall as rain or snow. The region immediately in the lee of this range usually sees appreciably less rain (i.e., is rain shadowed).
Examples of notable rain shadowing include:

Asia

★ The peaks of the Caucasus Mountains to the west, the Alborz mountains to the south and the ranges tied to the Himalaya to the east rain shadow the Karakum and Kyzyl Kum deserts east of the Caspian Sea.

★ The Dasht-i-Lut in Iran is in the rain shadow of the Elburz and Zagros Mountains and is one of the most lifeless areas on the entire Earth.

★ The Himalaya and connecting ranges also contribute to arid conditions in Central Asia including the Gobi desert of Mongolia and the Taklamakan Desert in China.

★ The Ordos Desert is rain shadowed by mountain chains including the Kara-naryn-ula, the Sheitenula, and the In Shan mountains, which link on to the south end of the Great Khingan Mountains.

★ The Great Indian Desert or Thar desert is bounded and rain shadowed by the Aravalli ranges to the south-east, the Himalaya to the northeast, and the Kirthar and Sulaiman ranges to the west.

South America

★ The Atacama desert of Chile and Peru is the driest desert on Earth because it is blocked from moisture on both sides (by the Andes mountains to the east and a high pressure cell over the Pacific at that latitude which keeps moisture from coming in from the west).

★ Patagonia is rain shadowed from the prevailing westerly winds by the Andes range and is arid (e.g., in Santa Cruz few spots are capable of cultivation, the pastures being poor, water insufficient and salt lagoons fairly numerous).

North America

★ The deserts of the Basin and Range Province in the United States and Mexico, which includes the dry areas east of the Cascade Mountains of Oregon and Washington and the Great Basin, which covers almost all of Nevada and parts of Utah are rain shadowed.

★ The Mojave, Black Rock, Sonoran, and Chihuahuan deserts all are in regions which are rain shadowed.

★ The aptly-named Death Valley in the United States is another good example; it is behind both the Pacific Coast Ranges of California and the Sierra Nevada range, and is one of the driest places on the planet.

★ The eastern side of the Coast Mountains and the Cascade Range in the province of British Columbia, Canada is a rain shadow. The Okanagan Valley which sits in this rain shadow is home to Osoyoos, Canada's only hot desert.

★ Although much more humid than any obvious deserts or steppes, the Shenandoah Valley mostly in western Virginia, lying between the Blue Ridge and the Appalachian Mountains, is decidedly drier than areas to the east and west and is the driest part of the United States of America east of the Mississippi River because the modest mountains reduce rainfall within the valley. The lesser rainfall than that of surrounding areas makes it paradoxically a richer agricultural area than rainier areas nearby. [ftp://ftp.ftw.nrcs.usda.gov/pub/prism/maps/va.pdf][1]

Europe

★ The Cantabrian Mountains make a sharp divide between "Green Spain" to the north, and the dry central plateau. The north facing slopes receive heavy rainfall from the Bay of Biscay, whereas the southern slopes are in rain shadow. The most evident effect in the Iberian Peninsula occurs in the Almería, Murcia and Alicante areas, with an average rainfall of 300mm. and the dryest spot in Europe (see Cabo de Gata) mostly due to the mountainous range running through their western side which blocks the westerlies.

★ Some valleys in the inner Alps are also strongly rainshadowed by the high surrounding mountains.

★ Athens is shielded strongly by mountains from the strong moisture-bearing winds of the Adriatic Sea and receives only a quarter the rainfall of most of Albania.

★ Skjåk, a municipality in Norway, lies in a deep valley and is rain shadowed such that it sees less annual precipitation than the Sahara desert.

Africa

★ The windward side of the island of Madagascar, which sees easterly on-shore winds, is wet tropical, while the western and southern sides of the island lie in the rain shadow of the central highlands and are home to thorn forests and deserts.

★ The formation of the Atlas Mountains have been deemed as, at least, partially responsible for the climatic change which eventually created the Sahara desert. There is an undoubted rain shadow effect to the south side of the mountains.

Oceania

★ New Caledonia lies astride the Tropic of Capricorn, between 19° and 23° south latitude. The climate of the islands is tropical, and rainfall is brought by trade winds from the east. The western side of the Grande Terre lies in the rain shadow of the central mountains, and rainfall averages are significantly lower.

★ Hawaii also has rain shadows, with some areas of the islands being desert, much to the surprise of many tourists. Orographic lifting produces the world's highest annual percipitation record, 12.7 meters (500 inches), on the island of Kauai; the leeward side is understandably rain shadowed^[1]. The entire island of Kahoolawe lies in the rain shadow of Maui's East Maui Volcano.

★ New Zealand can boast of one of the most remarkable rain shadows of any where on Earth. On the South Island, the Southern Alps intercept moisture coming off the Tasman Sea. The mountain range is home to significant glaciers and 250 to 350 inches liquid water equivalent per year. To the east and down slope of the Southern Alps, scarcely 30 miles from the snowy peaks, yearly rainfall drops to less than 30 inches and some areas less than 15.

★ In Tasmania, the central Midlands region is in a strong rain shadow and receives only about a fifth as much rainfall as the highlands to the west.

★ In New South Wales and Victoria, the Monaro is shielded by both the Snowy Mountains to the northwest and coastal ranges to the southeast. Consequently, parts of it are as dry as the wheat-growing lands of those states.

See also

★ Foehn wind

External links and references

:
★ USA Today on rain shadows
:
★ Weather pages on rain shadows
1. Mountain Meteorology: Fundamentals and Applications, Whiteman, C. David, , , Oxford University Press, 2000, ISBN 0-19-513271-8
2. Mountain Meteorology: Fundamentals and Applications, Whiteman, C. David, , , Oxford University Press, 2000, ISBN 0-19-513271-8