Rain shadow
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A rain shadow (or more accurately, precipitation shadow) is a dry region on the surface of the Earth that is leeward or behind a mountain with respect to the prevailing wind direction. A rain shadow area is dry because, as moist air masses rise to top a mountain range or large mountain, the air cools and water vapor condenses as rain or snow, falling 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, and a region that is quite arid.
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 Caucus 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 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 one of the driest deserts on Earth because it is blocked from moisture on both sides (by the Andes mountains to the east and by coastal mountains to 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, Sonoran, and Chihuahuan deserts all are in regions which are rain shadowed.
- The aptly-named Death Valley in the US 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.
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.
- 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 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.
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. The entire island of Kahoolawe lies in the rain shadow of Maui's East Maui Volcano.
External links and references
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