Rain shadow
A rain 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.
Encyclopedia
A
rain 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 .
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 .
- Patagonia is rain shadowed from the prevailing westerly winds by the Andes range and is arid .
North America
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 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.
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 , 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.
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