Rain shadow
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.
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.:*USA Today on rain shadows:*Weather pages on rain shadows]
