Precipitation shadow

Rain shadow

For the Australian television series see Rain Shadow (TV series).

A rain shadow or rainshadow, or more accurately, precipitation shadow, is a dry region of land that is leeward of a mountain range or other geographic feature, with respect to prevailing wind direction. The mountains block the passage of rain-producing weather systems, casting a "shadow" of dryness behind them.

Description

A rainshadow is warm and dry because as moist air masses rise to the top of a mountain range or large mountain, the air cools and it's temperature decreases until it reaches it's dew point, the point at which the condenses as rain, and then falls either on the windward side or atop the mountain. This is called orographic lifting precipitation. The effect of this phenomenon is the creation of an arid region on the leeward side of the mountains. Also, the warm air absorbs moisture from the already dry and warm air (see Foehn winds). The land gets little precipitation because all the moisture is lost on the mountains. Furthermore, the warm air absorbs moisture from the already dry land.

Regions of notable rain shadow

There are regular patterns of prevailing winds found in bands round 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 30 and 50 degrees latitude.

Examples of notable rain shadowing include:

Asia

South America

  • The Atacama Desert in Chile is the driest desert on Earth because it is blocked from moisture on both sides (by the Andes mountains to the east and high pressure over the Pacific at a 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

Most rainshadows in the western United States are due to mountain ranges, notably the Sierra Nevada and Cascades, that intercept rain and snowfall that would otherwise reach a valley in the lee of the mid-latitude prevailing westerlies.

Europe

  • The Cantabrian Mountains make a sharp divide between "Green Spain" to the north and the dry central plateau. The northern-facing slopes receive heavy rainfall from the Bay of Biscay, but the southern slopes are in rain shadow. The most evident effect on the Iberian Peninsula occurs in the Almería, Murcia and Alicante areas, each with an average rainfall of 300 mm. 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.
  • The Plains of Limagne and Forez in the northern Massif Central, France, are also relatively rainshadowed (mostly the plain of limagne, shadowed by the Chaîne des Puys (up to 2000mm of rain a year on the summits and below 600mm on Clermont-Ferrand, which is one of the dryest place in the country.
  • 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 same is true for the island of Réunion.
  • The formation of the Atlas Mountains has been deemed at least partially responsible for the climatic change which eventually created the Sahara. There is a strong 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. The entire island of Kahoolawe lies in the rain shadow of Maui's East Maui Volcano.
  • New Zealand boasts one of the most remarkable rain shadows anywhere 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 liquid water equivalent per year. To the east and down slope of the Southern Alps, scarcely from the snowy peaks, yearly rainfall drops to less than and some areas less than 15.
  • In Tasmania, one of the states of Australia, 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 (both states of Australia), 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

References

External links

* USA Today on rain shadows
* Weather pages on rain shadows

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