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From Wikipedia, the free encyclopedia

An example of a rain shadow.

A rain shadow is a dry area on the mountainside facing away from the direction of the wind. The mountains block the passage of rain-producing weather systems, casting a "shadow" of dryness behind them.



The condition exists because as warm moist air rises through "orographic lifting" to the top of a mountain range or large mountain, where due to decreasing atmospheric pressure with increasing altitude, it has expanded and adiabatically cooled to the point that the air reaches its dew point. At the dew point, moisture condenses onto the mountain and it precipitates on the top and windward sides of the mountain. The air descends on the leeward side, but due to the process of precipitation, it has lost much of its initial moisture. Typically, descending air also gets warmer (see Foehn winds) down the leeward side of the mountain, creating an arid region.[1]

Regions of notable rain shadow

The Tibetan Plateau (top), perhaps the best example of a rain shadow. Rain does not make it past the Himalayas, leading to an arid weather on the leeward side of the mountain range.

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.[citation needed]

Examples of notable rain shadowing include:



The Agasthiyamalai hills cut off Tirunelveli (India) from the monsoons, creating a rainshadow region

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,[2] that intercept rain and snowfall that would otherwise reach a valley in the lee of the mid-latitude prevailing westerlies.


  • The Pennines of Northern England, the Welsh Mountains, and the Highlands of Scotland create a large rain shadow that covers almost the entirety of the Eastern United Kingdom, with Glasgow and Manchester for example receiving around double the rainfall of Edinburgh and York respectively. The contrast is even stronger further north, where Aberdeen gets around a third the rainfall of Fort William or Skye. The Fens of East Anglia receive similar rainfall amounts to Seville.[3]
  • 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 driest 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 driest places in the country).
  • The Piedmont wine region of northern Italy is rainshadowed by the mountains that surround it on nearly every side; Asti receives only 527 mm of precipitation each year, making it one of the driest places in mainland Italy.[4]
  • 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.
  • The Scandinavian Mountains create a rain shadow for lowland areas east of the mountain chain and prevents the Oceanic climate from penetrating further east; thus Bergen west of the mountains receives 2,250 mm precipitation annually while Oslo receives only 760 mm, and Skjåk, a municipality situated in a deep valley, receives only 280 mm.


  • 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.


  • 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 second-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 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 350 inches (8,900 mm) liquid water equivalent per year. To the east and down slope of the Southern Alps, scarcely 30 miles (48 km) from the snowy peaks, yearly rainfall drops to less than 30 inches (760 mm) 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.
  • Also in Victoria, the area around Port Phillip Bay is in the rain shadow of the Otway Ranges. The area between Geelong and Werribee is the driest part of southern Victoria: whereas the crest of Otway Ranges receives 2,000 millimetres (79 in) of rain per year, the area around Little River receives as little as 420 millimetres (17 in) annually, which is as little as Nhill or Longreach.

See also


  1. ^ a b Whiteman, C. David (2000). Mountain Meteorology: Fundamentals and Applications. Oxford University Press. ISBN 0-19-513271-8. 
  2. ^ USA Today. How mountains influence rainfall patterns. Retrieved on 2008-02-29.
  3. ^
  4. ^

External links

Simple English

Picture that has wind blowing up a mountain. The water falls out and dries the air. When the air goes down the other side, it is much drier and warmer.

A rain shadow is an area of land that lies behind a mountain that gets almost no rainfall.


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