Rain shadow

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An example of a rain shadow.

A rain shadow is a dry area on the lee side of a mountainous area (away from the wind). The mountains block the passage of rain-producing weather systems and cast a "shadow" of dryness behind them.

As shown by the diagram to the right, the incoming warm and moist air is drawn by the prevailing winds towards the top of the mountains, where it condenses and precipitates before it crosses the top. The air, without much moisture left, advances behind the mountains creating a drier side called the "rain shadow".

Description[edit]

The condition exists because warm moist air rises by orographic lifting to the top of a mountain range. As atmospheric pressure decreases with increasing altitude, the air has expanded and adiabatically cooled to the point that the air reaches its adiabatic dew point (which is not the same as its constant pressure dew point commonly reported in weather forecasts). At the adiabatic 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 precipitation it has lost much of its moisture. Typically, descending air also gets warmer because of adiabatic compression (see Foehn winds) down the leeward side of the mountain, which increases the amount of moisture that it can absorb and creates an arid region.[1]

Regions of notable rain shadow[edit]

The Tibetan Plateau (top), perhaps the best example of a rain shadow. Rain does not make it past the Himalayas, leading to an arid climate 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:

Asia[edit]

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

South America[edit]

  • The Atacama Desert in Chile is the driest desert on Earth because it is blocked from moisture on both sides (because of the Andes Mountains to the east and high pressure over the Pacific at a latitude which keeps moisture from coming in from the west).
  • The Argentinian wine region of Mendoza is almost completely dependent on irrigation, using water drawn from the many rivers that drain glacial ice from the Andes. The nearby Chilean wine region of Valle Central on the other hand, is situated on the Chilean side of the Andes and experiences a maritime climate.
  • 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).
  • The Guajira Peninsula in northern Colombia is in the rain shadow of the Sierra Nevada de Santa Marta and despite its tropical latitude is almost arid, receiving almost no rainfall for seven to eight months of the year and being incapable of cultivation without irrigation.

North America and the Caribbean[edit]

On the largest scale, the entirety of the North American Interior Plains are shielded from the prevailing Westerlies carrying moist Pacific weather by the North American Cordillera. More pronounced effects are observed, however, in particular valley regions within the Cordillera, in the direct lee of specific mountain ranges. Most rainshadows in the western United States are due to the Sierra Nevada and Cascades.[2]

Europe[edit]

  • The Pennines of Northern England, Welsh mountains, Lake District and Highlands of Scotland create a rain shadow that includes most of the eastern United Kingdom, due to the prevailing south-westerly winds. Manchester and Glasgow, for example, receive around double the rainfall of Sheffield and Edinburgh respectively (although there are no mountains between Edinburgh and Glasgow). 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 form 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, which are the driest spots in Europe (see Cabo de Gata) mostly a result of the mountain 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 areas of Gap and Briançon in France, the district of Zernez in Switzerland.
  • The eastern part of the Pyrenean mountains in the south of France (Cerdagne).
  • 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 2000 mm of rain a year on the summits and below 600mm at 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 per year, making it one of the driest places in mainland Italy.[4]
  • The valley of the Vardar River and south from Skopje to Athens is in the rain shadow of the Prokletije and Pindus Mountains. On its windward side the Prokletije has the highest rainfall in Europe at around 5,000 millimetres (200 in) with small glaciers even at mean annual temperatures well above 0 °C (32 °F), but the leeward side receives as little as 400 millimetres (16 in).
  • 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.

Africa[edit]

  • 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.
  • In Western Cape Province, the Breede River Valley and the Karoo lie in the rain shadow of the Cape Fold Mountains and are arid; whereas the wettest parts of the Cape Mountains can receive 1,500 millimetres (59 in), Worcester receives only around 200 millimetres (8 in) and is useful only for grazing.
  • 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[edit]

  • 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.
  • On the South Island of New Zealand is to be found one of the most remarkable rain shadows anywhere on Earth. The Southern Alps intercept moisture coming off the Tasman Sea, precipitating about 6,300 mm (250 in) to 8,900 mm (350 in) liquid water equivalent per year and creating large glaciers. To the east of the Southern Alps, scarcely 50 km (30 mi) from the snowy peaks, yearly rainfall drops to less than 760 mm (30 in) and some areas less than 380 mm (15 in).
  • 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 western side of 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: the crest of the Otway Ranges receives 2,000 millimetres (79 in) of rain per year and has myrtle beech rainforests much further west than anywhere else, whilst the area around Little River receives as little as 425 millimetres (16.7 in) annually, which is as little as Nhill or Longreach and supports only grassland.
  • Western Australia's Wheatbelt and Great Southern regions are shielded by the Darling Range to the west: Mandurah, near the coast, receives about 700 millimetres (28 in) annually. Dwellingup, 40 km inland and in the heart of the ranges, receives over 1,000 millimetres (39 in) a year while Narrogin, 130 km further east, receives less than 500 millimetres (20 in) a year.

See also[edit]

References[edit]

  1. ^ a b Whiteman, C. David (2000). Mountain Meteorology: Fundamentals and Applications. Oxford University Press. ISBN 0-19-513271-8. 
  2. ^ "How mountains influence rainfall patterns". USA Today. 2007-11-01. Retrieved 2008-02-29. 
  3. ^ "UK Rainfall averages". 
  4. ^ "Asti weather". weatherbase.com. 

External links[edit]