Rain shadow

Enlarge Sierra Nevada (U.S.) Washington (state)
Effect of a rain shadow

A rain shadow is a dry area on the leeward 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. Wind and moist air are 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 across the mountains creating a drier side called the "rain shadow".

Description

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

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

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

South America

North America

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.[3]

Europe

Africa

Tenerife, Madeira and other Macaronesian islands have a laurisilva forest where the rain shadow effect is not present.

Oceania

See also

References

  1. ^ a b Whiteman, C. David (2000). Mountain Meteorology: Fundamentals and Applications. Oxford University Press. ISBN 0-19-513271-8.
  2. ^ Bruniard, Enrique D. (1982). "La diagonal árida Argentina: un límite climático real". Revista Geográfica (in Spanish): 5–20.
  3. ^ "How mountains influence rainfall patterns". USA Today. 2007-11-01. Retrieved 2008-02-29.
  4. ^ Glossary of Meteorology (2009). "Westerlies". American Meteorological Society. Archived from the original on 2010-06-22. Retrieved 2009-04-15.
  5. ^ Sue Ferguson (2001-09-07). "Climatology of the Interior Columbia River Basin" (PDF). Interior Columbia Basin Ecosystem Management Project. Archived from the original (PDF) on 2009-05-15. Retrieved 2009-09-12.
  6. ^ http://www.cocorahs.org/Media/docs/ClimateSum_VA.pdf
  7. ^ "UK Rainfall averages". Archived from the original on 2010-02-18.
  8. ^ "Asti weather". weatherbase.com.
  9. ^ S.A, Wirtualna Polska Media (2016-02-02). "Kujawy - najsuchsze miejsce w Polsce". turystyka.wp.pl (in Polish). Retrieved 2020-01-31.
  10. ^ "Spør meteorologen!". www.miljolare.no. Retrieved 2019-05-07.
  11. ^ Giambelluca, Tom; Sanderson, Marie (1993). Prevailing Trade Winds: Climate and Weather in Hawaií. University of Hawaii Press. p. 62.