Effect of a rain shadow
Effect of a rain shadow
The Tibetan Plateau (center), perhaps the best example of a rain shadow. Rainfalls from the southern South Asian monsoon do not make it far past the Himalayas (seen by the snow line at the bottom), leading to an arid climate on the leeward (north) side of the mountain range and the desertification of the Tarim Basin (top).
The Tibetan Plateau (center), perhaps the best example of a rain shadow. Rainfalls from the southern South Asian monsoon do not make it far past the Himalayas (seen by the snow line at the bottom), leading to an arid climate on the leeward (north) side of the mountain range and the desertification of the Tarim Basin (top).

A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side.

Evaporated moisture from water bodies (such as oceans and large lakes) is carried by the prevailing onshore breezes towards the drier and hotter inland areas. When encountering elevated landforms, the moist air is driven upslope towards the peak, where it also condenses into nimbuses and starts to precipitate. If the landforms are tall and wide enough to block or sufficiently delay the passage of these rain-producing weather fronts, most (if not all) of the humidity will be lost to precipitation over the windward side (also known as the rainward side) before ever making it past the top, and the air also forms foehn winds on the leeward side that absorb moisture downslope, therefore casting a broad "shadow" of dry climate region behind the mountain crests, usually in the form of shrub–steppe, xeric shrublands or even deserts.

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

There are regular patterns of prevailing winds found in bands round 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.[2] 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.[3] Some of the strongest westerly winds in the middle latitudes can come in the Roaring Forties between 30 and 50 degrees latitude.[4]

Examples of notable rain shadowing include:

Africa

Northern Africa

The Atlas mountains' (top) rain shadow effect makes the Sahara even drier.
The Atlas mountains' (top) rain shadow effect makes the Sahara even drier.

Southern Africa

Asia

Central and Northern Asia

Eastern Asia

Southern Asia

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

Western Asia

Most of Iran is rain-shadowed by the Alborz mountains in the north (just south of the Caspian sea), hence the country's mostly (semi) arid climate.
Most of Iran is rain-shadowed by the Alborz mountains in the north (just south of the Caspian sea), hence the country's mostly (semi) arid climate.

Europe

Central Europe

Northern Europe

Southern Europe

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.[10]

Caribbean

Northern America

Oceania

Australia in Oceania[18] showing on a map.
Australia in Oceania[18] showing on a map.

Australia

Pacific Islands

South America

The Andes mountains block rain and moisture from the Amazon basin to the west (Bolivia).
The Andes mountains block rain and moisture from the Amazon basin to the west (Bolivia).

See also

References

  1. ^ a b Whiteman, C. David (2000). Mountain Meteorology: Fundamentals and Applications. Oxford University Press. ISBN 0-19-513271-8.
  2. ^ Glossary of Meteorology (2009). "trade winds". Glossary of Meteorology. American Meteorological Society. Retrieved 4 July 2021.
  3. ^ Glossary of Meteorology (2009). "westerlies". Glossary of Meteorology. American Meteorological Society. Retrieved 4 July 2021.
  4. ^ Glossary of Meteorology (2009). "roaring forties". Glossary of Meteorology. American Meteorological Society. Retrieved 4 July 2021.
  5. ^ "Asti weather". weatherbase.com.
  6. ^ S.A, Wirtualna Polska Media (2016-02-02). "Kujawy - najsuchsze miejsce w Polsce". turystyka.wp.pl (in Polish). Retrieved 2020-01-31.
  7. ^ "UK Rainfall averages". Archived from the original on 2010-02-18.
  8. ^ "Spør meteorologen!". www.miljolare.no. Retrieved 2019-05-07.
  9. ^ "Iberian Climatic Atlas" (PDF). IPMA, AEMET. Retrieved 24 December 2020.
  10. ^ "How mountains influence rainfall patterns". USA Today. 2007-11-01. Retrieved 2008-02-29.
  11. ^ Glossary of Meteorology (2009). "Westerlies". American Meteorological Society. Archived from the original on 2010-06-22. Retrieved 2009-04-15.
  12. ^ 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.
  13. ^ http://www.cocorahs.org/Media/docs/ClimateSum_VA.pdf
  14. ^ "Precipitation Variability | Western North Carolina Vitality Index".
  15. ^ "Answer Man: Asheville a 'temperate rainforest' in wake of record rain?".
  16. ^ "Gorges State Park | NC State Parks".
  17. ^ "Canada's only desert is in B.C. But not where you think it is".
  18. ^ "Oceania | Definition, Population, & Facts".
  19. ^ Rain Shadows by Don White. Australian Weather News. Willy Weather. Retrieved 24 May 2021.
  20. ^ And the outlook for winter is … wet by Kate Doyle from The New Daily. Retrieved 24 May 2021.
  21. ^ Giambelluca, Tom; Sanderson, Marie (1993). Prevailing Trade Winds: Climate and Weather in Hawaií. University of Hawaii Press. p. 62. ISBN 9780824814915.
  22. ^ Bruniard, Enrique D. (1982). "La diagonal árida Argentina: un límite climático real". Revista Geográfica (in Spanish): 5–20.