Rain shadow


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 bodies of water 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 expands, cools, and its moisture condenses and starts to precipitate. If the landforms are tall and wide enough, most of the humidity will be lost to precipitation over the windward side before ever making it past the top. As the air descends the leeward side of the landforms, it is compressed and heated, producing Foehn winds that absorb moisture downslope and cast a broad "shadow" of dry climate region behind the mountain crests. This climate typically takes the form of shrub–steppe, xeric shrublands, or deserts.
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. 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 down the leeward side of the mountain, which increases the amount of moisture that it can absorb and creates an arid region.

Notably affected regions

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. 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. Some of the strongest westerly winds in the middle latitudes can come in the Roaring Forties of the Southern Hemisphere, between 30 and 50 degrees latitude.
Examples of notable rain shadowing include:

Africa

Northern Africa

  • The Sahara is made even drier because of a strong rain shadow effects caused by major mountain ranges. To the northwest, the Atlas Mountains, covering the Mediterranean coast for Morocco, Algeria and Tunisia. On the windward side of the Atlas Mountains, the warm, moist winds blowing from the northwest off the Atlantic Ocean, which contain a lot of water vapor, are forced to rise, lift up and expand over the mountain range. This causes them to cool down, which causes an excess of moisture to condense into high clouds and results in heavy precipitation over the mountain range. This is known as orographic rainfall and after this process, the air is dry because it has lost most of its moisture over the Atlas Mountains. On the leeward side, the cold, dry air starts to descend and to sink and compress, making the winds warm up. This warming causes the moisture to evaporate, making clouds disappear. This prevents rainfall formation and creates desert conditions in the Sahara.
  • Desert regions in the Horn of Africa such as the Danakil Desert are all influenced by the air heating and drying produced by rain shadow effect of the Ethiopian Highlands.

Southern Africa

Asia

Central and Northern Asia

Eastern Asia

  • The Ordos Desert is rain shadowed by mountain chains including the Kara-naryn-ula, the Sheitenula, and the Yin Mountains, which link on to the south end of the Great Khingan Mountains.
  • The central region of Myanmar is in the rain shadow of the Arakan Mountains and is almost semi-arid with only of rain, versus up to on the Rakhine State coast.
  • The plains around Tokyo, Japan – known as Kantō Plain – during winter experiences significantly less precipitation than the rest of the country by virtue of surrounding mountain ranges, including the Japanese Alps, blocking prevailing northwesterly winds originating in Siberia.

Southern Asia

Western Asia

Europe

Central Europe

Northern Europe

  • The Pennines of Northern England, the mountains of Wales, the Lake District and the 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 Leeds and Edinburgh respectively. The contrast is even stronger further north, where Aberdeen gets around a third of the rainfall of Fort William or Skye. In Devon, rainfall at Princetown on Dartmoor is almost three times the amount received to the east at locations such as Exeter and Teignmouth. The Fens of East Anglia receive similar rainfall amounts to Seville.
  • Iceland has plenty of microclimates courtesy of the mountainous terrain. Akureyri on a northerly fiord receives about a third of the precipitation that the island of Vestmannaeyjar off the south coast gets. The smaller island is in the pathway of Gulf Stream rain fronts with mountains lining the southern coast of the mainland.
  • 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 and a place like Brekke in Sogn, west of the mountains, receive an annual precipitation of, respectively, while Oslo receives only, and Skjåk Municipality, a municipality situated in a deep valley, receives only. Further east, the partial influence of the Scandinavian Mountains contribute to areas in east-central Sweden around Stockholm only receiving annually. In the north, the mountain range extending to the coast in around Narvik and Tromsø cause a lot higher precipitation there than in coastal areas further east facing north such as Alta or inland areas like Kiruna across the Swedish border.
  • The South Swedish highlands, although not rising more than, reduce precipitation and increase summer temperatures on the eastern side. Combined with the high pressure of the Baltic Sea, this leads to some of the driest climates in the humid zones of Northern Europe being found in the triangle between the coastal areas in the counties of Kalmar, Östergötland and Södermanland along with the offshore island of Gotland on the leeward side of the slopes. Coastal areas in this part of Sweden usually receive less precipitation than windward locations in Andalusia in the south of Spain.

Southern Europe

Caribbean

  • Throughout the Greater Antilles, the southwestern sides are in the rain shadow of the trade winds and can receive as little as per year as against over on the northeastern, windward sides and over over some highland areas. This is most apparent in Cuba, where this phenomenon leads to the Cuban cactus scrub ecoregion, and the island of Hispaniola, which results in xeric semi-arid shrublands throughout the Dominican Republic and Haiti.

North American mainland

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. This includes much of the Basin and Range Province in the United States and Mexico.
The Pacific Coast Ranges create rain shadows near the West Coast:
Most rain shadows in the western United States are due to the Sierra Nevada mountains in California and Cascade Mountains, mostly in Oregon and Washington.
The Colorado Front Range is limited to precipitation that crosses over the Continental Divide. While many locations west of the Divide may receive as much as of precipitation per year, some places on the eastern side, notably the cities of Denver and Pueblo, Colorado, typically receive only about 12 to 19 inches. Thus, the Continental Divide acts as a barrier for precipitation. This effect applies only to storms traveling west-to-east. When low pressure systems skirt the Rocky Mountains and approach from the south, they can generate high precipitation on the eastern side and little or none on the western slope.
Further east:

Oceania

Australia

Pacific Islands

  • Hawaii also has rain shadows, with some areas being desert. Orographic lifting produces the world's second-highest annual precipitation record,, 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 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 found one of the most remarkable rain shadows anywhere on Earth. The Southern Alps intercept moisture coming off the Tasman Sea, precipitating about liquid water equivalent per year and creating large glaciers on the western side. To the east of the Southern Alps, scarcely from the snowy peaks, yearly rainfall drops to less than and some areas less than..

South America

  • The Atacama Desert in Chile is the driest non-polar desert on Earth because it is blocked from moisture by the Andes Mountains to the east while the Humboldt Current causes persistent atmospheric stability.
  • Cuyo and Eastern Patagonia is rain shadowed from the prevailing westerly winds by the Andes range and is arid. The aridity of the lands next to eastern piedmont of the Andes decreases to the south due to a decrease in the height of the Andes with the consequence that the Patagonian Desert develop more fully at the Atlantic coast contributing to shaping the climatic pattern known as the Arid Diagonal. The Argentinian wine region of Cuyo and Northern Patagonia is almost completely dependent on irrigation, using water drawn from the many rivers that drain glacial ice from the Andes.
  • 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.
  • The Guianan savanna is in the rain shadow of the Guiana Highlands and stands in stark contrast to the surrounding rainforest environment.