A rain shadow is an area of significantly reduced behind a region, on the side facing away from prevailing winds, known as its leeward side.
Evaporated moisture from bodies of water (such as and large ) is carried by the prevailing sea breeze towards the drier and hotter inland areas. When encountering elevated , the moist air is orographic lift towards the summit, where it expands, cools, and its moisture and starts to Precipitation. If the landforms are tall and wide enough, most 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. As the air descends the leeward side of the landforms, it is compressed and heated, producing that absorb moisture downslope and cast a broad "shadow" of arid region behind the ridge. This climate typically takes the form of shrub–steppe, xeric shrublands, or .
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 isobaric process 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 (as with foehn winds) 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
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
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The Sahara is made even drier because of a strong rain shadow effects caused by major mountain ranges (whose highest points can culminate up to more than 4,000 meters; 2½ miles high). 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.
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Desert regions in the Horn of Africa (Ethiopia, Eritrea, Somalia and Djibouti) 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
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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 drying effect of the cental highlands extends into southeastern Mozambique in spite of the Mozambique Channel in between. The same is true for the island of Réunion.
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On Tristan da Cunha, Sandy Point on the east coast is warmer and drier than the rainy, windswept settlement of Edinburgh of the Seven Seas in the west.
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In Western Cape Province, the Breede River Valley and the Karoo region lie in the rain shadow of the Cape Fold Mountains and are arid; whereas the wettest parts of the Cape Mountains can receive , Worcester receives only around and is useful only for grazing.
Asia
Central and Northern Asia
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The Himalayas and connecting ranges also contribute to arid conditions in Central Asia including Mongolia's Gobi desert, as well as the semi-arid steppes of Mongolia and north-central to north western China.
Eastern Asia
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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.
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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.
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The plains around Tokyo, Japan – known as Kanto 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
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The eastern side of the Sahyadri ranges on the Deccan Plateau including: Vidarbha, North Karnataka, Rayalaseema and western Tamil Nadu.
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Gilgit and Chitral, Pakistan, are rainshadow areas.
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The Thar Desert is bounded and rain shadowed by the Aravalli Range ranges to the southeast, the Himalaya to the northeast, and the Kirthar and Sulaiman ranges to the west.
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The Central Highlands of Sri Lanka rain shadow the northeastern parts of the island, which experience much less severe summer monsoon rains and instead have precipitation peaks in autumn and winter.
Western Asia
Europe
Central Europe
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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; 80" of rain a year on the summits and below 600mm; 20" at Clermont-Ferrand, which is one of the driest places in the country).
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The Piedmont wine region of northern Italy is rainshadowed by the mountains that surround it on nearly every side: Asti receives only of precipitation per year, making it one of the driest places in mainland Italy.
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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.
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The Kuyavia and the eastern part of the Greater Poland has an average rainfall of about because of rainshadowing by the slopes of the Kashubian Switzerland, making it one of the driest places in the North European Plain.
Northern Europe
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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 (although there are no mountains between Edinburgh and Glasgow). 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.
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Iceland has plenty of 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.
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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,
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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 County, 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
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The Cantabrian Mountains form a sharp division 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 other most evident effect on the Iberian Peninsula occurs in the Almería, Murcia and Alicante areas, each with an average rainfall of , 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.
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The Norte Region in Portugal has extreme differences in precipitation with values surpassing in the Peneda-Gerês National Park to values close to in the Douro Valley. Despite being only apart, Chaves has less than half the precipitation of Montalegre.
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The eastern part of the Pyrenees in the south of France (Cerdagne).
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In the Northern Apennines of Italy, Mediterranean city La Spezia receives twice the rainfall of Adriatic city Rimini on the eastern side. This is also extended to the southern end of the Apennines that see vast rainfall differences between Naples with above on the Mediterranean side and Bari with about on the Adriatic side.
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The valley of the Vardar River and south from Skopje to Athens is in the rain shadow of the Accursed Mountains and Pindus Mountains. On its windward side the Accursed Mountains has the highest rainfall in Europe at around with small glaciers even at mean annual temperatures well above , but the leeward side receives as little as .
Caribbean
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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 contains the Caribbean's highest mountain ranges), 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:
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The Dungeness Valley around Sequim and Port Angeles, Washington lies in the rain shadow of the Olympic Mountains. The area averages of rain per year. The rain shadow extends to the eastern Olympic Peninsula, Whidbey Island, parts of the San Juan Islands, and Victoria, British Columbia which receive between of precipitation each year. Seattle is also affected by the rain shadow, albeit to a much lesser effect.
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The east slopes of the Coast Ranges in central and southern California cut off the southern San Joaquin Valley from enough precipitation to ensure desert-like conditions in areas around Bakersfield.
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San Jose, and adjacent cities are usually drier than the rest of the San Francisco Bay Area because of the rain shadow cast by the highest part of the Santa Cruz Mountains.
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The Sonoran Desert is bounded to the west by the Peninsular Ranges, but extends even along part of the east coast of the Gulf of California.
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The Sierra Madre Occidental in Mexico are west of the Chihuahuan Desert.
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.
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The Cascades create a rain-shadowed Columbia Basin area of Eastern Washington and valleys in British Columbia, Canada - most notably the Thompson River and Nicola River Valleys which can receive less than of rain in parts, and the Okanagan Valley (particularly the south, nearest to the US border) which receives anywhere from 12 to 17 inches of rain annually.
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The endorheic Great Basin of Utah and Nevada is in the rain shadows of the Cascades and Sierra Nevada.
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The Mojave Desert is rain-shadowed by the Sierra Nevada and the Transverse Ranges of southern California.
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The Black Rock Desert is in the rain shadows of the Cascades and Sierra Nevada.
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California's Owens Valley is rain-shadowed by the Sierra Nevada.
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Death Valley in the United States, behind both the Pacific Coast Ranges of California and the Sierra Nevada range, is the driest place in North America and one of the driest places on the planet. This is also due to its location well below sea level which tends to cause high pressure and dry conditions to dominate due to the greater weight of the atmosphere above.
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:
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The Shenandoah Valley of Virginia, wedged between the Ridge-and-Valley Appalachians and the Blue Ridge Mountains and partially shielded from moisture from the west and southeast, is much drier than the very humid remainder of Virginia and the American Southeast.
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Asheville, North Carolina sits in the rain shadow of the Balsam, Smoky Mountains, and Blue Ridge Mountains. While the mountains surrounding Asheville contain the Appalachian temperate rainforests, with areas receiving over an annual average precipitation of , the city itself is the driest location in North Carolina, with an annual average precipitation of only .
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Ashcroft, British Columbia, the only true desert in Canada, sits in the rain shadow of the Coast Mountains of Canada.
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Yellowknife, the capital and most populous city in the Northwest Territories of Canada, is located in the rain shadow of the mountain ranges to the west of the city.
Oceania
Australia
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In New South Wales and the Australian Capital Territory, 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. For comparison, Cooma receives of rain annually, whereas Batlow, on the western side of the ranges, receives of precipitation. Furthermore, Australia's capital Canberra is also protected from the west by the Brindabellas which create a strong rain shadow in Canberra's valleys, where it receives an annual rainfall of , compared to Adjungbilly's . In the cool season, the Great Dividing Range also shields much of the southeast coast (i.e. Sydney, the Central Coast, the Hunter Valley, Illawarra, the South Coast) from south-westerly polar vortex that originate from the Southern Ocean.
[ Rain Shadows by Don White. Australian Weather News. Willy Weather. Retrieved 24 May 2021.][ And the outlook for winter is … wet by Kate Doyle from The New Daily. Retrieved 24 May 2021.]
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In Queensland, the land west of Atherton Tableland in the Tablelands Region lies on a rain shadow and therefore would feature significantly lower annual rainfall averages than those in the Cairns Region. For comparison, Tully, which is on the eastern side of the tablelands, towards the coast, receives annual rainfall that exceeds , whereas Mareeba, which lies on the rain shadow of the Atherton Tableland, receives of rainfall annually.
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In Tasmania, 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.
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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 of rain per year and has myrtle beech rainforests much further west than anywhere else, while the area around Little River receives as little as annually, which is as little as Nhill or Longreach and supports only grassland. Also in Victoria, Omeo is shielded by the surrounding Victorian Alps, where it receives around of annual rain, whereas other places nearby exceed .
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Western Australia's Wheatbelt and Great Southern regions are shielded by the Darling Scarp to the west: Mandurah, near the coast, receives about annually. Dwellingup, inland and in the heart of the ranges, receives over a year while Narrogin, further east, receives less than a year.
Pacific Islands
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Hawaii also has rain shadows, with some areas being desert.
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New Caledonia lies astride the Tropic of Capricorn, between 19° and 23° south latitude. The climate of the islands is tropics, 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.
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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 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 . (see Nor'west arch for more on this subject).
South America
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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.
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Cuyo and 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.
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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.
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The Guianan savanna is in the rain shadow of the Guiana Highlands and stands in stark contrast to the surrounding rainforest environment.
See also
External links
