Evaporatedmoisture 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 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 (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
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 even
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 (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 (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.[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 of the Southern Hemisphere, between 30 and 50 degrees latitude.[4]
Examples of notable rain shadowing include:
Africa
Northern Africa
The
Sahara is made even drier because of two strong rain shadow effects caused by major mountain ranges (whose highest points can culminate up to more than 4,000 meters high). To the northwest, the
Atlas Mountains, covering the
Mediterranean coast for
Morocco,
Algeria and
Tunisia as well as to the southeast with the
Ethiopian Highlands, located in
Ethiopia around the
Horn of Africa. 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. The same phenomenon occurs in the Ethiopian Highlands, but this rain shadow effect is even more pronounced because this mountain range is larger, with the tropical
Monsoon of South Asia coming from the
Indian Ocean and from the
Arabian Sea. These produce clouds and rainfall on the windward side of the mountains, but the leeward side stays rain shadowed and extremely dry. This second extreme rain shadow effect partially explains the extreme aridity of the eastern Sahara Desert, which is the driest and the sunniest place on the planet.
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.[citation needed]
The Himalaya 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.[citation needed]
The
Verkhoyansk Range in eastern
Siberia is the coldest place in the
Northern Hemisphere, because the moist southeasterly winds from the Pacific Ocean lose their moisture over the coastal mountains well before reaching the
Lena River valley, due to the intense
Siberian High forming around the very cold continental air during the winter. One effect in the
Sakha Republic (Yakutia) is that, in
Yakutsk,
Verkhoyansk, and
Oymyakon, the average temperature in the coldest month is below −38 °C (−36 °F). These regions are synonymous with extreme cold.[citation needed]
The central region of
Myanmar is in the rain shadow of the
Arakan Mountains and is almost semi-arid with only 750 millimetres (30 in) of rain, versus up to 5.5 metres (220 in) on the
Rakhine State coast.
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.
The
Thar Desert is bounded and rain shadowed by the
Aravalli ranges to the southeast, the Himalaya to the northeast, and the Kirthar and Sulaiman ranges to the west.[citation needed]
The
Central Highlands of Sri Lanka rain shadow the northeastern parts of the island, which experience much less severe
summer monsson rains and instead have precipitation peaks in autumn and winter.
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.[5]
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
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
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 of the rainfall of
Fort William or
Skye. In Devon, rainfall at
Princetown on Dartmoor is almost three times the amount received 48 kilometres (30 mi) to the east at locations such as
Exeter and
Teignmouth.
The Fens of East Anglia receive similar rainfall amounts to
Seville.[7]
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 2,250 millimetres (89 in) and 3,575 millimetres (141 in), respectively,[8] while
Oslo receives only 760 millimetres (30 in), and
Skjåk, a
municipality situated in a deep valley, receives only 280 millimetres (11 in). Further east, the partial influence of the Scandinavian Mountains contribute to areas in east-central
Sweden around
Stockholm only receiving 550 millimetres (22 in) 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 377 metres (1,237 ft), 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.[9]
Southern Europe
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 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.
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 1,000 millimetres (39 in) on the Mediterranean side and
Bari with about 560 millimetres (22 in) on the Adriatic side.
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 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).[citation needed]
Caribbean
Throughout the
Greater Antilles, the southwestern sides are in the rain shadow of the trade winds and can receive as little as 400 millimetres (16 in) per year as against over 2,000 millimetres (79 in) on the northeastern, windward sides and over 5,000 millimetres (200 in) 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.[citation needed]
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.[11] This includes much of the
Basin and Range Province in the
United States and
Mexico.
The Dungeness Valley around
Sequim and
Port Angeles,
Washington lies in the rain shadow of the
Olympic Mountains. The area averages 10–15 inches 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 18-24 inches of precipitation each year.
Seattle is also affected by the rain shadow, albeit to a much lesser effect.[12] By contrast,
Aberdeen, which is situated southwest of the Olympics, receives nearly 85 inches of rain per year[13]
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.
The Cascades create a rain-shadowed
Columbia Basin area of
Eastern Washington and valleys in British Columbia, Canada - most notably the
Thompson and
Nicola Valleys which can receive less than 250 millimetres (10 in) of rain in parts, and the
Okanagan Valley (particularly the south, nearest to the US border) which receives anywhere from 12-17 inches of rain annually.[14][15]
The
Black Rock Desert is in the rain shadows of the Cascades and Sierra Nevada.
California's
Owens Valley is rain-shadowed by the Sierra Nevada.
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.[citation needed]
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 1,000 millimetres (40 in) 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.
Asheville, North Carolina sits in the rain shadow of the
Balsam,
Smoky, and
Blue Ridge Mountains. While the mountains surrounding Asheville contain the
Appalachian Temperate Rainforests, with areas receiving over an annual average precipitation of 2,500 millimetres (100 in) , the city itself is the driest location in North Carolina, with an annual average precipitation of only 940 millimetres (37 in) .[18][19][20]
Ashcroft, British Columbia, the only true desert in Canada, sits in the rain shadow of the
Coast Mountains of Canada.[21]
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.
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 4,000 millimetres (160 in), whereas
Mareeba, which lies on the rain shadow of the Atherton Tableland, receives 870 millimetres (34 in) of rainfall annually.
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
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. Also in Victoria,
Omeo is shielded by the surrounding
Victorian Alps, where it receives around 650 millimetres (26 in) of annual rain, whereas other places nearby exceed 1,000 millimetres (39 in).
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 kilometres (81 mi) further east, receives less than 500 millimetres (20 in) a year.
Pacific Islands
Hawaii also has rain shadows, with some areas being desert.[24] Orographic lifting produces the world's second-highest annual precipitation record, 12.7 meters (500 inches), on the island of
Kauai; the leeward side is understandably rain-shadowed.[1] The entire island of
Kahoolawe lies in the rain shadow of Maui's
East Maui Volcano.[citation needed]
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.
In 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 on the
western side. 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). (see
Nor'west arch for more on this subject).
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.[25] 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.