Evaporatedmoisture frombodies of water (such asoceans and largelakes) is carried by the prevailingonshore breezes towards the drier and hotter inland areas. When encountering elevatedlandforms, the moist air isdriven upslope towards thepeak, where it expands, cools, and its moisturecondenses and starts toprecipitate. If the landforms are tall and wide enough, most of thehumidity will be lost to precipitation over thewindward side (also known as therainward side) before ever making it past the top. As the air descends the leeward side of the landforms, it is compressed and heated, producingFoehn winds thatabsorb moisture downslope and cast a broad "shadow" ofdry climate region behind themountain crests. This climate typically takes the form ofshrub–steppe,xeric shrublands, ordeserts.
The condition exists because warm moist air rises byorographic lifting to the top of a mountain range. As atmospheric pressure decreases with increasing altitude, the air has expanded andadiabatically cooled to the point that the air reaches its adiabaticdew point (which is not the same as itsconstant pressure dew point commonly reported in weather forecasts). At the adiabatic dew point, moisture condenses onto the mountain and it precipitates on the top andwindward 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 ofadiabatic compression (as with foehn winds) down the leeward side of the mountain, which increases the amount of moisture that it can absorb and creates anarid region.[1]
There are regular patterns ofprevailing winds found in bands round Earth'sequatorial region. The zone designated thetrade winds is the zone between about 30° N and 30° S, blowing predominantly from the northeast in theNorthern Hemisphere and from the southeast in theSouthern Hemisphere.[2] Thewesterlies are the prevailing winds in themiddle latitudes between 30 and 60 degreeslatitude, 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 theRoaring Forties of the Southern Hemisphere, between 30 and 50 degrees latitude.[4]
TheSahara 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, theAtlas Mountains, covering theMediterranean coast forMorocco,Algeria andTunisia. On thewindward side of theAtlas Mountains, the warm, moist winds blowing from the northwest off theAtlantic 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 asorographic rainfall and after this process, the air is dry because it has lost most of its moisture over theAtlas Mountains. On theleeward 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 mountain ranges on the eastern side ofMadagascar provide a rain shadow for the country's western portion.
The windward side of the island ofMadagascar, 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 ofRéunion.[citation needed]
TheVerkhoyansk Range in easternSiberia is the coldest place in theNorthern Hemisphere, because the moist southeasterly winds from thePacific Ocean lose their moisture over the coastal mountains well before reaching theLena River valley, due to the intenseSiberian High forming around the very cold continental air during the winter. One effect in theSakha Republic (Yakutia) is that, inYakutsk,Verkhoyansk, andOymyakon, 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 ofMyanmar is in the rain shadow of theArakan Mountains and is almost semi-arid with only 750 millimetres (30 in) of rain, versus up to 5.5 metres (220 in) on theRakhine State coast.
The plains around Tokyo, Japan – known asKanto plain – during winter experiences significantly less precipitation than the rest of the country by virtue of surrounding mountain ranges, including theJapanese Alps, blocking prevailing northwesterly winds originating in Siberia.
TheThar Desert is bounded and rain shadowed by theAravalli ranges to the southeast, the Himalaya to the northeast, and the Kirthar and Sulaiman ranges to the west.[citation needed]
TheCentral Highlands of Sri Lanka rain shadow the northeastern parts of the island, which experience much less severesummer monsoon rains and instead have precipitation peaks in autumn and winter.
Most of Iran is rain-shadowed by theAlborz mountains in the north (just south of theCaspian Sea), hence the country's mostly (semi) arid climate.Lake Urmia (centre) and surrounds rain-shadowed by the snowyZagros mountains to the west.
The Plains ofLimagne andForez in the northernMassif Central, France are also relatively rainshadowed (mostly the plain of Limagne, shadowed by theChaîne des Puys (up to 2000 mm; 80" of rain a year on the summits and below 600mm; 20" atClermont-Ferrand, which is one of the driest places in the country).
ThePiedmont wine region of northern Italy is rainshadowed by the mountains that surround it on nearly every side:Asti receives only 527 mm (20¾") of precipitation per year, making it one of the driest places in mainland Italy.[5]
Some valleys in the innerAlps are also strongly rainshadowed by the high surrounding mountains: the areas ofGap andBriançon inFrance, the district ofZernez inSwitzerland.
ThePennines of Northern England, the mountains ofWales, theLake District and theHighlands of Scotland create a rain shadow that includes most of the eastern United Kingdom, due to the prevailing south-westerly winds.Manchester andGlasgow, for example, receive around double the rainfall ofLeeds andEdinburgh respectively (although there are no mountains between Edinburgh and Glasgow). The contrast is even stronger further north, whereAberdeen gets around a third of the rainfall ofFort William orSkye. In Devon, rainfall atPrincetown on Dartmoor is almost three times the amount received 48 kilometres (30 mi) to the east at locations such asExeter andTeignmouth.The Fens of East Anglia receive similar rainfall amounts toSeville.[7]
Iceland has plenty ofmicroclimates courtesy of the mountainous terrain.Akureyri on a northerlyfiord receives about a third of the precipitation that the island ofVestmannaeyjar off the south coast gets. The smaller island is in the pathway ofGulf Stream rain fronts with mountains lining the southern coast of the mainland.
TheScandinavian Mountains create a rain shadow for lowland areas east of the mountain chain and prevents theOceanic climate from penetrating further east; thusBergen and a place likeBrekke inSogn, west of the mountains, receive an annual precipitation of 2,250 millimetres (89 in) and 3,575 millimetres (141 in), respectively,[8] whileOslo receives only 760 millimetres (30 in), andSkjåk Municipality, amunicipality 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-centralSweden aroundStockholm only receiving 550 millimetres (22 in) annually. In the north, the mountain range extending to the coast in aroundNarvik andTromsø cause a lot higher precipitation there than in coastal areas further east facing north such asAlta or inland areas likeKiruna across the Swedish border.
TheSouth 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 theBaltic 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 ofKalmar,Östergötland andSödermanland along with the offshore island ofGotland on the leeward side of the slopes. Coastal areas in this part of Sweden usually receive less precipitation than windward locations inAndalusia in the south of Spain.[9]
Cantabrian Mountains in the north, which rain-shadow most of Spain
TheCantabrian Mountains form a sharp division between "Green Spain" to the north and the dry central plateau. The northern-facing slopes receive heavy rainfall from theBay of Biscay, but the southern slopes are in rain shadow. The other most evident effect on the Iberian Peninsula occurs in theAlmería,Murcia andAlicante areas, each with an average rainfall of 300 mm (12"), which are the driest spots in Europe (seeCabo de Gata) mostly a result of the mountain range running through their western side, which blocks the westerlies.
In the NorthernApennines ofItaly, Mediterranean cityLa Spezia receives twice the rainfall of Adriatic cityRimini on the eastern side. This is also extended to the southern end of the Apennines that see vast rainfall differences betweenNaples with above 1,000 millimetres (39 in) on the Mediterranean side andBari with about 560 millimetres (22 in) on the Adriatic side.
The valley of theVardar River and south fromSkopje toAthens is in the rain shadow of theAccursed Mountains andPindus 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]
Throughout theGreater 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 inCuba, where this phenomenon leads to theCuban cactus scrub ecoregion, and the island ofHispaniola (which contains the Caribbean's highest mountain ranges), which results in xeric semi-arid shrublands throughout theDominican Republic andHaiti.[citation needed]
On the largest scale, the entirety of the North AmericanInterior Plains are shielded from the prevailingWesterlies carrying moist Pacific weather by theNorth 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 theBasin and Range Province in theUnited States andMexico.
The Dungeness Valley aroundSequim andPort Angeles,Washington lies in the rain shadow of theOlympic Mountains. The area averages 10–15 in (250–380 mm) of rain per year. The rain shadow extends to the eastern Olympic Peninsula,Whidbey Island, parts of theSan Juan Islands, andVictoria, British Columbia which receive between 18–24 in (460–610 mm) 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 in (2,200 mm) of rain per year[13]
The east slopes of the Coast Ranges in central and southern California cut off the southernSan Joaquin Valley from enough precipitation to ensure desert-like conditions in areas aroundBakersfield.
The Cascades create a rain-shadowedColumbia Basin area ofEastern Washington and valleys in British Columbia, Canada - most notably theThompson andNicola Valleys which can receive less than 250 millimetres (10 in) of rain in parts, and theOkanagan Valley (particularly the south, nearest to the US border) which receives anywhere from 12–17 inches of rain annually.[14][15]
TheBlack Rock Desert is in the rain shadows of the Cascades and Sierra Nevada.
California'sOwens Valley is rain-shadowed by the Sierra Nevada.
Death Valley in the United States, behind both thePacific Coast Ranges ofCalifornia and theSierra 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 belowsea level which tends to cause high pressure and dry conditions to dominate due to the greater weight of the atmosphere above.[citation needed]
TheColorado Front Range is limited to precipitation that crosses over theContinental 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 ofDenver andPueblo, Colorado, typically receive only about 12 to 19 inches. Thus, theContinental Divide acts as a barrier for precipitation. This effect applies only to storms traveling west-to-east. When low pressure systems skirt theRocky 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 theBalsam,Smoky, andBlue Ridge Mountains. While the mountains surrounding Asheville contain theAppalachian 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 theCoast Mountains of Canada.[21]
Yellowknife, the capital and most populous city in theNorthwest Territories ofCanada, is located in the rain shadow of the mountain ranges to the west of the city.
InQueensland, the land west ofAtherton Tableland in theTablelands Region lies on a rain shadow and therefore would feature significantly lower annual rainfall averages than those in theCairns 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), whereasMareeba, which lies on the rain shadow of the Atherton Tableland, receives 870 millimetres (34 in) of rainfall annually.
InTasmania, 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.
InVictoria, the western side ofPort Phillip Bay is in the rain shadow of theOtway Ranges. The area betweenGeelong andWerribee is the driest part of southern Victoria: the crest of the Otway Ranges receives 2,000 millimetres (79 in) of rain per year and hasmyrtle beech rainforests much further west than anywhere else, while the area aroundLittle River receives as little as 425 millimetres (16.7 in) annually, which is as little asNhill orLongreach and supports only grassland. Also in Victoria,Omeo is shielded by the surroundingVictorian Alps, where it receives around 650 millimetres (26 in) of annual rain, whereas other places nearby exceed 1,000 millimetres (39 in).
Western Australia'sWheatbelt andGreat Southern regions are shielded by theDarling Range to the west:Mandurah, near the coast, receives about 700 millimetres (28 in) annually.Dwellingup, 40 km (25 miles) inland and in the heart of the ranges, receives over 1,000 millimetres (39 in) a year whileNarrogin, 130 kilometres (81 mi) further east, receives less than 500 millimetres (20 in) a year.
Hawaii also has rain shadows, with some areas being desert.[24]Orographic lifting produces the world's second-highest annual precipitation record, 12,700 mm (500 in), on the island ofKauai; the leeward side is understandably rain-shadowed.[1] The entire island ofKahoolawe lies in the rain shadow of Maui'sEast Maui Volcano.[citation needed]
New Caledonia lies astride theTropic of Capricorn, between 19° and 23° south latitude. The climate of the islands istropical, 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 theSouth Island ofNew Zealand is found one of the most remarkable rain shadows anywhere on Earth. TheSouthern 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 largeglaciers on thewestern 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). (seeNor'west arch for more on this subject).
The Andes mountains block rain and moisture from theAmazon basin to the west (Bolivia).
TheAtacama Desert inChile is the driest non-polar desert on Earth because it is blocked from moisture by theAndes Mountains to the east while theHumboldt Current causes persistent atmospheric stability.
Cuyo andEastern Patagonia is rain shadowed from the prevailing westerly winds by theAndes 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 thePatagonian Desert develop more fully at the Atlantic coast contributing to shaping the climatic pattern known as theArid 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 theAndes.
TheGuajira Peninsula in northern Colombia is in the rain shadow of theSierra 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.
