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 (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 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.
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 (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. 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
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 central highlands extends into southeastern Mozambique in spite of the Mozambique Channel in between. The same is true for the island of Réunion. 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. 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 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 northwestern China.
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
The eastern side of the Sahyadri ranges on the Deccan Plateau including: Vidarbha, North Karnataka, Rayalaseema and western Tamil Nadu. Gilgit and Chitral, Pakistan, are rainshadow areas. 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. 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
The peaks of the Caucasus Mountains to the west and Hindukush and Pamir to the east rain shadow the Karakum and Kyzyl Kum deserts east of the Caspian Sea, as well as the semi-arid Kazakh Steppe. They also cause vast rainfall differences between coastal areas on the Black Sea such as Rize, Batumi and Sochi contrasted with the dry lowlands of Azerbaijan facing the Caspian Sea. The semi-arid Anatolian Plateau is rain shadowed by mountain chains, including the Pontic Mountains in the north and the Taurus Mountains in the south. The High Peaks of Mount Lebanon rain-shadow the northern parts of the Beqaa Valley and Anti-Lebanon Mountains. The Judaean Desert, the Dead Sea and the western slopes of the Moab Mountains on the opposite (Jordanian) side are rain-shadowed by the Judaean Mountains. The Dasht-i-Lut in Iran is in the rain shadow of the Elburz and Zagros Mountains and is one of the most lifeless areas on Earth. The peaks of the Zagros Mountains rain-shadow the northern half of the West Azerbaijan province in Iranian Azerbaijan (above Urmia), as manifested by the province's dry winters relative to those in the windward part of the region (i.e. Kurdistan Region and Hakkâri Province in Turkey). Much of the Mesaoria Plain of Cyprus is in the rain shadow of the Troodos Mountains and is semi-arid.
Europe Central Europe 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 600 mm; 20" 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 of precipitation per year, making it one of the driest places in mainland Italy. 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 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 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. 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
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. 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. The eastern part of the Pyrenees in the south of France (Cerdagne).