rain shadow

An example of a rain shadow.

A rain shadow is a dry area on the lee back side of a mountainous area. The mountains block the passage of rain-producing weather systems casting a "shadow" of dryness behind them.

As shown by the diagram to the right, the incoming warm and moist air is "pulled" by the prevailing winds towards the top of the mountains where it condenses and precipitates before it crosses the top. The air, without much moisture left, advances behind the mountains creating a dryer side called "rain shadow."

Description

The condition exists because warm moist air rises through "orographic lifting" to the top of a mountain range. Because of decreasing atmospheric pressure 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 because of the process of precipitation, it has lost much of its initial moisture. Typically, descending air also gets warmer because of adiabatic compression (see Foehn winds) down the leeward side of the mountain, creating an arid region.^[1]

Regions of notable rain shadow

The Tibetan Plateau (top), perhaps the best example of a rain shadow. Rain does not make it past the Himalayas, leading to an arid climate on the leeward side of the mountain range.

There are regular patterns of prevailing winds found in bands round the 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. The strongest westerly winds in the middle latitudes can come in the Roaring Forties between 30 and 50 degrees latitude.^{[citation needed]}

Examples of notable rain shadowing include:

Asia

The Agasthiyamalai hills cut off Tirunelveli (India) from the monsoons, creating a rainshadow region

• 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.
• The Judean Desert and the Dead Sea are rain shadowed by the Judean Hills.
• 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 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.
• 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 Great Indian Desert or Thar desert is bounded and rain shadowed by the Aravalli ranges to the south-east, the Himalaya to the northeast, and the Kirthar and Sulaiman ranges to the west.
• Eastern Side of Sahyadri ranges on Deccan Plateau of India.

South America

• The Atacama Desert in Chile is the driest desert on Earth because it is blocked from moisture on both sides (by the Andes Mountains to the east and high pressure over the Pacific at a latitude which keeps moisture from coming in from the west).
• The Argentinian wine region of Mendoza is almost completely dependent on irrigation, using water drawn from the many rivers that drain glacial ice from the Andes. The nearby Chilean wine region of Valle Central on the other hand, is situated on the Chilean side of the Andes and experiences a maritime climate.
• Patagonia is rain shadowed from the prevailing westerly winds by the Andes range and is arid (e.g., in Santa Cruz few spots are capable of cultivation, the pastures being poor, water insufficient and salt lagoons fairly numerous).
• 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.

North America and the Caribbean

Most rainshadows in the western United States are due to the Sierra Nevada and Cascades,^[2] that intercept rain and snowfall that would otherwise reach a valley in the lee of the mid-latitude prevailing westerlies.

• The deserts of the Basin and Range Province in the United States and Mexico, which includes the dry areas east of the Cascade Mountains of Oregon and Washington and the Great Basin, which covers almost all of Nevada and parts of Utah are rain shadowed.
• The east slopes of the Coast Ranges in central and southern California also cut off the southern San Joaquin Valley from enough precipitation to ensure desert-like conditions in areas around Bakersfield.
• San Jose, California is usually drier than the rest of the San Francisco Bay Area because of the rain shadow cast by the Santa Cruz Mountains.
• The Dungeness Valley around Sequim, Washington lies in the rain shadow of the Olympic Mountains. The area averages 10–15 inches of rain per year, less than half of the amount received in nearby Port Angeles and approximately 10% of that which falls in Forks on the western side of the mountains.
• The dry precipitation regime of the Great Plains of Western Canada and the central USA can be attributed in large part to the rain shadow of the North American Cordillera.
• The Mojave, Black Rock, Sonoran, and Chihuahuan deserts all are in regions which are rain shadowed.
• The aptly named Death Valley in the United States, behind both the Pacific Coast Ranges of California and the Sierra Nevada range, is one of the driest places on the planet.
• The Colorado Front Range is limited to the rainfall that makes it over the continental divide. While many places west of the divide receive as much as 40 inches (1,000 mm) of precipitation throughout the year, some places on the east side, notably Evergreen, Colorado and Denver, Colorado only receive about 17-19 inches. Thus, the continental divide acts as a barrier for the precipitation. This effect only applies to west to east storms. When low pressure areas skirt the Rocky Mountains to the south, they can give high precipitation to the east side and none to the west side.
• In the islands of Hispaniola, Cuba and Jamaica, 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.

Europe

• The Pennines of Northern England, the Welsh Mountains, and the Highlands of Scotland create a large rain shadow that covers almost the entirety of the Eastern United Kingdom, with Glasgow and Manchester for example receiving around double the rainfall of Edinburgh and York respectively. The contrast is even stronger further north, where Aberdeen gets around a third the rainfall of Fort William or Skye. The Fens of East Anglia receive similar rainfall amounts to Seville.^[3]
• The Cantabrian Mountains make a sharp divide 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 most evident effect on the Iberian Peninsula occurs in the Almería, Murcia and Alicante areas, each with an average rainfall of 300 mm and the driest spot in Europe (see Cabo de Gata) mostly a result of the mountainous range running through their western side, which blocks the westerlies.
• Some valleys in the inner Alps are also strongly rainshadowed by the high surrounding mountains.
• 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 2000mm of rain a year on the summits and below 600mm on 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 each year, making it one of the driest places in mainland Italy.^[4]
• The valley of the Vardar River from Skopje to Athens is in the rain shadow of the Prokletije, which on its windward side 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 on its leeward side receives as little as 400 millimetres (16 in).
• 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 west of the mountains receives 2,250 mm precipitation annually while Oslo receives only 760 mm, and Skjåk, a municipality situated in a deep valley, receives only 280 mm.

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 same is true for the island of Réunion.
• In Western Cape Province, the Breede River Valley and the Karoo lie in the rain shadow of the Cape Fold Mountains and are arid; whereas the wettest parts of the Cape Mountains can receive 1,500 millimetres (59 in), Worcester receives only around 200 millimetres (8 in) and is useful only for grazing.
• The formation of the Atlas Mountains has been deemed at least partially responsible for the climatic change which eventually created the Sahara. There is a strong rain shadow effect to the south side of the mountains.

Oceania

• Hawaii also has rain shadows, with some areas being desert, much to the surprise of many tourists.^{[citation needed]} 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.
• 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 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. 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).
• 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 New South Wales and Victoria (both states of Australia), the 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.
• Also 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.
• 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 km further east, receives less than 500 millimetres (20 in) a year(365 days).

See also

• Orographic precipitation
• Foehn wind
• Lake effect

References

External links

• USA Today on rain shadows
• Weather pages on rain shadows