A rain shadow is a dry area on the lee side of a mountainous area. The
mountains block the passage of rain-producing weather systems and cast
a "shadow" of dryness behind them. As shown by the diagram, the incoming
warm and moist air is drawn 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
drier side called the "rain shadow". Description
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. 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 down the leeward side of the
mountain, which increases the amount of moisture that it can absorb and creates
an arid region. Regions of notable rain shadow
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.
Examples of notable rain shadowing include:
= 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.
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 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 e.g. Northern Karnataka & Solapur, Beed, Osmanabad and Vidharba
Plateau of India. The central region of Myanmar is in the
rain shadow of the Arakan Mountains and is almost semi-arid with only 750
millimetres of rain as against as much as 5.5 metres on the Rakhine State
coast. The Tokyo, Japan plain in the winter
months experiences significantly less precipitation than the rest of the
country by virtue of surrounding mountain ranges, including the "Japan
Alps", blocking prevailing northwesterly winds originating in Siberia.
= South America= The Atacama Desert in Chile is the
driest non-polar desert on Earth because it is blocked from moisture on both
sides. 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.
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 concequence that the Patagonian Desert develop more fully at
the Atlantic coast contributing to shaping the climatic pattern known as
the Arid Diagonal. 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=
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. Most rainshadows in the western United States
are due to the Sierra Nevada and Cascades.
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 Cascades also cause 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 10 inches of rain in parts, and the Okanagan Valley which
receives anywhere from 12-17 inches of rain annually.
Hawaii also has rain shadows, with some areas being desert. Orographic lifting
produces the world's second-highest annual precipitation record, 12.7
meters, on the island of Kauai; the leeward side is understandably
rain-shadowed. The entire island of Kahoolawe lies in the rain shadow of
Maui's East Maui Volcano. 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 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.
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. To a lesser
extent, this rain shadow extends to other parts of the eastern Olympic
Peninsula, Whidbey Island, and parts of the San Juan Islands and southeastern
Vancouver Island around Victoria, British Columbia.
The Mojave, Black Rock, Sonoran, and Chihuahuan deserts all are in regions
which are rain shadowed. The Owens Valley in the United States,
behind the Sierra Nevada range in California.
The aptly named 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 40 inches 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. The Shenandoah Valley of Virginia,
wedged between the Appalachian Mountains 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.
On 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
per year as against over 2,000 millimetres on the northeastern,
windward sides and over 5,000 millimetres over some highland areas.
= Europe= The Pennines of Northern England, Welsh
mountains, Lake District and 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. The contrast is
even stronger further north, where Aberdeen gets around a third of the
rainfall of Fort William or Skye. The Fens of East Anglia receive similar
rainfall amounts to Seville. The Cantabrian Mountains form 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, which are the driest spots in Europe mostly a result of the
mountain 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 areas of Gap
and Briançon in France, the district of Zernez in Switzerland.
The eastern part of the Pyrenean mountains in the south of France.
The Plains of Limagne and Forez in the northern Massif Central, France, are
also relatively rainshadowed. 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. The valley of the Vardar River and south
from Skopje to Athens is in the rain shadow of the Prokletije and Pindus
Mountains. On its windward side the Prokletije has the highest rainfall in
Europe at around 5,000 millimetres with small glaciers even at mean annual
temperatures well above 0 °C, but the leeward side receives as little as 400
millimetres. 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, Worcester receives only around 200 millimetres and is useful
only for grazing. The Sahara Desert is made even drier
because of two strong rain shadow effects caused by some major mountains
ranges. 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. Similar levels of aridity and dryness are only seen in the Atacama
Desert, located in Chile and Peru. Desert regions in the Horn of Africa
such as the Danakil Desert are all influenced by the air heating and drying
produced by rain shadow effect of the Ethiopian Highlands, too.
= Oceania= 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 to 8,900 mm liquid water equivalent per
year and creating large glaciers. To the east of the Southern Alps, scarcely 50
km from the snowy peaks, yearly rainfall drops to less than 760 mm and some areas
less than 380 mm.. 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, 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
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 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 annually. Dwellingup, 40 km
inland and in the heart of the ranges, receives over 1,000 millimetres a year
while Narrogin, 130 km further east, receives less than 500 millimetres a
year. See also
Orographic precipitation Foehn wind
Lake effect References
External links USA Today on rain shadows
Weather pages on rain shadows