Rain Shadow deserts

Formally defined by Stephen Marshak in 2001, a desert is a region that is so arid or dry that it contains no permanent streams, except for rivers that bring water in from temperate regions elsewhere, and supports vegetation on no more than 15 percent of its surface. In general, a desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than 10 inches (25 centimeters) per year. A common definition distinguishes between true deserts, which receive less than 10 inches (250 millimeters) of average annual precipitation, and semi-deserts or steppes, which receive between 10 inches (250 millimeters) and 16 to 20 inches (400 to 500 millimeters). Deserts can also be described as areas where loss of water by evapotranspiration is more than gained as precipitation. Measurement of rainfall alone cannot provide an accurate definition of what a desert is because being arid also depends on evaporation, which depends in part on temperature.

Deserts are sometimes classified as hot and cold deserts. Cold deserts can be covered in snow or ice—frozen water unavailable to plant life. These are more commonly referred to as tundra if a short season of above-freezing temperatures is experienced, or as an ice cap if the temperature remains below freezing year-round, rendering the land almost completely lifeless. In some parts of the world, deserts are created by a rain shadow effect in which air masses lose much of their moisture as they move over a mountain range; other areas are arid by virtue of being very far from the nearest available sources of moisture. Deserts are also classified by their geographical location and dominant weather pattern as trade wind, mid-latitude, rain shadow, coastal, monsoon, or polar deserts. Former desert areas presently in nonarid environments are paleodeserts.

Deserts take up about one-fifth (20 percent) of the Earth's land surface. Hot deserts usually have a large diurnal and seasonal temperature range, with high daytime temperatures and low nighttime temperatures (due to extremely low humidity). In hot deserts the temperature in the daytime can reach 113 degrees F (45 degrees C) or higher in the summer, and dip to 32 degrees F (0 degrees C) or lower at nighttime in the winter.

Rain shadow deserts form when tall mountain ranges block clouds from reaching areas in the direction the wind is going. As the air moves over the mountains, it cools and moisture condenses, causing precipitation on the windward side. When that air reaches the leeward side, it is dry because it has lost the majority of its moisture, resulting in a desert. The air then warms, expands, and blows across the desert. The warm, desiccated air takes with it any remaining moisture in the desert.

What Is a Rain Shadow?

A rain shadow is an area of dry land that lies on the leeward (or downwind) side of a mountain. Winds carry air masses up and over the mountain range and as the air is driven upward over the mountain, falling temperatures cause the air to lose much of its moisture as precipitation. Upon reaching the leeward side of the mountain, the dry air descends and picks up any available moisture from the landscape below. The resulting profile of precipitation across the mountain is such that rainfall and moist air prevail on the windward side of a mountain range, while arid, moisture-poor air prevails on the leeward side of the mountain range. Mountain ranges act as barriers to the flow of air across the surface of the Earth. They act to squeeze moisture out of the air.

When a parcel of warm air reaches a mountain range, it is lifted up the mountain slope, cooling as it rises. This process is known as orographic lifting and the cooling of the air often results in large clouds, precipitation, and even thunderstorms. Orographic lifting is a fascinating process that keeps the windward sides of mountain ranges moist and filled with vegetation but the leeward sides dry and deserted.

The following locations are examples of dry, rain shadow regions and the mountain ranges that shield them:

• • The Gobi Desert lies in the rain shadow of the Himalayas.
• • The Atacama Desert lies in the rain shadow of the Andes.
• • The Patagonia region lies in the rain shadow of the Andes.
• • Death Valley lies in the rain shadow of the Pacific Coast Ranges of California and the Sierra Nevada.
• • The city of Spokane in the state of Washington lies in the rain shadow of the Cascade Mountain Range (Spokane receives little rainfall). Seattle, Washington, lies on the windward side of the Cascades (it receives generous amounts of rainfall).

The rain shadow condition exists because warm moist air rises through orographic lifting to the top of a mountain range or large mountain. Because atmospheric pressure decreases as altitude increases, 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 because of the process of precipitation, it has lost much of its initial moisture. Typically, descending air also gets warmer because of adiabatic compression down the leeward side of the mountain, creating an arid region.

There are regular patterns of prevailing winds found in bands around the Earth's equatorial region. The zone designated the trade winds is the zone between about 30 degrees north and 30 degrees south, 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.

Asia

Examples of rain shadowing in Asia include the Tirunelveli (India), cut off from the monsoons by the Agasthiyamalai hills, creating a rain shadow region. The peaks of the Caucasus Mountains to the west, the Alborz Mountains to the south, and the ranges tied to the Himalayas 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 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 northwestern China. The Great Indian Desert, or 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.

South America

The Atacama Desert in Chile is the driest non-Antarctic 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 that 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

Most rain shadows in North America are because of mountain ranges, notably the Sierra Nevada and Cascades, 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 dry precipitation regime of the Great Plains of western Canada and the central United States can be attributed in large part to the rain shadow of the North American Cordillera. The Mojave, Black Rock, Sonoran, and Chihuahuan Deserts are all 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. The Rocky Mountain Front Range in Montana shows areas of limited precipitation as long as the systems passing over the mountain range come from the west. The clouds dry out considerably by the time they reach the peaks of the mountain range. 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 desertlike conditions in areas around Bakersfield. The areas to the east of the Coast Mountains and the Cascade Range in the province of British Columbia, Canada, and the American states of Washington, Oregon, and Idaho are located in a rain shadow. 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 the amount received in nearby Port Angeles and approximately 10 percent of that which falls in Forks on the western side of the mountains.

The rain shadow effect even occurs in the eastern United States. Although much more humid than any obvious deserts or steppes, the Shenandoah Valley, mostly in western Virginia, lying between the Blue Ridge and the Appalachian Mountains, is drier than areas to the east and west because the modest mountains reduce rainfall within the valley.

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 farther north, where Aberdeen gets around a third the rainfall of Fort William or Skye. The Fens of East Anglia receive rainfall amounts similar to Seville. 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 less than 12 inches (300 millimeters) and the driest spot in Europe, 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 rain shadowed by the high surrounding mountains. The Plains of Limagne and Forez in the northern Massif Central, France, are also relatively rain shadowed (mostly the plain of Limagne, shadowed by the Chaîne des Puys, with up to 78 inches, or 2,000 millimeters, of rain annually on the summits and below 24 inches, or 600 millimeters, on Clermont-Ferrand, which is one of the driest places in the country). The Piedmont wine region of northern Italy is rain shadowed by the mountains that surround it on nearly every side; Asti receives only 20–21 inches (527 millimeters) of precipitation each year. Athens is shielded strongly by mountains from the strong moisture-bearing winds of the Adriatic Sea and receives only a quarter the rainfall of most of Albania.

The Scandinavian Mountains create a rain shadow for lowland areas east of the mountain chain and prevent the oceanic climate from penetrating farther east; thus Bergen west of the mountains receives almost 89 inches (2,250 millimeters) precipitation annually while Oslo receives only about 30 inches (760 millimeters), and Skjåk, a municipality situated in a deep valley, receives only 11 inches (280 millimeters).

Africa

The windward side of the island of Madagascar, which sees easterly onshore winds, is wet and 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. The formation of the Atlas Mountains has been deemed at least partially responsible for the climatic change that eventually created the Sahara. There is a strong rain shadow effect to the south side of the mountains.

Oceania

New Caledonia lies astride the Tropic of Capricorn, between 19 degrees and 23 degrees 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.

Hawaii also has rain areas of the islands classified as desert. Orographic lifting produces the world's second-highest annual precipitation record, 500 inches (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.

In New Zealand is to be found one of the most remarkable rain shadows anywhere on Earth. On the South Island, the Southern Alps intercept moisture coming off the Tasman Sea. The mountain range is home to significant glaciers and about 250 inches (6,300 millimeters) to 350 inches (8,900 millimeters) liquid water equivalent per year. To the east and down slope of the Southern Alps, scarcely 30 miles (50 kilometers) from the snowy peaks, yearly rainfall drops to less than 30 inches (760 millimeters) and in some areas to less than 15 inches (380 millimeters).

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 (also states of Australia), the Monaro is shielded by both the Snowy Mountains to the northwest and coastal ranges to the southeast. In Victoria, the area around 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: whereas the crest of Otway Ranges receives 79 inches (2,000 millimeters) of rain per year, the area around Little River receives as little as 17 inches (420 millimeters) annually, which is as little as Nhill or Longreach.

Western Australia's Wheatbelt and Great Southern regions are shielded by the Darling Range to the west: Mandurah, near the coast, receives about 28 inches (700 millimeters) annually. Dwellingup, about 24 miles (40 kilometers) inland and in the heart of the ranges, receives over 39 inches (1,000 millimeters) a year, while Narrogin, 80 miles (130 kilometers) farther east, receives less than 20 inches (500 millimeters) a year.