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A short introduction to deserts: they're among the most inhospitable places on Earth, typified by extremes of both temperature and aridity. Yet deserts are home to a wealth of fascinating wildlife, and have played host to numerous important developments in human civilisation.

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DRIEST OF THE DRY

Deserts are typified by their aridity and low rainfall. Many also experience other forms of precipitation--fog, dew, hail and snow--but the overall amount of atmospheric precipitation received on average each year is characteristically low. In extreme cases, such mean annual totals are less than ten millimetres. Quillagua, a village in Chile's Atacama desert, holds the title of 'driest place on Earth', with an annual average precipitation of just half a millimetre between 1964 and 2000. The Atacama is a very dry desert: 100 years of meteorological data from the town of Iquique gives an annual average precipitation of around two millimetres for the 20th century.

Another characteristic of rainfall in deserts is its great variability from year to year, which in many respects makes annual average statistics seem like nonsense. A very arid desert area may go for several years with no rain at all (Iquique received no precipitation whatsoever during the 1960s). It may then receive a whole 'average' year's rainfall in just one storm. For example, a period of heavy rainfall over a week in April 2006 at the coastal town of Luderitz in the Namib Desert brought a total of 102 millimetres, or about six times its annual average rainfall of 16.7 millimetres. Ltideritz received well above its annual average rainfall on each of three days that week.

SALT OF THE EARTH

Many of the lakes found in desert depressions are, at least to some degree, salty. Salt is common in deserts and tends not to be washed away through the soils because of the low rainfall. Hence, salts accumulate in certain parts of the landscape. When the lakes dry out, a hard saline crust often forms, and this crust is frequently covered by a large polygonal pattern of slightly raised cracks that are formed as the salt dries.

These saline desert lakes are frequently called 'playas', but numerous other names are also used. These include sabkha in Arabia, chott in North Africa and salar in South America. The Salar de Uyuni, 3,600 metres above sea level on the Altiplano in Bolivia, is the largest salt flat on Earth. It's very flat indeed, exhibiting less than a metre of vertical relief over its 9,000-square-kilometre area. Like many playas, Uyuni is usually flooded in the rainy season: from December to March in this part of the Altiplano.

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CULTURAL DESERTS

Several fundamental aspects of human culture have arisen from desert beginnings. These include the domestication of plants and animals, the creation of the city and the advent of at least three major world religions.

Several desert species were among the first to undergo domestication--the process of deliberate selection and breeding for human use. Some of the earliest cultivated food crops were wheat and barley--two desert annuals--in the so-called Fertile Crescent of the Near East around 7,000-9,000 years ago. Cattle, sheep and goats were also first domesticated in the same area at around the same time. This early transition in society, from hunting and gathering to herding and farming, was also associated with the emergence of the world's first urban civilisations on some of the great desert rivers: the Nile, Tigris-Euphrates and Indus.

The deserts of the Middle East also gave rise to three major religions. Judaism, Christianity and Islam developed from desert visionaries whose profound religious experiences each formed the basis of a faith.

THE HOTTEST PLACE ON THE PLANET

Desert interiors regularly experience air temperatures in excess of 40[degrees]C over several consecutive days thanks to clear skies and plentiful sunshine. The title of 'hottest place on Earth' has been claimed by several locations, all of them in deserts. Death Valley, in California, USA, held the record for the highest recorded air temperature of 56.7[degrees]C from 1913 to 1922, but lost the world record in September 1922 when a temperature of 58[degrees]C was recorded at El Azizia in northern Libya. Another place that often appears in compilations of meteorological records is Dallol, in Ethiopia's Danakil Desert, which holds the record for the highest average annual air temperature (34.5[degrees]C).

Hotter temperatures may well have occurred elsewhere in desert locations without instruments to record them. Sensors carried on satellites have the advantage over fixed weather stations of being able to collect data with continuous geographical coverage. Attempts to locate the 'hottest place on Earth' using satellite data have focused on measuring the temperature at the Earth's surface, which is typically hotter than the temperature of the air.

Global satellite maps of the highest annual maximum land-surface temperatures confirm that the planet's hottest places are in arid, sparsely vegetated landscapes. Land-surface temperatures regularly exceed 60[degrees]C in parts of the Sahara, Middle East, Australia and western North America. The highest land-surface temperature documented by satellite was in the Lut desert, Iran, where 70.7[degrees]C was recorded in 2005.

Nonetheless, the direct measurement of ground surfaces has returned still greater temperatures. A bare sand surface at Repetek, a desert research station in the Karakum in Turkmenistan, has been measured at 79.4[degrees]C, and in the Red Sea hills, north of Port Sudan, a sand temperature of 83.5[degrees]C has been recorded.

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WAVES IN A SEA OF SAND

The most iconic desert landscapes are made up of sand dunes. Their sleek forms and fluid curves come in a diversity of sizes, ranging from small accumulations tens of centimetres in height and length to huge mounds measured in hundreds of metres. Shapes vary considerably too, dictated mainly by the amount of sand and the pattern of winds.

The most common form of desert dune is a long, slightly sinuous ridge known as a linear dune. They can extend nearly 200 kilometres in length and reach heights of 200-300 metres. Good examples can be found in the Namib, the southwest Kalahari, parts of the Sahara and in several Australian deserts.

Dunes take on more complex shapes in regions where winds blow from numerous different directions. One of the most distinctive is the star dune, a peak of sand shaped like a pyramid with radiating sand ridges. These are found in parts of the Sahara, the Arabian Peninsula and the Namib. The world's highest dunes are the star dunes of the Badain Jaran in Chinese Inner Mongolia, which are more than 400 metres high.

Most of the world's dunes are concentrated in vast areas known as sand seas, or ergs, a North African Arabic word. The largest of these is the Rub' al-Khali, the Empty Quarter of Arabia, which covers about 600,000 square kilometres, an area larger than France. Several different types of dune are found in the Rub' al-Khali, including linear and star dunes. A distinct hierarchy of sand shapes are found in ergs, often producing a compound pattern. The largest--so-called mega dunes are typically covered with smaller dunes, and these smaller dunes themselves are covered with sand ripples.

Dunes are not only found in the world's deserts--they occur on beaches, on the seabed, in snow, and on the planet Mars--but they are certainly very distinctive features of many desert landscapes. Around one third of the Arabian subcontinent is covered by sandy deserts, for example, but elsewhere deserts dunes are much less common. There are no large ergs in any North or South American desert, where dunes cover less than one per cent of the arid zone.

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DESERT RESOURCES

A number of natural resources have been exported from desert regions for millennia. The ancient Egyptians sent expeditions to the Land of Punt, today part of Somalia, to bring back gum resins from trees such as frankincense and myrrh. These products were transported to Europe during the times of the ancient Greeks and Romans by camel caravans along the so-called Incense Route across the deserts of Arabia. They were highly prized for religious ceremonies, as well as being used to fumigate clothes, in medicine, cosmetics and cooking.

This trading network included the remote Indian Ocean island of Socotra (today part of Yemen), also a source of frankincense and myrrh but probably best known in the ancient world as the sole supplier of cinnabar, or dragon's blood. This deep-red liquid oozes from the injured bark of the Socotra dragon tree, a member of the genus Dracaena, and was used as a pigment in paint, to stain the wood for Italian violins and even to fasten loose teeth. Roman soldiers and gladiators appreciated its disinfectant and healing properties, using it to treat wounds.

Salt mined from the Danakil depression in northern Ethiopia also has a lengthy history. Bars of Danakil rock salt, known as amole in Amharic, were used as currency in Ethiopia for more than 1,000 years. When the Italians invaded Ethiopia during the 1930s, they were very disappointed to find bank vaults stacked not with gold, but with amole bars. Blocks of salt are still mined by hand in the Danakil and carried on camel trains up to the Ethiopian Highlands, although the salt is used today only for culinary purposes.

Minerals and reserves of fossil energy are the most widely used modern desert commodities, of course. Some of the world's largest fields of oil and natural gas lie beneath the deserts of the Middle East and North Africa. These energy reserves are non-renewable resources that will eventually run out. In the long term, probably beyond the current century, solar power is expected to meet most of our global energy needs. If so, the high levels of sunshine typical of so many deserts will enable them to maintain their central position on the world's energy scene.

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A STING IN THE TAIL

Scorpions are remarkably resilient creatures, well adapted to the rigours of desert life. A body covering of wax makes them more tolerant of high temperatures and arid conditions than most animals (although scorpions aren't restricted solely to deserts). The wax reduces water loss, which has been measured in one species at just one part per 10,000 of its body moisture, the lowest recorded for any animal. Scorpions are also remarkably tolerant of dehydration--they are able to survive losing up to 40 per cent of their body fluid.

Scorpions don't have to eat regular meals, thanks to their low metabolic rate and lengthy periods of inactivity. Some have been known to survive without food for more than a year.

Despite these capabilities, scorpions typically spend the daylight hours in burrows or seek some form of cover, and restrict their predatory surface activities to the night-time. This combination of behavioural traits is probably the most important adaptive mechanism for scorpions inhabiting desert areas, as it is for many other creatures.

WATER HOARDERS

Swollen or fleshy tissue designed to store water is a common feature of many plants adapted to arid lands. These are the 'succulents', which have developed fleshy, column-shaped stems (providing water-storage capacity), reduced leaves (limiting transpiration and water loss) and spines (protecting the leaves from herbivores).

Probably the best known desert succulents are the plants from the cactus family, which are native only to the Americas (with one exception among some 2,000 species). Although some cacti have been introduced to other parts of the world, desert plants elsewhere that look like cacti to the untrained eye aren't cacti at all. The apparently similar members of the euphorbia and milkweed families occur mainly in the deserts of Asia and Africa, where the ecological role they play is analogous to that of the cacti in the North and South American deserts.

Deserts: A Very Short Introduction by Nick Middleton, published by Oxford University Press (RRP 7.99 [pounds sterling]), is available to readers at the special price of 6 [pounds sterling] (including free postage and packing in the UK only). Please call 01536 741 017 and quote reference WEBDES10. Please allow 14 days for delivery. Offer ends 31 March 2010, subject to availability

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Title Annotation:physical geography: DESERTS
Comment:A short introduction to deserts: they're among the most inhospitable places on Earth, typified by extremes of both temperature and aridity.
Author:Middleton, Nick
Publication:Geographical
Article Type:Essay
Geographic Code:4EUUK
Date:Mar 1, 2010
Words:1997
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