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.
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. …