Both hot and cold deserts have areas of low rainfall. The driest areas fall into the hyper-arid category, which encompasses 4.2 percent of the world's total land area. Rainfall in hyper-arid regions seldom is over 100 mm (4 inches) per year, is irregular, and sometimes doesn't fall for several years. Reasons for aridity include distance from oceanic sources of moisture, isolation from weather-making storm systems, and geographic features such as high mountain ranges or cold offshore ocean currents that harvest moisture from the air.
The driest area on Earth is within the Atacama Desert of Peru and Chile. This coastal desert is 600 miles long, going from the Pacific inland to the pampas grasslands and the dry highland altiplano. Areas of absolute desert in the center of the Atacama are without recorded rainfall, at least during the time humans have been recording it. Annual precipitation is 10 mm (0.04 inches), mostly from fog. Rainfall occurs two to four times a century. Frequent fogs keep the temperatures relatively cool, averaging about 18 degrees Celsius (65 degrees Fahrenheit), and results in high relative humidity of about 75 percent. Large areas are without vegetation of any kind.
The Sahara Desert of northern Africa is the largest desert in the world. This hot desert has recorded a high temperature of 58 degrees Celsius (136.4 degrees Fahrenheit) at Al-Aziziya, Libya. Rainfall averages about 10 cm (4 inches) annually, with many areas receiving less, sometimes none for 100 years or more. Many areas have little to sparse vegetation. A second very dry African desert, the Namib, exists along the coast of western Namibia. Rainfall varies from an average of 5 mm (0.19 inches) in the west to about 85 mm (3.3 inches) in the east. Fog is also common in the Namib.
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Called the Empty Quarter, the Rub al-Khali desert of Arabia is the largest sand desert in the world. Most of it has an average annual rainfall below 50 mm (2 inches), but an area in the south of this desert has a mean annual rainfall of less than 16 mm (0.6 inches). The Rub al-Khali falls within the Arabian Desert that covers almost all of Saudi Arabia and extends into nearby Middle Eastern countries. Rainfall in the Arabian Desert is usually less than 100 mm (4 inches) a year.
Antarctica's very dry, cold desert gets most of its precipitation as snow, with an equivalent of about 150 mm (6 inches) of water annually. Over the center of the land mass, less than 50 mm (1.9 inches) of snowfall occurs. Cold winter deserts of central Asia include the Gobi Desert of China and Mongolia, which averages about 178 mm (7 inches) of rain yearly. Central areas receive about 25 to 50 mm (1 to 2 inches) of rain annually. China's Taklamakan Desert has an average of about 20 mm (0.78 inches) annually in its center, with 50 mm (2 inches) occurring along the edges. North America's driest spot, Death Valley, is in the cold-winter Mojave Desert. It has an average rainfall of less than 5 cm (2 inches). No rain fell during 1929 or 1953.