Although desert climates vary, they are always extreme.
In hot deserts, days are usually sunny and skies cloudless. During the summer, daytime air temperatures between 105° and 110°F (43.8° and 46.8°C) are not unusual. A record air temperature of 136.4°F (62.6°C) was measured in the Sahara Desert on September 13, 1922, and that was in the shade! Because there is little vegetation, rocks and soil are exposed to the Sun, which may cause ground temperatures in the hottest deserts to exceed 175°F (80°C). Nights, however, are much cooler. The lack of cloud cover allows heat to escape and the temperature may drop 25 degrees or more after the Sun sets. At night temperatures of 50°F (10°C) or less are common, and they may even drop below freezing.
Winters in cold deserts at latitudes midway between the polar and equatorial regions can be bitter. In the Gobi Desert, for example, temperatures below freezing are common. Blizzards and violent winds often accompany the icy temperatures.
Rainfall also varies from desert to desert and from year to year. The driest deserts may receive no rainfall for several years, or as much as 17 inches (430 millimeters) in a single year. Rainfall may be spread out over many months or fall within a few hours. In the Atacama Desert of Chile, considered the world's driest desert, more than half an inch (12.5 millimeters) of rain fell in one shower after four years of drought. Such conditions often cause flash floods, which sweep vast quantities of mud, sand, and boulders through dry washes, gullies, and dry river beds (sometimes called wadis or arroyos). The water, however, soon evaporates or disappears into the ground. The Atacama is also the site of the world's longest drought, where no rain fell for 400 years (from 1571 until 1971).
In coastal deserts, fog and mist may be common. Fog occurs when cold-water ocean currents cool the air and moisture condenses.
Source: "Desert." U·X·L Encyclopedia of Biomes, Vol. 1. U·X·L, Detroit: 2000.