Extreme dryness is one of the Sahara's chief characteristics Except in a few of the higher mountainous areas, the average annual rainfall nowhere exceeds 5 inches (130 mm). Most areas receive much less, and the annual total is highly variable Some areas may have no rain for several years and then receive 5 inches or more m one brief downpour. The chief cause of the Sahara's dryness is the northeast trade winds, which blow toward the Equator all year As the air moves southward it becomes warmer (which increases its water-holding capacity) and quickly absorbs any available moisture. Under these conditions, precipitation is rare.
Daytime temperatures, especially in summer, are among the hottest in the world. Highs of more than 100° F. (38° C.) are common. Azizia, Libya, holds the world record high temperature of 136.4° F. (58° C.). Because the air is so dry and has so few clouds, the temperature drops quickly after sunset. Differences of as much as 50° F. (28° C.) between day and night occur regularly, and overnight freezes are common during winter.