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to 50 feet (15 meters) a year. Sand grains are blown up the back slope of the dune, and tumble over the front or “slip face.” Since sand moves from the back to the front, the dune keeps moving in the same direction as the wind. The barchan is the most common type of dune. It is cres- cent-shaped, with its “horns” pointing downwind. Barchan dunes can reach a length of 1,200 feet (350 meters). The rather similar parabolic dunes are also “horned,” but their long horns point into the wind rather than away from it. Transverse dunes lie at right-angles to the wind, and occur where there is plenty of sand but little vegetation. Some trans- verse dunes in the Sahara are 62 miles (100 km) long. Longitudinal dunes occur where wind blows from two direc- tions. They lie in line with the prevailing wind, and may be nearly 300 feet (90 meters) high. Water in the Desert Deserts are the driest places on the planet, but despite this they are shaped more by water than by wind. Water is a very pow- erful force, and an occasional rainstorm may have more effect on the landscape than months of wind. A desert may go without rain for years, then suddenly receive a year’s supply of precipitation all at once. When these rainstorms strike, flash floods rush like miniature tidal waves along dry watercourses carved by similar floods over thousands of years. These stream beds, which are usually bone-dry, are called arroyos in America, and wadis in North Africa and the Middle East. Fed by rainwater cascading off the hills and rocks above, each flood bears a heavy load of debris. The water is

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Deserts