extremes of hot and cold can split rocks apart, letting the wind in to batter and re-shape them. One of the most powerful forces shaping any desert is the wind. Hot winds in the Sahara and other deserts regularly reach 60 miles per hour (100 kilometers per hour). These winds blow away loose material, such as rock flakes loosened by the intense heat or cold, and carve and polish rock surfaces with sand grains like a giant sand-blasting machine. The wind cannot pick up sand grains, but it can roll them along the ground. If they are rolling fast enough, sand grains bounce up into the air when one grain bumps into another. Once in the air, they are blown along by the wind before they fall to the ground again. The dark and choking “sand storms” that occur in deserts are actually dust storms. Dust particles are much smaller than sand grains, and they can be blown thousands of feet into the sky. But blown sand stays close to the ground. You could sit in a chair and see above a sand storm if there were no dust parti- cles also blowing in the storm. Desert winds can clear sand from large areas, leaving boul- ders and pebbles behind. The cleared areas are called desert pavements. Ancient Peruvians made giant desert pictures by moving the stones to reveal paler soil beneath. Sand Dunes Dunes are formed when wind that is carrying blown sand slows down, or meets an obstacle. The sand falls to the ground as a small heap, but it doesn’t just sit there. Unless plants grow to bind it, the dune can move across the desert at a rate of up

What Is a Desert?


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