Around 30° N. and S., the poleward flowing air begins to descend toward the surface in subtropical high-pressure belts. The sinking air is relatively dry because its moisture has already been released near the Equator above the tropical rain forests. Near the center of this high-pressure zone of descending air, called the “Horse Latitudes,” the winds at the surface are weak and variable. The name for this area is believed to have been given by colonial sailors, who, becalmed sometimes at these latitudes while crossing the oceans with horses as cargo, were forced to throw a few horses overboard to conserve water. The surface air that flows from these subtropical high-pressure belts toward the Equator is deflected toward the west in both hemispheres by the Coriolis Effect. Because winds are named for the direction from which the wind is blowing, these winds are called the northeast trade winds in the Northern Hemisphere and the southeast trade winds in the Southern Hemisphere. The trade winds meet at the doldrums. Surface winds known as “westerlies” flow from the Horse Latitudes toward the poles. The “westerlies” meet “easterlies” from the polar highs at about 50-60° N. and S. Near the ground, wind direction is affected by friction and by changes in topography. Winds may be seasonal, sporadic, or daily. They range from gentle breezes to violent gusts at speeds greater than 300 kilometers/hour. [Illustration: These dunes in the Algodones Sand Sea of southeastern California move as much as 5 meters per year. The dunes in this photograph, looking south, move toward the east (left) (photograph by Peter Kresan).] [Illustration: Searles Lake, California (photograph courtesy of Kerr-McGee, Inc.).] Where Deserts Form Dry areas created by global circulation patterns contain most of the deserts on the Earth. The deserts of our world are not restricted by latitude, longitude, or elevation. They occur from areas close to the poles down to areas near the Equator. The People’s Republic of China has both the highest desert, the Qaidam Depression that is 2,600 meters above sea level, and one of the lowest deserts, the Turpan Depression that is 150 meters below sea level. Deserts are not confined to Earth. The atmospheric circulation patterns of other terrestrial planets with gaseous envelopes also depend on the rotation of those planets, the tilts of their axes, their distances from the Sun, and the composition and density of their atmospheres. Except for the poles, the entire surface of Mars is a desert. Venus also may support deserts. [Illustration: The Garlock fault, near the bottom of this Landsat image, is generally considered to be the geologic border between the Mojave Desert in the south and the Great Basin Desert in the north. The Great Basin contains more than 150 discrete desert basins separated by more than 160 mountain ranges.] Types of Deserts