Air and moisture movement patterns in the atmosphere are determined by the spinning of the Earth and the radiation from the sun. Most of the world's deserts occur in two belts of high pressure called the horse latitudes that lie about 20-30 degrees north and south of the equator. Warm moist air from the equator rises and moves toward the poles. This air eventually loses its moisture and cools. This air then moves down toward the Earth's surface at the horse latitudes, thus preventing surface moisture from rising into the air to form clouds. So these areas at the horse latitudes are perpetually dry and prime areas for desert formation.