Most of the dry, warm air that descends at 25 to 30° north or south loops back to move at surface level in the reverse direction of’ its outward journey. Gradually it is drawn back toward the tropics, is How deserts form 15 The Namib Desert, shown here in a satellite image, is maintained by the cold, dry Benguela current. recharged with heat and moisture, and so continues its movement. The end result is a cycle of air moving from the surface at the tropics up into the atmosphere, and then either northeast or southeast, sinking back to the surface at 25 to 30° north or south; this cooler air then returns toward the tropics from the northeast or southeast in the form of the trade winds. This air circulation forms corkscrewlike spirals north and south of the equator,
known to climatologists as Hadley Cells.
The Hadley Cells are not self-contained. Some of the warm air descending at 25 to 30° north or south flows, not toward the equator again, but away from it, toward the middle latitudes. There it mixes with cold air coming from the polar regions, creating “battlefields” between warm and cold known as fronts. The fronts provide many temperate regions with their changeable weather.