Distribution is especially uneven over the continents because of mountain ranges. Heavy rain often drenches slopes where the air rises, leaving dry the slopes where the air descends. The convergence causes the air to rise, producing clouds and rain. Certain regions alternate between rainy and dry seasons due to shifting winds. In regions near the tropics, winds known as monsoons blow in one direction in the winter and in the opposite direction in the summer.
Monsoon winds bring extremely heavy rain to southern Asia in the summer.
On both sides of the equator, generally along the Tropic of Cancer and the Tropic of Capricorn, are regions where the air usually sinks to the surface. Rainfall is therefore light in those areas, and so deserts have formed in them. Sinking air is responsible for the Sahara and the Kalahari Desert in Africa and a band of deserts in the western and central parts of Australia. Away from the tropics, the western coasts of large land masses have more rainfall than their central regions. To the east lies the Great Basin, a desert area that covers parts of Oregon, California, Nevada, Idaho, Utah, and Wyoming.