The Sahara Desert in Northern Africa, which is categorized as a subtropical hot desert, was formed primarily by the effects of dry, hot subtropical air forced into the middle latitudes by atmospheric currents. The scarcity of soil is a result of the lack of moisture in the air, which severely limits the chemical weathering that soil formation requires. Because hot air rises, cooler air enters to replace it in the form of wind, which can be extreme, scorching and heavily dust-laden in the Sahara Desert.
The Sahara Desert has been in its present state since about 1600 B.C., and it began forming after temperatures were increased significantly by new weather patterns caused by shifts in the Earth's axis. About 4,300 years ago, the region in which the Sahara Desert is now located is believed to have been moister and supportive of vegetation.
Covering more than 3 million square miles, the Sahara Desert is almost as large as the United States or China. Sand dunes can reach heights of almost 600 feet. Landforms in the desert have been shaped by the dry, hot winds and include salt flats, gravel plains, dry valleys and stone plateaus. Oases are formed when underground aquifers are able to reach the surface and are capable of supporting some forms of life. The central portion of the desert contains little, if any vegetation and in some areas, years can pass without rainfall.