Deserts and Grasslands

The Sahara, covering roughly 9.2 million square kilometers of North Africa, is the largest hot desert on Earth, formed primarily by its position beneath the descending, moisture-shedding air of the subtropical high-pressure belt near 30 degrees north, where the Hadley circulation suppresses cloud formation and rainfall across most of the year. Its landscape is far from uniformly sandy: true sand seas, or ergs, such as the Erg Chebbi in Morocco, cover a minority of its area, while rocky plateaus known as hamada and gravel plains called reg dominate much larger portions. Life in the Sahara depends on extreme physiological and behavioral adaptation, exemplified by the fennec fox, whose oversized ears dissipate heat, and by the dromedary camel, capable of tolerating substantial dehydration and rehydrating rapidly when water becomes available. Oases fed by underground aquifers, such as those in Egypt's Western Desert, have historically supported human settlement and trade routes across an otherwise largely uninhabitable expanse.