Which desert covers most of northern Africa?

The Sahara stretches from the Atlantic coast of Mauritania and Western Sahara to the Red Sea, and from the Mediterranean margins down to the semi-arid Sahel, forming a belt of arid land across almost the entire width of the continent. It sits where it does because of atmospheric circulation rather than accident: air rising over the equator sheds its moisture as tropical rain, then descends around thirty degrees north as dry, warming air that suppresses cloud formation. The same band of subsiding air explains the Arabian and Sonoran deserts at similar latitudes. Contrary to the picture most people carry, only a minority of the Sahara is sand sea; much of it is stony plateau, gravel plain and bare rock, with mountain ranges such as the Ahaggar and Tibesti rising high enough to catch occasional snow. Rock art across the region records a much wetter period a few thousand years ago, when hunters and herders lived among lakes and grassland.

Dust blown off the Sahara crosses the Atlantic and fertilises the Amazon rainforest with mineral nutrients.