This photograph shows the continent of Africa and the Arabian Peninsula.
Clouds cover much of central Africa.
When Arab merchants and soldiers arrived in North Africa around 650 a.d., shortly after the Islamicization of Arabia, they became aware that the Sahara's southern boundary advanced and receded like oceanic tides. Desert climates are dry, but a lack of precipitation is not the end of the story. For a region to be a desert, the annual potential evaporation must be greater than the annual precipitation. In fact, it is better to refer to deserts as drylands . The drylands of the world can be classified along a continuum ranging from subhumid to hyperarid, as the figure below shows.