Saharan dust had spread across the Canary Islands and Madeira. The dust started blowing a couple of days earlier in Algeria and Mali, and traveled hundreds of kilometers toward the northwest. Over the Atlantic Ocean, the dust made a giant turn toward the east, in the direction of the Mediterranean Sea.
Sand seas provide ample material for dust storms, and such seas extend across substantial portions of Algeria, Mali, and Mauritania. Sand seas also cover parts of Western Sahara. In addition to sandy desert, this region is covered with impermanent rivers, and the fine sediments of dry riverbeds can also be borne aloft by winds. (EarthObservatory)