From NASA's Earth Observatory:
Thick dust is seen blowing through the Sahara Desert this month, extending over parts of Algeria, Mali, Mauritania and Morocco.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite captured the image on June 23. The dust plume spanned hundreds of kilometers north to south and east to west.
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.