Saharan dust formed giant arcs off the west coast of Africa on July 1, 2009. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite took this natural-color picture the same day. The camel-colored dust forms a series of arcs, one east of Cape Verde, one west of the archipelago, and one to the south. The dust plume nearest the coast is both the smallest and most concentrated.
Source points for this dust storm are not obvious in this image, and the dust may have traveled from far inland. Spikes in land surface temperature in the Sahara Desert can lead to instability in the lowest layer of the atmosphere, lofting dust particles in the air. Westward-moving dust can continue heating as it travels over the Sahara. Once it moves off Africa’s west coast, the air layer is undercut by a cooler air layer from the Atlantic Ocean. The hot, dusty air layer over the cool, moist air causes a temperature inversion that prevents atmospheric mixing. As a result, dust often travels across the Atlantic relatively intact.
NASA image courtesy MODIS Rapid Response Team, Goddard Space Flight Center. Caption by Michon Scott.
