Winter is a dusty season off the coast of North Africa. As temperatures drop and high pressure builds on the continent, strong winds known as the harmattan blow east across the Sahara. They pick up sand and dust from the desert and loft it over the ocean.
In late January 2018, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite caught a glimpse of Saharan dust bathing the Cape Verde islands, which lie about 650 kilometers (400 miles) off the coast of Senegal. The images were acquired on January 22 and 23.
Note the wind shadows, wakes, and vortices—areas with less dust density—on the leeward side of the islands. Winds blowing from the northeast and east run into the high volcanic peaks of the islands, which block some of the dust and alter the air flow. Note, too, how much denser the dust plume grows on the second day.
Hundreds of millions of tons of dust blow out of Africa every year, crossing the Atlantic all the way to the Caribbean and the Gulf of Mexico. This dust fertilizes the ocean with nutrients that can promote plankton growth, but it can also carry fungus and disease-causing microorganisms that damage coral reefs.