From the National Oceanic and Atmospheric Administration’s description of this image of Saharan dust drifting out over the eastern Atlantic:
The dust is part of the Saharan Air Layer (SAL), a mass of very dry, dusty air which forms over the Sahara Desert during the late spring, summer, and fall months and usually moves over the tropical North Atlantic Ocean every 3 to 5 days. Associated with large amounts of dust and strong winds, the SAL can have a significant impact on the intensity and formation of tropical cyclones. Its dry air can actually weaken tropical cyclones by promoting downdrafts around the storm, while its strong winds can substantially increase the vertical wind shear in and around the storm environment. Although it is not yet clear what effect the SAL’s high dust content has on a storm’s intensity, recent studies have suggested that it can affect the formation of clouds.
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