File:Saharan Dust between Hurricanes Issac and Helene (MODIS 2018-09-12).jpg

English: On September 9, 2018, the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a true-color image of two storm systems spinning westward across the Atlantic Ocean in a dusty atmosphere. Tropical Storm Isaac leads the parade in the west and the larger, more intense Hurricane Helene spins in the east. A river of dust, originally from the Saharan Desert, flows between the two systems and some dust also appears to be entrained in the outer convective bands of Helene.

Massive Saharan dust storms as well as many tropical storms and hurricanes can thank their origins from a similar source – the strong winds that howl across relatively flat northern Africa. As these winds roar across the desert, they pick up massive amounts of dust, which can become entrained in what is known as the Saharan Air Layer (SAL) and travel across the ocean, reaching South (and sometimes North) America. If the winds roar a little further south they pick up less dust, but air that pours off the coast of Africa near the Cape Verde Islands does pick up moisture – and moist air plus rotation are ingredients for the formation of tropical systems. Most of the storms that threaten North America travel across the Atlantic Ocean from origins near the Cape Verde Islands.

Recent research suggests that as storm systems run into the Saharan Air Layer they begin to struggle. Storms need moist air in order to intensify, and the SAL is exceedingly dry and tends to suck the moisture – and the life – out of smaller storm systems. A large storm, such as Hurricane Helene which carries maximum sustained winds of 110 mph (175 km/h) and tropical-storm-force winds extending more than 115 miles (185 km) from center, can draw moisture from dust-free southern waters even while fighting the dry air effect of the SAL. Hurricane Helene is forecast to increase in intensity somewhat before turning northward later in the week, so any dampening effect of the dry, hot, dust-laden winds on the strength of this storm will be minimal.