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Cloudy Curls

Cloudy Curls

Appearing like swirls of soft-serve ice cream, the twists and twirls of these clouds off the western coast of Africa are a demonstration of the artistic nature of our planet and of fluid dynamics.

These cloud patterns—known as von Kármán vortices—are a familiar atmospheric phenomenon, especially in areas where trade winds are prevalent. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this image of vortices created by the passage of winds and air masses over Madeira Island (north of the Canary Islands) on July 9, 2020. Satellites have also spotted von Kármán vortices in this region at night.

Such vortices can form nearly anywhere that fluid flow—including an air mass, as shown here—is disturbed by a solid object. In this case, winds blowing across the ocean are disturbed by a small island (Madeira Island) poking above the surface of the North Atlantic. The air mass, and clouds moving with it, blows around instead of over the island.

Physicist Theodore von Kármán, a co-founder of NASA’s Jet Propulsion Laboratory, was the first to describe the physical processes that create these chains of spiral eddies. As winds are rerouted around the landmasses, the flow slows down and creates a vertical wall of whirling air—with faster wind flowing past slower wind below. These sheets can wrap themselves into vortices and shed alternately off the two sides of the island. The clouds within the air stream, acting like “dusting fingerprints,” help show the details of the interrupted air flow. The pattern of the spirals depends on the intensity of the wind.

NASA Earth Observatory image by Joshua Stevens, using MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview. Story by Kasha Patel.

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