NASA Earth Observatory image by Joshua Stevens, using VIIRS day-night band data from the Suomi National Polar-orbiting Partnership. Story by Kasha Patel, with image interpretation by George Young.

“The basic idea is that flow over, and around, a mountainous island slows down,” said George Young, professor of meteorology at Penn State University. This 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. They can subsequently travel downwind from the island to create “vortex streets,” as seen in this image. The pattern of the spirals depends on the intensity of the wind.

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Nighttime Swirls