Spin up the clouds: Sky spirals, explained

September 20, 2011

Andrew Prince

USGS
Air flows create sheets of von Karman vortices as they cross Alaska's Aleutian Islands. This image was acquired by the Landsat 7 Enhanced Thematic Mapper. View larger version.
Von Karman vortices over the Pacific Ocean, captured by Skylab on Aug. 1, 1973.

Heard Island is a very remote place. About two-thirds of the way between Madagascar and Antarctica, deep in the Indian Ocean, the ice-covered island boasts a population of zero and a 9,006-foot volcanic peak. And on a cloudy day, if we zoom out — way out, as only NASA can do — we see that the lonely island can make its mark in the skies.

This photo, added to NASA's Flickr feed on Monday, shows how wind blowing across Heard Island hits the mountain and leaves vortices in its wake. The swirls of air form just as spinning eddies do in a river. The clouds take the same shape as the spinning air, drawing neat, white spirals.

Update: A couple of sharp commenters pointed out that this phenomenon is called a "von Karman vortex street," and it's a well-studied concept in the field of fluid dynamics. (NASA has a more technical explanation of why it happens here.) Below, we've added a few more examples from the NASA archives.

Copyright 2011 National Public Radio.