A series of huge breaking waves lined the horizon in Birmingham, Ala., on Friday (Dec. 16), their crests surging forward in slow motion.

Amazed Alabamans took photos of the clouds and sent them to their local weather station, wondering what they were.

Experts say the clouds were pristine examples of “Kelvin-Helmholtz waves.”

This type of turbulence forms when a fast-moving layer of fluid slides on top of a slower, thicker layer, dragging its surface.

Chris Walcek, a meteorologist at the Atmospheric Sciences Research Center at the State University of New York, Albany, says that fast-moving air high in the sky can drag the top of slow-moving, thick clouds underneath it in much the same way.

“In the pictures [of the Birmingham sky] there is probably a cold layer of air near the ground where the wind speed is probably low. That is why there is a cloud or fog in that layer,” Walcek told reporters.

“Over this cloudy, cold, slow-moving layer is probably a warmer and faster-moving layer of air.”

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