For a morning, the sky looked like a surfer's dream: 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 are these tsunamis in the sky?"

Experts say the clouds were pristine examples of "Kelvin-Helmholtz waves." Whether seen in the sky or in the ocean, this type of turbulence always forms when a fast-moving layer of fluid slides on top of a slower, thicker layer, dragging its surface.