Rare Kelvin-Helmholz clouds in Alabama

Yahoo reports, "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."

NOAA defines the wave-clouds as "Kelvin-Helmholtz clouds are a result of wind shear in a stable atmosphere. The wind speed at the top of the cloud layer is faster than that at the bottom, which rolls the cloud into wave shapes. These clouds form in stable layers of the atmosphere, such as near a temperature inversion. While Kelvin-Helmholtz waves are not uncommon in the atmosphere, it is relatively rare to get the "breaking wave" cloud shapes shown here."

As a personal note, in all the Kelvin-Helmholtz clouds I viewed online, including the particular ones NOAA says are very rare, the photo of the ones in Alabama blows them all away. They are stunning. I believe the perfection of their formation is as rare as the hole punch cloud. Or this hole punch cloud. Or the tubular cloud.

Anyway, signs in the sky...
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