Clouds form tsunami in the sky

It's the kind of scene you'd expect to see at the beach. But for Alabamans, the sky turned on its own show, with clouds appearing in the shape of a number of breaking waves.

Excited Birmingham residents grabbed their cameras to catch the natural phenomenon, known as "Kelvin-Helmholtz waves", reported.

The amazing visual display, most commonly seen at the beach, occurs when fast-moving liquid slides over the top of slower moving liquid, creating a kind of turbulence that drags the surface of the slow liquid.

"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 science website Life's Little Mysteries. "Over this cloudy, cold, slow-moving layer is probably a warmer and faster-moving layer of air."

Normally, the difference between the fast and slow layers is too small for the effect to occur. But when the wind speed reaches just the right level, the two layers burst into random turbulence like those seen in waves at the beach.

For Alabamans, the aerial spectacular came about when the two layers came together in a fast-slow combination that was just right.

"What [these pictures] show is air between these two atmospheric layers that is just very close to that threshold for turbulence, and mixing to mix the two layers together," he said.