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Sky Surf: Earth Images of the Week January 13, 2012
Kelvin-Helmholtz waves
Kelvin-Helmholtz waves around Birmingham, Alabama, looked like something out of a science-fiction movie to some residents.
Residents around Birmingham, Alabama, were startled on December 16 by cloud formations unlike anything they had ever seen before.

What looked like something out of a surfer’s dream, a line of wave-shaped clouds paraded across the horizon in slow-motion.

The city’s local ABC television affiliate received phone calls from puzzled residents who asked: ”What are these tsunamis in the sky?”

Meteorologists later explained that they were actually seeing a well-known phenomenon in fluid dynamics known as Kelvin-Helmholtz waves.

More common in water, they occur in the ocean when the difference between the speed of the wind and surface water increased to the point that waves “break,” with their crests lurching forward.

In the sky, they similarly occur when winds are higher at the top of the clouds than they are at the base, in a atmospheric process known as velocity shear.

The lighter-density and faster-moving parts of the clouds slide on top of slower, thicker layers, dragging out the surface and creating the effect of a wave rolling on top of water.

The resulting series of waves can look like surf crashing on the coast.

The phenomenon is named after Lord Kelvin, a Scottish baron who along with German physicist Hermann von Helmholtz came up with an explanation for the freak occurrence.

Short video of the wave clouds