Residents of Birmingham, Alabama woke last Friday to skies that looked more like an ocean, with a series of huge "wave" shaped crests rolling slowly across the sky. Experts have now explained the phenomenon as perfect examples of "Kelvin-Helmholtz" waves, when a fast layer moves over a slower layer and drags the top along creating a curled shape.
You'll recognize Kelvin-Helmholtz waves when put in context of the ocean. This kind of turbulence forms when the top layer — air — moves faster than the water layer below. This creates what we all know as wave crests or breaks when the water surges forward.
The phenomenon is less common in the sky, but when it does occur the principle is the same.
In the case of last week's clouds, Chis Walcek, a meteorologist at the Atmospheric Science Research Center at the State University of New York, Albany explained to Life's Little Mysteries, "In the picture 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. Over this cloudy, cold, slow-moving layer is probably a warmer and faster-moving layer of air."
Generally the differences between the air speed and the temperature of the layers of the atmosphere tend to run from one extreme to the other. In most cases the difference between the layers are small so they will merely glide on top of one another. On the other end of the spectrum if the difference between the layers is too great the meeting of the layers turns into random turbulence.
The Kelvin-Helmholtz waves are only formed when atmospheric conditions between the two layers are 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," according to Walcek.