Several eagle-eyed readers captured photos of what looked like rolling surf in the skies over Seattle Thursday evening. But it wasn't Mother Nature "Hanging 10", they're known as Kelvin Helmholtz clouds.
I'm not sure if Lord Kelvin and German scientist Hermann von Helmholtz were avid surfers (famous physicists are well known for their happenin' parties on the beach, right?), but if so, they would probably be happy to know these wave-like clouds bear their names.
The clouds are caused when you have wind shear -- that is, layers of air moving in different directions. As those layers interact with clouds, you can get turbulence that causes these impressive wave-like formations to occur.
Or, more detailed, from the University of Illinois, Champaign website: "Billow clouds are created in the upward branch of each of the eddies if the air within this branch has a high enough relative humidity that, upon lifting, the air parcel reaches saturation. Individual billow clouds generally have life times of a few minutes. The presence of billow clouds provides a visible signal to aviation interests of potentially dangerous turbulence."
Or, an even more detailed physical explanation from The University of Reading in the UK:
"When (potentially) warm air lies above colder air, the interface between the two air masses is statically stable. However, if a vertical wind shear exists across the interface, so the two air masses are in relative motion, then the interface can become dynamically unstable. The waves that develop are known as Kelvin-Helmoltz billows, and may occur in stably stratified layers when the Richardson Number (Ri) falls below 0.25. The Richardson number is defined as the ratio of the static stability, N2, to the square of the vertical wind shear."
OR... how about just some compelling video evidence that demonstrates this quite clearly?
Here is time lapse video captured from atop the University of Washington Atmospheric Sciences Building. You can see how two cloud layers are moving in almost exactly the opposite direction. As the day gets into evening, you can see some of the tube-type clouds form, and then just before sunset, watch in the bottom left corner and you'll see the "breaking-wave" cloud formations:
And here is another animation that shows how these clouds form.
Lord Kelvin, of course, is better known for the Kelvin temperature scale that is based on absolute zero, but I don't know -- I think these clouds are cooler, if not literally, then figuratively :)
Here are a few more photographs we received, courtesy of Laura Johnson: