Bet you never expected to see a title like that while looking through fall foliage pictures! But that's what happens when your photographer is also a weather geek. :) Kelvin-Helmholtz instability, named after physicists Lord Kelvin and Hermann von Helmholtz, occurs when two adjacent fluids are moving at different speeds, creating shear, causing one of the fluids to become disturbed and form wave-like features. This is the process that causes waves to form on the ocean, because the wind is moving faster than the water, but Kelvin-Helmholtz waves can also be seen in the atmosphere under very specific conditions.
So, those rippling clouds that look like waves really
waves. This phenomenon is very rare, and I considered myself lucky that it occurred on a day when I was out shooting (and especially, on a day where I was starting to complain to myself about it being too cloudy...)
This is a seven-shot panorama, but about half of it was cropped from the right so that the main subject here was more visible. There's a tiny sliver of Echo Lake visible in the background.