These rare Kelvin-Helmholtz clouds look like waves on the ocean.

In fact they are breaking waves generated by the same fluid dynamics that creates wind-driven waves on water.

Both are caused by Kelvin-Helmholtz instability which occurs at the boundary where two fluids flow by each other at different speeds or densities.  The air above these clouds is moving faster left-to-right than the air below them.  The boundary is very turbulent and becomes more so when the waves break.

Kelvin-Helmholz instability can be described mathematically and its effect plotted over time.  This silent video by VanjaZ shows a yellow fluid on top flowing faster than the black fluid on the bottom.  Talk about turbulence!

We rarely see K-H clouds because the atmosphere has to be just right to make them stand alone. The curling waves disappear in seconds, wiped out by chaos as soon as they break.

(photo by GrahamUK on Wikimedia Commons; video by VanjaZ on YouTube)