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.