Something lovely to contemplate over the weekend.
They’re called Kelvin-Helmholtz clouds:
When two different layers of air are moving at different speeds in the atmosphere, a wave structure will often form. The upper layers of air are moving at higher speeds and will often scoop the top of the cloud layer into these wave-like rolling structures. The clouds often form on windy days where there is a difference in densities of the air, such as in a temperature inversion.