Kelvin-Helmholtz clouds (also classified as fluctus by the WMO) display a series of curling, breaking wave crests. They are among the most spectacular and short-lived cloud formations, typically lasting only a few minutes.
Shear instability
They are produced by Kelvin-Helmholtz instability: when two adjacent air layers move at very different speeds, the interface undulates and the crests curl over and break. For pilots, their presence indicates severe wind shear and turbulence at that level. The same phenomenon occurs in oceans, rivers, and the atmospheres of Jupiter and Saturn.