Matthew Cobb called my attention to this link from GeekOSystem, which shows an amazing cloud formation near Birmingham, Alabama a few days ago:
A meterorologist on Reddit explains how this occurs:
Meteorologist here. These are indeed Kelvin-Helmholtz waves. What is happening is that the nocturnal near-surface layers (lowest 50-100m) of the atmosphere are much more stable than the layers above it in the mornings. Until the ground heats up due to daytime heating, the surface layers stay more stable than the air over it. Kelvin-Helmholtz waves occur when the wind shear between the layers destabilizes the topmost portion of that stable layer, and entrains the air into the unstable layer. What you see is stable air being lifted, cooled, and condensed so that this process becomes visible, though this commonly happens many places without being visible.
The same phenomenon occurs on other planets; here’s a photo from Wikipedia of the phenomenon on Saturn:
Here’s a video of the formation: