The undulatus variety features parallel bands or crests that give the cloud layer a wavy, sea-like appearance. It can be observed in stratocumulus, altocumulus, altostratus, and cirrocumulus.
Origin of the waves
The undulations are produced by atmospheric gravity waves — disturbances that propagate along the boundary between air layers of differing density or speed. Wind generates crests and troughs; cloud forms at the crests (where air rises and cools) and dissipates in the troughs. Typical wavelength ranges from 1 to 10 km.