(This page blatantly ripped off from the University of Illinois Urbana-Champaign
Weather World 2010 project.  I am ashamed.)

Kelvin-Helmholtz Wave Clouds

These clouds are formed between two layers of air, with different densities, traveling at different speeds. Therefore, if a warm, less dense layer exists over a layer of colder, denser air, and the wind shear across the two layers is strong enough, eddies will develop along the boundary.

Consequently, the clouds will take on a wave-like appearance, as in the picture above. These waves are better known as Kelvin-Helmholtz or shear-gravity waves. However, if the air layers are traveling at the same speed, such waves will not occur.