(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.