If the differences between the air speed and the temperatures of the layers of the atmosphere are small they will glide over each other. If the difference between the layers is too great then where the layers meet turns into turbulence. When the atmospheric conditions between the two layers are just right, Kelvin-Helmholtz waves form. The example in the photo is one where the air between the two atmospheric layers is close to the threshold for turbulence.
The photo was taken from the Alabama Air National Guard facility at the Birmingham-Shuttlesworth International Airport, in December 2011. The Kelvin-Helmholtz waves seen in Alabama occurred when there was a cold layer of air near the ground, where the wind speed is low. A warmer and faster moving layer of air was probably moving over this cold and slow-moving layer.
Animation of Kelvin Helmholtz Instability:http://bit.ly/11lJQS7
Vortex formation in free jet caused by Kelvin-Helmholtz instability: http://bit.ly/VlnEmS
Video of the Kelvin-Helmholtz wave clouds over Birmingham: http://bit.ly/tjtWWG
More cloud photos and information here: http://