Yesterday, while talking to my wife, I noticed that Kelvin-Helmholtz Clouds were forming in the sky behind her. These are cirrus clouds that due to the Kelvin-Helmholtz instability grow vortex billows. They arise from very large shears in the velocity fields, which is this example arise just below the jet stream.
The phenomenon is very similar to the excitation of sea waves by the wind, or waves on the surface of the small water flow coming out of a faucet (the later formation of drops is due to surface tension, an altogether different phenomenon).
Here is a picture I took, while a time lapse movie I made can be found here. Due to the lack of any prior notice, I used a very low tech method to capture the movie's frames... a still camera, a stop watch and my finger. This is also the reason for the slight jitter in the movie. (The time lapse movie is 360 times faster than real time).