What is Kelvin-Helmholtz Instability
                                                            - Eddy happens when flows shear: Kelvin-Helmholtz Instability

Kelvin-Helmholtz Instability (after Lord Kelvin and Hermann Von Helmholtz) is the instability happens when there is velocity shear in a single coutinuous fluid, or when there is a velocity difference across the interface between two fluids. (according to Wiki)

An example is wind blowing over water: The instability manifests in waves on the water surface.


  An example of K-H instability            An example of K-H cloud
             http://i.livescience.com/images/i/000/022/847/i02/k-h-clouds-1.jpg?1324322111                                                   http://upload.wikimedia.org/wikipedia/commons/e/e3/Wavecloudsduval.jpg