The Kelvin-Helmholtz instability tends to arise when there is a friction/shear velocity at the interface between the two fluid layers. The shear velocity of one fluid moving in an opposing direction induces a shear stress on the other, once a certain velocity has been exceeded. When this occurs, an instability along the interface is generated, resulting in a pattern appearing like a series of overturning ocean waves is generated at the boundary between the two oppositely moving fluids.
Many examples of the Kelvin-Helmholtz instability can be seen in nature, shown in the images below.
Weather satellites take advantage of the Kelvin-Helmholtz instability to measure wind speeds over large bodies of water. Waves are generated by the wind which shears the water at the air/water interface and so are a result of the Kelvin-Helmholtz Instability between the wind and the ocean. The computers on board the satellites determine the roughness of the ocean by measuring the wave height. This is done by using radar, where a radio signal is transmitted to the surface and the delay is measured in its reflection, called ‘the time of flight’. From this data, a 4D graphical plot can be generated to help meteorologists to predict the weather.