Imaged by Markus Selmke in Leipzig, Germany. Air layers of different density slip over one another. Small irregularities at their junction magnify into regular waves.
Images ©Markus Selmke, shown with permission
K - H waves tens of thousands of miles across between different velocity cloud belts on Jupiter.
Image courtesy NASA
Kelvin-Helmholtz instability waves can arise when two fluids are travelling at different speeds.
The fluids can be, as here, air layers at different temperature and density with the upper layer moving faster. They can be layers of different density inside the oceans. They can be the sea surface and the windy air above. They can be swiftly moving belts on mighty Jupiter.
Under some circumstances the layers’ interface becomes unstable. Imagine a small hill forming. The situation then has some similarity to flow over an aircraft wing. The net pressure produced by the two fluids moving past the hill is upwards (Bernoulli effect) causing the hill to grow. Similarly, a depression is forced downwards. Irregularities grow, they then roll up to become mature Kelvin-Helmholtz waves.
As well as forming rare and beautiful clouds, Kelvin-Helmholtz instabilities are of major importance in the transfer of momentum and energy between the atmosphere and oceans.