Kelvin-Helmholtz undulations or Kelvin-Helmholtz instabilities are a rare formation, although they occur with various types of clouds. They appear as a thin band with ripples, similar to the waves of the sea. However, this formation usually tends to dissipate only one or two minutes after formation and, as a result, is rarely observed. The shape of this type of ripple is the result of a particular type of wind shear.
In general, shear winds occur when one layer of air passes through another layer moving at a different velocity or in a different direction (or both). This gives rise to vertical swirls that produce a regular pattern of air waves. In the case of the Kelvin-Helmholtz formation, the vortices are more powerful, causing the cloud to rise to the "crest" and descend on the other side, so that the waves "break" like the waves of the sea.
This phenomenon was first described in the nineteenth century by Baron Kelvin (1824-1907), Scottish physicist, and Hermann von Helmholtz (1821-1894), German physicist - hence the name of the cloud. Kelvin-Helmholtz waves are quite common in the upper troposphere, but there is usually not enough moisture present to generate clouds and make the pattern visible.
These clouds are often good indicators of atmospheric instability and show turbulence for aircraft.
Kelvin-Helmholtz instability occurs not only in clouds, but also in the ocean (the waves of the sea), Saturn, Jupiter, and the Sun's atmosphere.