We've had some very strange examples of the Kelvin-Helmholtz effect, when clouds produce a billowing wave pattern just like sets of cross-sectioned waves breaking across the sky, writes Spike.
According to the scientific people, the rather rare Kelvin-Helmholtz waves occur when there is a strong vertical shear between upper and lower level streams of air travelling at different speeds in the same direction. Just like ocean waves, the energy at the top causes the 'wave' to topple.
Okay weakish anaology but it is true in the sense that when a wave breaks onto shore, the bottom slows down as it drags along the seafloor and the top pitches over because it overtakes the bottom part of the wave.
To strengthen the metaphor, however, "the instability is not exclusively associated with clouds. It can occur wherever there is a velocity difference across the interface between two fluids. The most obvious example is wind blowing over water, in which fast-moving air can create the waves on the slower-moving water," says the UK Met Office on their website. Ah ha!
The height of the base can be any height, but it will occur at relatively lofty levels above the surface. To actually see them is probably a lot more rare than the number of occurances, since these may be blocked by other clouds, or do their wavey thing beyond sight.
The repeating curling wave pattern are named after two physicists who studied turbulent air flow, messers Hermann von Helmholtz and Lord Kelvin, whose real name was the much more lowly William Thomson
So to unpack the wind shear part, Kelvin-Helmholtz clouds, resembling evenly spaced rolling ocean waves, are caused by the top layer stronger flowing air in our atmosphere scoop up the top of an existing cloud layer into the wavey pattern.
We've seen a good few examples of these waves on Wavescape, including from the cams back in the day, which were screenshotted by readers and sent in, or from readers who spotted the phenomenon. Some have been quite eery and apt as waves marched across the sky above the ocean waves marching in below.