If you ever needed a reminder that the atmosphere behaves like a liquid, then surely these breaking waves in cloud form are it.
What are they called?
Named after two physicists, Kelvin-Helmholtz clouds are extremely rare and represent billowing ocean waves rolling and breaking in a long line.
They usually occur on windy days, tend to be high level and do not bring rain or any other precipitation.
How do they form?
To answer that question, we need to think about how air moves throughout the depth of our atmosphere.
Wind never blows in the same direction everywhere all of the time - take a look at this sped-up timelapse footage showing clouds moving in different directions.
This difference in wind speed and direction with height is called "shear".
Kelvin-Helmholtz clouds form when two layers of air sit on top of each other, the upper one moving faster, scooping up air from the lower layer and building it into visible wave shapes.
We do not just see this phenomenon in clouds either - it can happen in any fluid.
The easiest way to think about it is when air blows over water. The faster moving wind moves the slower moving water beneath it into little waves on the surface.