Meteorologist here, because gases like the atmosphere are fluids, too. The only real difference between a gas and a liquid is that the individual molecules don't connect to each other at all.

But you can apply most laws of fluid dynamics on liquids just as much as on gases.

In this case, there is a strong temperature difference between two layers of air. The lower one with the clouds is a decent bit colder, so all the humidity condensates. Above that, there is an inversion layer. Inversions are phenomena where the temperatures rises with the altitude again.

Specifically in this case, this might be the tropopause, the border between the Troposphere and Stratosphere. Within the stratosphere temperatures usually go up with the altitude again.

Now, imagine one fluid that is colder and heavier (because of the water) and therefore denser, and one that is warmer and lighter. And these two get dragged along each other. When two different things rub along each other, they usually build up tension and interact with each other. They start to oscillate (vibrate steadily) along the way and create these kinds of patterns.

If you press your hand against the window and drag it in just the right angle across a window or table, you'll notice it starts vibrating, too and might make a noisd. That's the same principle, except with two fluids instead of two solid objects. Or, even simpler, look at the sea. Waves are just a resilt of two fluids interacting at the borders with each other: a gas (the atmosphere) drags along a liquid (water) in the form of wind and creates waves.

And since gases are fairly inert, this happens on a much bigger scale than with other things.