The wave-like clouds are called Fluctus clouds, but are more commonly known as Kelvin-Helmholt clouds. The clouds are formed by the Kelvin-Helmholtz instability, which is due to different layers and the density of the air. And they are rare.
– And this is an unusually beautiful specimen, usually there are maybe five waves and they are a bit half broken, says Nils Holmqvist.
See pictures of the clouds, and hear how they are formed, in the video