Picture taken in August 2022. Fluctus, also known as Kelvin-Helmholtz wave clouds, form when wind shear causes instability in the (usually the upper) surface of a cloud. The formation is short lived and relatively rare. It can also only be seen well from the side like here. This video contains a good explanation of the physics involved here.
This community is for pictures and videos of clouds, weather and all sorts of atmospheric phenomena.
Some good links for identifying stuff:
The International Cloud Atlas is a great resource on observing the sky and the weather. The definitive authority on cloud classification.
Atoptics is a more thorough resource on atmospheric optical phenomena.
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