Kelvin-Helmholtz clouds
are also known as “rotor clouds” and their spiraling pattern is the effect of a cloud generating a billowing wave pattern, which is a very rare occurrence. They happen when there is a severe vertical shear between two air streams, producing the upper-level winds to blow faster than the lower-level winds.
Images © WSLS 10, Dave Throup, Vivian Knezevich
(via grumpy-gh0st)