we did this on the film Mrs Henderson Presents for the Blitz scenes and we have 4 different layers of clouds. So we had four spheres that fitted inside each other (there was a bit of a gap inbetween each on to give some paralax). each one had a cloud map mapped to it and so we slowly rotated them around the same point very slowly! The problem with twisting the sphere is that your UVs will stretch so it's better to rotate the whole sphere.
let me know if you need some more help.
A
Technical Director - Framestore
Currently working on: Your Highness
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