Thanks Henry

Frank is doing a real good job on these type of clouds.
At least 80-90% what you see here is his work (base fractal and lighting).
I only slightly added 2 extra fractals and made minor tweaks to the lighting to get lighter shadows.
I rendered this with RTE enabled, detail 1, GI 2/4 and quite some samples for atmosphere and clouds to get noise-free results.
Therefore it all in all took 13 hours to render, but I cropped a bit from the original version, say 25%.
I'll try to make another one the next few days.
Cheers,
Martin