As wildman2 says above: Have you considered making clouds out of terrains? It's been done by some of the Bryce masters... Use two-sided terrain object (between the rock object and the metaball object on the Create menu bar), then go into the Terrain editor, load a fractal-like pic, then use the clipping function to 'open' holes in the cloud. Then add a cloud material. For the sun-rays...volumetric lights positioned strategically should do the trick.

RosemaryR
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"This...this is magnificent!"
"Oh, yeah. Ooooo. Aaaaah. That's how it starts.
Then, later, there's ...running. And....screaming."