This isn't antialiased, I ran out of patience. At 800 x 600, I think it would have been somewhere around an 8 to 10 hr. render! All clouds are volumetric spheres. The cloud in front of the mountain is made up of 3 or 4 spheres slightly offset to give it more density. The key to this image is lighting. The clouds behind the mounain were pretty flat until I aimed some spots at them from above and put some big point lights inside them. The point light really bring out a lot more detail in the clouds. These clouds could really be useful if they didn't take so darn long to render.