Some render artifacts on this one, apparently where the spheres overlapped. I think this was about a 45 min render, 800 x 600. To make the cloud material, I started with a cumulous vol. cloud mat, made the color white, and messed with the function. I added some rotation to the smooth noise. This makes the clouds look windblown. I also added a fractal layer. Then I had to play with the ambient, diffuse and luminosity till it looked right. Different atmospheres need different settings. The clouds are all primitive spheres that are a little flattened. I think the size of the mat was set to 20. Playing with the size will change the look of the clouds greatly. I'm trying to figure out how to make a good thunderhead. Not there yet!