what do you think about that,2 spheres a real big one and a small one in the center of the big one.the big one is for the sky color and the small one is only for the clouds.you can make a shader for the cloud sphere in carrara or make a texture map with a transmap in paint program.so you have some cool shadows from the clouds on the ground if you want,if not then turn it off.it looks realistic and you can animate the small cloudsphere without the skycolor,or animate the skycolor from blue to black or something else.