you can fake it with a few spheres and some texturing.

Create four spheres inside each other and scale them differently. These are going to be your different layers. By using different layers we are going to fake the perspective and give some paralax (this is what we did on the movie "Mrs Henderson Presents" for the big london bombing shots). Now paint some cloud textures for your sky, make sure to use your alpha channels to give transparency. Now when you map these onto your spheres it should start to look a bit better than just having the one sphere. You can also slowly key the rotation in y to give the clouds some movement to enhance the effect.


A