Great question!  I agree with the last post from dhama, but you may want to start from the lowest source to the next ball being a little higher and larger placed behind the previous one (sort of like a stair step.) this one may be slightly more transparent than the first one.

The example here is from a piece that I did a while ago. I started with a small ball of "cloud" and then behind that I duplicated that cloud and raised it and then enlaged it a little. The next one I did the same thing but I made it more transperant and so on, remembering to repeat the same for the other nostril on the other side of the dragons snout..

 Be careful not to let the ball of clouds intersect one another because the results will not look good at all! Hope this helps you!

Rick KIng