REALLY nice work! I like how you go right through some clouds and then around others... nice camera direction.
How many different clouds do you have? Or, are they all one instance of a volumetric cloud?
What is your scale? Very large with a large distance of camera motion? Or, very short camera motion with small scale clouds? (I'm curious as to how much detail is really available within a volumetric cloud.)
Sorry for all the questions, but I do love to pick the minds of folks who have created nice looking animations.