A method that doesn’t rely on mathematical formulas from the reality but instead only tries to mimic it’s look. The good result is possible by a large number of sprites that looks like clouds which are encapsulated in a defined bounding mesh. It’s said to use a light-curve instead of an light scattering algorithm.

Link to more details from the author (he says a demo should be available soon).
http://www.gamedev.net/community/forums/topic.asp?topic_id=511184