Are you talking about the "Cloud-tank" shot from ID4 right?
Well, that was done by pouring a mix of milk and fullers-earth into a tank of water, and shot with the frame-rate changed to emphisize a sense of scale.
In your case, I would suggest fluids, as it is a LOT less messy
Check out the fluid presets in your visor for something cloudy to start from, and add a dynamic dummy-object to fly through it, and use that as a starter for your composite.
Does that help a little?
PS, welcome to SM!
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS