Guess it would be hard to make with planes... I wouldn't try it that way... I suppose those clouds have something to do with field objects (like radiation fields or toxic fields), with animated texture (I guess the object itself is immobile). If I would be doing that I would start trying to create a permanent radiation field all over a map. I guess it is possible, since we all have seen such permanent fields in challenge campaign against doctor Thrax, where he had bunkers permanently surrounded by toxin fields... Guess that's it - a high altitude permanent field object with textures animated in a different manner...
