Using several cloud layers (6 in total) and a distance shader, I've managed to create "storm fronts". This is the first shot, the real beauty of this is only visible in 360° unfortunately. I'll post a Unity webplayer shortly that shows it all. A second scene, with a much closer and more detailed storm front is currently rendering. Takes about 9 hours per image, 6 images total for a 360° view...
Very nice use of the distance shader. Look forward to the 3D view.
3D Player available here:
http://web.lemuria.org/~tom/unity/Storm1.html
It's a Unity 3D webplayer. If you haven't got the Unity plugin installed, it should automatically direct you to the proper site, otherwise check http://unity3d.com