Just as an idea for cloud self-shadows (as well as for cloud grand shadows)
There was a short "PATHS OF HATE" that was presented at Siggraph 2011 (By Autodesk).
Basically one of the key moments there was a cloud creation:
http://youtu.be/LBovH7_um_s?t=17m5sEven this is a render - some of it's techniques might have a chance in real-time rendering.
In few words:
1) Clouds are 3D Geometry - which in Outerra:
- can have different LOD's
- Can cast shadows to ground
- Can be lit and have self -shadows
2) However the user sees the smoke (sprites or any way you did the smoke in Outerra) attached to 3D geometry.
And each "sprite" gets the illumination from the vertex colour (as in Video above) or getting it from geometry surface.
Of course with some adjustments to get Translucidity effect (also covered in video).
Here's a exact part of his fast explanation how it works: