OK. I think I figured how to make ground fog.
Compo file: GroundFog1.compo (11.4 KB)
Here is what I did:
1. Changed the color of the main compo background
2. Added alpha component to the material layer with a 3d linear mask
3. Added mask to the sky layer with a 2d linear mask.
Is this the right workflow? Is there an easier solution?
Thanks in advance.