1. That dark effect is made by negative lights(at least it is in JKA), not fog.
2. Patch-meshes? Eww. If your using it for terrain, I suggest you make it yourself vertex by vertex, or use EasyGen. If your using it for detail, brushes can do the same job as a patch mesh.
Edit: And, Patch Meshes have a limit. It's 6 patchmeshes per map, I think. This may only be JKA, though.