For the low altitude fog you can just play around 'Fog & Haze' (falloff & altitude) in atmosphere editor. But, if you need the fog in only at certain place, you could use few volumetric spot light (w/light gel) layed down horizontaly... but this works only at night scene though. I know "smallspace" had a tutorial on this, in this forum, unfortunately I couldn't find the link anywhere. PS. eb, that's a very tempting offer from forester, I wouldn't say no to it. :-)