I'd suggest a very low density cloud with a Coverage Adjust (I suggest doing this in the Density Fractal on the Density tab). Experiment with the height and depth to get the effect you want. Remember that Cloud Depth is centered on the Cloud Height and extends both up and down from there, so a cloud of 1000 thickness at altitude 1000 (meters), will extend from 500 meters to 1500 meters from the ground. So for ground fog, you'd want the height to be at least tall enough to intersect the ground.
- Oshyan