You can do this with the atmosphere using the standard atmosphere model. The attached image shows a series of renders using fog density = 0 (yes, zero) and with colour set to a light grey. The fog is set to gather at low altitudes.

The top three renders have falloff set to 95% with different altitudes - this seems to be very altitude-sensitive. The lower three have the altitude set to 25 and the falloff varying from 80% to 90% - this gives much more subtle control. Interestingly, by making the fog gather at high altitudes you can get fog clinging to the ceiling but clear on the ground - could be useful for smoke in a fire!

Steve