If you search for fog_start in the manual you'll find this:

view.fog_start
view.fog_end

Quote:

Alternatively to controlling the fog strength trough the fog parameter, these parameters give a start and end range for fog in D3D mode. There won't be any fog closer than fog_start; the world will be all foggy beyond fog_end. This way a very sharp fog boundary can be given, for instance for completely hiding everything outside the clip_range.



Set your for color in map properties and place something simliar to this at the end of the main function in your level script:

Code:
fog_color = 1;

clip_range = 15000;

camera.fog_start = 0.5 * clip_range;

camera.fog_end = 0.9 * clip_range;


This method works pretty well for general fog. Good luck.