Fog is due to atmosphere mie/rayleigh settings, higher numbers are more foggy.
They're in the track's special.ini, in gfx.fog section, or in the track's /tod folder, as copies of data/renderer/tod/rayleigh.crv & mie.crv, depending when the track was updated.
You can also open the console (~) and write 'time 1400' to change the time of day, there's usually less fog in the afternoon (it uses a 24 hour clock, so 0000 to 2359)