I am not going to explain this in detail as it's pretty simple.
It's possible to open .ini files on your computer with notepad / wordpad.
If you transfer your .ini to your computer, and then you search it for something like [Weather Clear], you will see a list of values under that.
The fog ones will be obvious. They say something similar to "Fog Depth Daytime", "Fog Depth Night Time". You can decrease these numbers to make less fog, as someone posted much earlier in a more cryptic way. There are fog values for each type of weather. There are also fog values above that point, but I have no idea what they do.