Dead easy.
So go into where the 'weathers' are kept so under 'weather' in AC root folder i think.
In there is all the weathers it can use.
So for arguments sake lets say we want to fiddle with ilija's default weather...
So copy that folder, paste and rename...remember to rename the manifest.ini as well with the new name so maybe add a '1' onto it...
OK, then in that new folder find 'src' folder, then find 'conditions_converter'
Once in there you will see a bunch of text and some hints onto how to change stuff.
For example
v[T.FewClouds] = { fog = 0.04, clear = 1.0, clouds = 0.1, fogTint = rgb(1, 1, 1) }
So from that you can see that my 'few clouds' has 0.04 fog, full fog is 1.0, for example...
Clear 1.0 is blue sky, clear 0.2 will turn the sky grey.
Clouds at 0.1 if few clouds... overcast is 1.0, 0.5 is mid clouds etc etc...
There is more stuff within it to fiddle with.
But for me the fog was always too much, knock fog down to 0.7 or bit lower and it is more realistic.
Beware.... its a rabbit hole.
But provided you are only messing with the copied folder, no problem.
Hope this helps and i am assuming you know how to find the 'weather' folder already.