I recently updated with Stiebler's add-on (patches?) for H.SIE's V16B1. One of the add-ons is the weather meter (It's fantastic, by the way). Now when I start a mission and there is light fog I get a fog warning (i.e. visibility is getting worse!) and the weather meter is a dull white which should mean medium fog.
Unfortunately, I had it all working nicely before then had a melt down and can't recapture the magic!

I also had around 16km visibility after installing a 20km environment in light fog. It just worked out that way?!?! That to went bye-bye in the recent implosion.
I'm hoping if I can tweak the way the fog limits my watch crew it everything else will fall into place.