For what's it worth, I did a side by side comparison between a clear day and a dense fog day and this is what it looks like.
https://www.dropbox.com/s/ycfhmvqpc7uyzpx/fog.png
I trust that the LOS of the troops are adjusted correctly, but visually it looks more like overcast than anything else. Or light foggy at best. I have a ATI HD5850.