[Murph]

One way of increasing "fog" effect is to open the mission in FMB, zoom in anywhere on the map, select "view" then "landscape". Move the "brightness" slider to the left until you feel visibility is poor enough.
Set weather conditions as hazy. This won't give you a full-on blind fog, but pilots did'nt fly in such conditions in WWII anyways. It should allow you to build missions where visibility is far less than ideal.