The problem lies in that fog and rain are related, but different. Both are a result of water particles within the air being no longer stable, and thus dropping out. This is a function of the temperature of the air changing - in this case becoming colder, and thus unable to hold as much water vapor.
Fog is a fine mist - thus easily subject to wind conditions. Rain is denser, and can come in everything from a drizzle to a full-on hurricane as far as wind speeds go.
The phenomenon of wave tops being blown off (known as spindrift) and having an effect on visibility come into play only at high wind speeds - Beaufort scale 9 (Strong Gale) and up.
Visibility is effected by all three, of course - but I really don't know how to adjust for the graphic effects aside from the crude one of limiting visibility. Thus the wish that wind speed effects "fog" only at lower and higher ends, while the middle area is only effected by rain.
In game terms, of course, there may be no way to distinguish between them.
Just for reference:
http://www.spc.noaa.gov/faq/tornado/beaufort.html