Stu,
I appreciate that your original question hasn't been answered...
I had a trawl through the FAA sites and cannot find a proper quantifiable definition of 'fog' - just the usual 'water droplets, reduced surface visibility, cloud within 50 ft of the surface' stuff. It's almost as if they don't want to join in with the rest of us and admit to a definition based on metric measurements.
HOWEVER, do go to
http://www.faa.gov/avr/AFS/afs400/ac00-45e.pdf
and look at their METAR constructions for RVR reporting in 'statute' measurements.
If someone else can point to an FAA definition, I'd be pleased to see it!!!!
The Odd One