Toecutter747
Can't find a reference either, but the definitions of Fog & Mist from the BOM site are:
Fog: Suspension of very small water droplets in the air, reducing visibility at ground level to less than a kilometre.
Mist: Similar to fog, but visibility remains more than a kilometre.
I would have thought that its the reduced visibility, as opposed to fog, as such, that requires the alternate.
As an example, a DRW ATIS earlier this year had a component, "cloud base 1500, visibility 1500m in fog"
Couldn't have been fog
if the vis was 1500m.
If the vis was 1500m then the aerodrome was open for landings and an alternate wasn't required.