The Aim Might have the answer!!!! I found this when talking about a metar coding. read with caution. Im no Mr. Spock and my logic can be flawed.
Obstructions to visibility. There are eight types of obscuration phenomena in the METAR code (obscurations are any phenomena in the atmosphere, other than precipitation, that reduce horizontal visibility):
FG fog (vsby less than 5/8 mile)
HZ haze
FU smoke
PY spray
BR mist (vsby 5/8 - 6 miles)
SA sand
DU dust
VA volcanic ash
NOTE-
Fog (FG) is observed or forecast only when the visibility is less than five-eighths of mile, otherwise mist (BR) is observed or forecast.
Also found this!
For aviation purposes, the ceiling is the lowest broken or overcast layer, or vertical visibility into an obscuration. Also there is no provision for reporting thin layers in the METAR code. When clouds are thin, that layer shall be reported as if it were opaque.
Now here is Vertical Visibility (Whiteman AFB loves to use VV)
Vertical Visibility (indefinite ceiling height). The height into an indefinite ceiling is preceded by "VV" and followed by three digits indicating the vertical visibility in hundreds of feet. This layer indicates total obscuration.
EXAMPLE-
1/8 SM FG VV006 - visibility one eighth, fog, indefinite ceiling six hundred.
So this is my conclusion. Fog has no ceiling but can have a Vertical visibility value. (kind of like an RVR value only vertical) Fog is when the ground visibility is being obstructed below 5\8ths of a mile. Therefore if you have good visibility on the ground then it is not fog but mist if it is over 5\8 of a mile. When good visibility prevails on the ground (5/8 mile +) and there is Vertical Visibility value this changes it from fog to a cloud. Keep in mind that this is based on relatively flat terrain and VV does not mean that is where the bases start. Just where visibility gets bad again. So you could have the bases start at 2 feet with 6/8 mile visibility under the 2 feet and 4/8 mile visibility above 2 feet and a VV of 600 feet. It is now deemed a cloud( if youwere laying under it getting that 6\8 mile visibility, standing up its still fog) On a mountain I guess its all relative to where you are. In it and the right conditions are met, its fog. Out of it, it’s a cloud. 
Make any sense at all??? I hope it did…. 
YEAH, what he said!!!!!!!!!!!!!!!!
Only, I like the one about going pole to pole-- but, that's how Bill Graham's pilot was navigating when they imbedded the helicopter in an extra tall power tower which carried the wires over a river.
