How about treating it the same as a Sandstorm?
"A sandstorm reduces visibility to 1d10 × 5 feet and provides a –4 penalty on Perception checks."
We did this by rerolling the visibility distance at the start of every combat round.
In real life:
"The international definition of fog is a visibility of less than 1 kilometre (3,300 ft); mist is a visibility of between 1 kilometre (0.62 mi) and 2 kilometres (1.2 mi) and haze from 2 kilometres (1.2 mi) to 5 kilometres (3.1 mi)."
So good visibility is being able to see more than 3 miles and a third of normal would still make it possible to see things a mile away.