On weather reports they will often say sometrhing like
VISIBILITIES REDUCED TO AS LOW AS 1/4 MILE.

I've been in fog that was so dense that I could not see an object 3 feet
from me. I'm guessing that would be ZERO VISIBILITY. But the question
I've always has is CAN THERE BE A NEGATIVE VISIBILITY? Like -1/4 MILE?
Or does it stop at ZERO?