Originally posted by Zeppion:

Hmm, this is strange. I wonder if there's different types of fog. Maybe some are thicker than others? I was in the Black Forest when I did my experiment. I tested it like five times and it always worked. There was fog all around except in the radius of my campfire.

There are indeed two types of fog. There is the heavy fog that pretty much wipes out all vision ("Misty"), and then there is a thin, ground-hugging fog that usually only appears in the Black Forest ("DeepForest Mist"). Fires will push back the thin fog in a radius around them, but I don't think they will do anything for the heavy fog.