Tule Fog = Radiation Fog. It forms when the ground cools rapidly by radiation into clear winter skies. Usually starts forming around sunset, gradually gets deeper through the night, then burns off after the sun comes up though sometimes it can stick around for days forming a low overcast if the inversion layer gets thick enough. Really bad stuff for breathing because that shallow layer not only produces fog but also traps pollutants - carbon monoxide, particulates, ammonia commonly peak in low-inversion conditions.

Once, I had to drive to Sac from Fresno early on a winter morning - the *&*(^&^ insisted on an 0830 meeting instead of the usual 1000 that allowed me to take the train. Basically followed the trucks at 35-45 mph on 99 until we got past Modesto and things lightened up a bit. Idiots in SUVs were cruising past in the left lane at 75-80 as usual; on instruments? Another time was getting across Fresno on one of those dense nights where you really couldn't see past the end of your hood. Headlights even on low beam made a bright wall there. Bundled up the jacket and gloves and left the windows open - had to listen for traffic at intersections because you couldn't see it. Though in a way it was better at night because you could see a glow in the fog from lights and signals even if you couldn't see the lights themselves. At certain intersections, you would see your light turn green, but wait and listen a bit, then do a drag race across the intersection assuming the cross traffic wouldn't see the signal until too late and you wanted to be out of there. Then, where I am now, there's a soccer field next to a small creek and some vernal pools; often, on winter nights, the fog starts forming low and dense, and when the top of the fog is at knee or hip height with the field lights shining down on it they're playing soccer by braille because nobody can actually see the ball when it's on the ground.

I do get the impression that some leeway is now granted for timekeeping in really dense fog. San Joaquins do run slightly slower and fall behind schedule systematically. I've also noticed light rail trains running slower in the fog - and using the horn at some grade crossings that are otherwise in quiet zones. So unlike the olde dayze there seems to be some allowance for (lack of) visibility now.