Originally posted by Beep:

in my life I have known only once a fog so violent that I literally saw not beyond 1m50 (around 5 feet for no metric people) in front of me in my garden and in the streets of my villages it was really terrifying

Here in Oregon we get a little bit of heavy fog every fall, winter and early spring. It's not constant heavy fog, but rather a couple of days here and there. Mostly in the mornings and evenings.