I've once ridden in freezing fog, in November a couple of years back. I'd gone out for a ride to ramsbottom with a guy I know. It was cold to begin with, but manageable. We came back over rivington, and it was icy but we just about got down in one piece, though I did skid on a huge patch of ice and nearly pooped my shorts. Then heading along the a6 towards home a load of fog suddenly appeared. Visibility was down to about 10m, it was horrendous. I was terrified we'd get hit by some idiot driving too fast in those conditions. I got home and had ice all down the front of my gloves, my head tube, forks, cables... It was awful.