"It'll burn off"
Slightly off-topic, but besides the legality, fog, unlike the dark (which tends to start and end at predictable times), can be full of false promise. I planned a trip to La Rochelle with a friend. Checked the Met including the TAFs, starting foggy clearing by 9am. It WAS a foggy morning but on the drive to my airfield I see patches of blue sky with the sun peeping through and think "yes, it'll burn off".
I get to the airfield and although it looks a little hazy, we think we are within IMC rules and take off. Within less than a minute I have climbed above the blanket of fog and cannot see the ground at all. Those holes I saw from the car on the way to the airport have all gone. I am flying to La Rochelle, so continue with the belief that "it'll burn off" ('by 9'). The blanket continues across the channel, across Cherbourg, across Mont-St-Michel, across Rennes.
My friend (ex-army chopper pilot) and I are now adjusting our route to avoid towns and wondering what to do if a) the engine fails and b) we get to La Rochelle and the blanket of fog is still around. We radio La Rochelle and to our relief they have glorious sunshine.
The blanket of fog cleared about 30 miles north of La Rochelle after almost 3 hours flying. The flight over fog was beautiful and nerve-racking, and was also a good lesson in the unpredictable nature of fog clearing!
The return trip was one of the clearest flights I've experienced.