I'm sure Genghis knows the books stuff, as do the people who write the forecasts. But the problem is these things are not forecast. I think what happens is that slack surface pressure gradients unexpectedly allow air close to its dewpoint to flow over land. Then given a temperature drop of a degree or so and a pressure drop you get instant fog. I have seen fog like this form over the vale of Aylesbury and a few years later at STN. On both days the forecast was CAVOK and on both days the viz dropped to less than 100M with little notice. But the first occasion was the most stunning. The fog "front" moved at about 50 knots when the surface wind was only about was just over 10 knots. I saw it from just above the top of Dunstable Downs in a Ka18.
We also suffer from Fret where we live, again rarely forecast. Summer's day convection is to blame. Again slack pressure gradients and light winds generally prevail. It often starts with inland rising due to convection. As it does so it is replaced by cold sea air full of salt nuclei to move across a cooler sea inland. I'll also guess that the lower layers become more saturated as they move across the parts of the sea closest to the coast. The slight rise of the air as it crosses the coast (due to vertical pressure gradient and topography) reduces its pressure sufficiently to allow it to reach its dewpoint.
Whichever sort of fog it is, it is still scary if not forecast.
PM
ps. Here's a question - Does an air mass reduce in pressure as it is made to move? I haven't a clue. I'll guess it does.