Here at Great Chishill, 142 metres asl in South Cambridgeshire,
radiation fog is a feature of life. But it's not quite the normal
radiation fog, ie not actually generated here.

So to give an example, currently temp is 4.2 and dewpoint also 4.2.
But we have clear skies. The scenario is that at the bottom of the
hill (circa 60 metres asl), radiation mist (becoming fog) will be
forming very soon (if not already). However, I am sure that when I
wake up at around 0600 hours on Wednesday, we will still be CAVOK. I
will be able to look out across the lower ground and see the fog
beneath me (only view by the way, is from the toilet window! As the
morning progresses, that fog will lift into low cloud. Here in Great
Chishill we will have an hour or so of mist/fog with visibilities of
circa 500 metres or less before it clears into weak, mist-affected
sunshine.

Jack