Do you live on high ground and the cold air has "drained away" into the valleys or frost hollows below ? Gatwick is a prime example of a frost hollow which explains why their early morning weather is often pants when all around is clear.
A fair amount of mixing is required for fog which is why the viz drops just after sunrise as the air gets stirred up and the fog deepens.
I used to get a thickening just before Dawn..... I wonder how she is these days
