Take temperature 7 dewpoint 6, strong possibility fog will form if temperature drops 1 or 2 degrees. However above the fog level it may well be clear for 1000s of ft. even if the lapse rate is the relatively common 2C per 1000 ft.
Cumulus in the UK summer is usually 2500 to 4000 ft, in winter 1200 to 2000 ft, and stratus can be down to the ground summer and winter.