Some observations:

1200 UTC
Dunkeswell: T 4.4, Td -7.1, RH 43%
Plymouth: T 7.2, Td -10.4, RH 27%
Cardinham: T 6.3, Td -10.8, RH 28%
Camborne: T 6.7, Td -10.1, RH 29%
Culdrose: T 6.8, Td -3.9, RH 46%

1300 UTC
Dunkeswell: T 4.5, Td -5.1, RH 50%
Plymouth: T 7.3, Td -9.8, RH 29%
Cardinham: T 6.6, Td -8.0, RH 34%
Camborne: T 7.9, Td -12.4, RH 22%
Culdrose: T 7.3, Td -3.3, RH 47%

Good hydrolapse showing on the upper air soundings, so the breeze is causing the dry air aloft to be mixed down to the surface.

Will Hand's weather station and the one at Postbridge, both about 300m AMSL, are showing consistent dew points of -9 to -11, so the dry air isn't too far above sea level.

--
Freddie
Fishpool Farm, Powys
290m AMSL