On Thursday, 2 March 2017 09:25:30 UTC, Graham Easterling wrote:
I was alerted to the fact how low the relative humidity was last night. Below 50% for several hours at Culdrose.

A bit of a surprise to me for overnight relative humidity in a westerly in winter, even for 'cold sector' air. Obviously the air crossed the Atlantic at a fair speed. The synoptic setup at the time seems to offer few clues, so a backward trajectory would be useful, and I'd welcome any comments.

The ridge was developing as it came towards the UK which would suggest to me more descent and drying than in a normal "cold sector". 50% does seem very low for nocturnal Atlantic air, though, as you pointed out!