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.

Yesterdays Max 11.2C, Min 7.8C Rain to 09:00 0.7mm.

Getting much calmer after a really rough spell http://magicseaweed.com/Sennen-Surf-Report/4/

Graham
Sunny Penzance

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!

On Thursday, March 2, 2017 at 9:25:30 AM 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.

Yesterdays Max 11.2C, Min 7.8C Rain to 09:00 0.7mm.

Getting much calmer after a really rough spell http://magicseaweed.com/Sennen-Surf-Report/4/

Graham
Sunny Penzance

Using GFS back trajectories at 950mb(circa 2000ft), the air over southern Cornwall at about 0001Z on the 2nd March left the Labrador coast at about 1200Z on the 27th Feb, so took about 60 hours to cross the Atlantic.
The nearest radiosonde ascent in Labrador (Goose Bay) showed a 950mb dew point of -20C, with a surface dew point of -18C. The Camborne ascent at 0001Z on the 2nd March had values of -4C and -1C respectively.
Interestingly, the 1200Z run of the Met Office Euro4 model on the 1st March predicted a narrow zone of surface dew points of between 0C and -2C just west of Cornwall at both 2100Z and 0001Z.

Dick Lovett

On Friday, March 3, 2017 at 8:34:28 PM UTC, wrote:
On Thursday, March 2, 2017 at 9:25:30 AM 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.

Yesterdays Max 11.2C, Min 7.8C Rain to 09:00 0.7mm.

Getting much calmer after a really rough spell http://magicseaweed.com/Sennen-Surf-Report/4/

Graham
Sunny Penzance

Using GFS back trajectories at 950mb(circa 2000ft), the air over southern Cornwall at about 0001Z on the 2nd March left the Labrador coast at about 1200Z on the 27th Feb, so took about 60 hours to cross the Atlantic.
The nearest radiosonde ascent in Labrador (Goose Bay) showed a 950mb dew point of -20C, with a surface dew point of -18C. The Camborne ascent at 0001Z on the 2nd March had values of -4C and -1C respectively.
Interestingly, the 1200Z run of the Met Office Euro4 model on the 1st March predicted a narrow zone of surface dew points of between 0C and -2C just west of Cornwall at both 2100Z and 0001Z.

Dick Lovett

Thanks for the replies.

Graham