I have just uploaded a wind and pressure chart for yesterday evening to my blog and when I look, it has turned out to be much more interesting than I thought it would be. There appears to be a minature col centred just south west of the upper reaches of the Humber located by the calm "spot" on the chart here. There are opposing winds towards a N/S line where the distance between the 1024 isobars narrows. I wish now that I had saved the chart applicable a couple of hours sooner!! which I recall showed the col more clearly and further west (I think). There is also the considerable low to the NW of the British Isles which it is possible that the birds might have been reacting to as they do later in the season.
I would suggest that the movement of Swifts which I noted in the hours before the chart time was in some way a weather related move to this "calm" but complex col situation. Any weather / Swift buffs out there who might like to comment?
Anyone else note unusual Swift moves late yesterday afternoon?
For location, Wilsden is in central northen England (4 miles west of Bradford for those that know the area) mid way between the upper reaches of the Humber and the southern half of Morecambe Bay