What I did see over the weekend was a 2400 ft shift between extremes, however the only things that really changed dramatically was the humidity and temperature. At my place, saturday morning, it was 600ft, and at newbury it was 1800ft. Sunday morning was much more 'dewy' and that read minus 250 ft, newbury being about the same.
So what it appears to suggest is that the humidity and temp when they get low can change air density as compared to a warm drier day.
Unfortunately I wasn't able to shoot at home at the weekend, but tests the night before indoors had the DA at 600ft, and there was no zero shift between that and 1800ft.
What would be interesting to see is what it's like when we get into the more extremes of winter, when zero shifts are more often seen.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep