Originally Posted by PeteM
Air pressure, humidity & temperature can change rapidly especially humidity, I watch the humidity rise & fall (quite dramatically) within minutes here, on a fairly sophisticated wireless weather station.
Humidity effects no more than a total of 1% change the the DA, when it goes from 1% to 100%.
Its generally not going to be humidity that gives you a 1 moa shift in the middle of a match. It probably more to do with a 60m climb, and warming air.
I've not discounted air moving up a hillside in warming conditions either. Adiabatic and anabatic winds in a valley such you were in Rob, could be a factor. Wind so gentle you can hardly feel it, but its moving uphill or downhill as it warms or cools, and is caused by sun heating the earth, or at sunset the cooling of the earth thus reversing the trend. Real subtle suff.