Air Density. This varies with altitude/heat/humidity/pressure.
Air has a set of "standards", set in 1973. The International standard atmosphere= sea level, 15 degC, 1013Mb, 50% humidty.
Pressure obviously changes (lessens) as you get higher above sea level, but add temperature/ humidity changes during a day and changes due to weather fronts/patterns,.....and the permutations are infinite.
The value for putting a "handle" on what it means to us, is "density altitude" . it is a way of saying,.....well compared to where I was at sea level this morning, since the weather/location changed I am now at the equivalent atmospheric condition of 3000ft above sea level. Then you have a method of calculating your trajectory change in advance of taking your shot.
Just for fun and because I was bored sitting here at home while you were all having fun, I worked out what my trajectory would have been in Italy based upon what some fellows told me during the match about temp, humidity, altitude etc and double checked it on google maps and weather sites etc. If I'd not gone to the zero range to change my zero for the conditions, I could have shot using a revised click chart devised from density altitude alone. I'm making assumptions though. Only those who went really know what it was like.
The problem being, you have to know how much shift in POI your rifle is going to have for a given change in density altitude. I know mine now as I recorded everything that occurred in SA 2009, and been nutting it out ever since.
What makes it so hard to go to a Worlds and perform, is that you have locations that are very different from home. Those who travel regularly to shoot the Worlds are more likely to see and understand these differences and adapt quicker during the Worlds match days. Those of us that stay at home can only guess at what is going on. But the real culprit is density altitude, and the changing atmosphere not only between locations, but also during the day at same said location.
Density Altitude induced POI shift will be significantly greater at higher altitude. At sea level or near it, the POI shift is almost negligible,......one could almost deny the existence of the effect if one did not shoot anywhere but home ground
Just one more comment. The reason why each individual rifle/scope/pellet combination is going to be affected differently, is the very same reason we say not to use a BC someone else has arrived at from their barrel. BC's are critical to the POI shift problem. BC is the inverse of the Coefficient of drag, so of course if your BC is unique to your barrel, then the POI shift will be too. For instance, I use a BC of 0.032 which works very well for me here at the bottom of the world. 8.4GR JSB Exact. What BC would you have thought the Exact was???? most UK based references to it have it at 0.022 - 0.025BC.
So there is the rub.....its a drag related problem and individual to the rifle/pellet. The effect may be consistent and predictable, but the answer to the question of how much air pressure will change POI is: ..... "depends".