I have been contemplating for a while now if I must add my findings as well.
I have been shooting big rifles for a couple of years now and from one shooting location to the next, the same phenomena is encountered. It is not just noticeable with air rifle shooting. I have been battling with this ever since. At 100- 150m there is normally no significant change to justify a change in scope settings. Further than that it becomes an issue and at distance further than 400m it becomes a major factor.
This is about in proportion to shooting at 15 - 18 m with an air rifle where atmospheric conditions have very little impact but at 20 - 25m it becomes noticeable and at distances further than 30 m you have to do something about it otherwise you miss.
Last year I bought myself a Kestrel 4500 to be more precise in my medium to long range shooting and to use it for air rifle shooting predictions as well.
Saturday past, we shot the Presidents shoot on Waterval shooting range in Rustenburg and it is the first time I went shooting there. I used my Kestrel 4500 to measure the conditions and then I did something that I see no one has mentioned in this thread before and what I found to be a very exact way of calculating beforehand what and how your trajectory is going to be.
The most crucial thing to influence external ballistics is the wind and air density. Now let us leave wind out for a moment and concentrate on air density. Air density is determined by atmospheric pressure, humidity and temperature. Up to now I have seen everybody realize there is a correlation between these 3 factors and the one goes up or down the POI shifts and so on.
Now as SD has said a little earlier, keep it simple. Now here is a simple solution to accurately know what your rifles performance will be at certain atmospheric conditions.
Firstly the correlation between these 3 factors is given in a formula:-
Air density in kg/m? is calculated using the formula AD = Air pressure/(Specific gas constant x Temp) where:
Air pressure = Pascal (29.53"Hg = 100 000Pa)
Spec gas constant = 287.05 for dry air
Spec gas constant = 293 for 100% humidity
Temp = Kelvin 273 + Celsius
Let's use an example
Air pressure is 102900 Pascal
Humidity is 50%, then use a figure approximately halfway between 287.05 and 293, that is 290
Temp is 28? Celsius
Lets calculate AD = 102900/(290x(273+28)) = 1.1788kg/m?
This 1.1788 is the factor you use for a specific range of conditions that gives you that calculated value. If the temperature changes, humidity changes or Air pressure changes to such an extent that the calculation still gives you 1.1788, THAT is the base figure you use for the external performance or bullet path determined and affected by the AIR DENikko StirlingITY prevalent at that time.
Yesterday at Waterval shooting range, I took all the readings early when I got there to determine a factor. The humidity went down from 76% to about 22% during the day, temp went up from 24 - 34 , air pressure went up from 105500 to 107110Pa. The actual effect of the air density prompted me to adjust an extra click on 300m shots to still hit home. All the other guys shooting next to me shot low. A friend of mine took my warning to heart to adjust an extra click, which he did and he shot a very good score. If he did not he would have missed.
Now how can you make these figures work for you?
What I do is I use a very good ballistics program written by Lapua to calculate a couple of ballistic tables with a variety of different atmospheric conditions. I get a couple of Air density values and I use them as a factor, starting from 1, 1.1, 1.15, 1.2, 1.25, 1.3. At a specific venue I will take the atmospheric condition measurements, calculate the Air density and from my cheat sheets I will select the right drop chart and shoot accordingly. So far I have seen it work for shooting at 5800ft in the Drakensberg mountains, Genl Piet Joubert, Heidelberg, Rustenburg, Delmas and a couple of place where I have put my hand to long distance shooting up 1000m. It worked every time to the click.
I have tried it with the air rifle as well and it also works in the same way. The change in conditions as measured above will cause you to miss a 40mm kill zone even with no wind @ 50m. The change in conditions was measured in a time span of 2 hours. So if you shoot in early and zero your rifle and only start shooting an hour or 2 later and with changing conditions you will have POI shift even with the most stable shot to shot velocity.
How legal it will be to quickly take a measurement and check which cheat sheet you must use during a shoot remains to be seen. It is fairly quick to take the measurement do the calculation and adapt. If you shoot in at say 20m the change in conditions will have no noticeable effect but further than that you may get some erratic and unexplainable behavior.
This is my findings with what I have been experimenting now for about a year and a half with and for me there is no other way to tackle a new venue or shooting conditions for any place.