Atmospherics change with H/L pressure systems

[Stanley basin]

Using the ICAO Standard for 6000 ft their table has 23.98 inches of mercury and 38 degrees. How much does a high or low pressure system moving into your area (6000 ft) change the InHG reading? I know the temp will rise and fall with the pressure change but how much of an impact will this have on bullet drop? This was a question that came up at my gun range. Thanks for comments.

 

[rammac]

I'm no expert since I rarely have to worry about this problem but from the research I've done it seems like a bullet will drop less about .7 MOA per 1000 feet of altitude increase and barometric pressure will drop about .8 inches per thousand feet of altitude increase. So I'd say that if your local barometric pressure changed by .8 in hg then your bullets will rise about .7 MOA. Extrapolating that you could say that in most cases you need to drop your elevation by 1 click (for a scope that uses 1/4 MOA per click) for a drop of .29 in hg in barometric pressure. If you don't mind loosing a little accuracy you could use a general rule of thumb that says to drop your elevation by 1 click for every .25 in hg drop in barometric pressure or for every 250 feet of altitude increase.

 

[CharlieNC]

I shoot a range that is around 4500ft, and there can be enough temperature change from only one day to the next to change the air density enough to cause target misses (steel) at 800-1000 yd. Easily on the order of >0.2 mil. I have not observed the BP changing that rapidly, more so between visits to the range and not quite to the same degree on the target. Get a ballistic app for your phone, enter the data appropriate for your load, and assess the types of differences you expect to encounter. Playing those cases will give you the experience to use the app at the range efficiently and become a better shooter.

 

[Bart B.]

People shooting 1000 yard matches with 308 Win ammo at the Whittington centers 6600 ft. elevation need come up about 5 MOA to shoot at 1000 on the Camp Perry range at 600 feet above sea level. Otherwise their first sighter will strike the safety berm low in front of the target.

 

[Stanley basin]

Thanks for the input from everyone. I figured it was a minor adjustment and I use the sigsauer kilo 2400abs. Seems to work fine beyond 1000 yards but the gentlemen shooting next to me was using a kestrel with the abs app and kept having some issues. That lead to the above posting. I appreciate the information.

 

[ShaunBrady]

Using the ICAO Standard for 6000 ft their table has 23.98 inches of mercury and 38 degrees. How much does a high or low pressure system moving into your area (6000 ft) change the InHG reading? I know the temp will rise and fall with the pressure change but how much of an impact will this have on bullet drop? This was a question that came up at my gun range. Thanks for comments.

High or low pressure systems moving through an area will change the absolute pressure 0.2 - 0.3 "Hg above and below the average.

How much that affects your impact will depend on the distance and bullet/velocity.

 

[ShaunBrady]

Thanks for the input from everyone. I figured it was a minor adjustment and I use the sigsauer kilo 2400abs. Seems to work fine beyond 1000 yards but the gentlemen shooting next to me was using a kestrel with the abs app and kept having some issues. That lead to the above posting. I appreciate the information.

A common problem for people not familiar with atmospheric properties or the use of Ballistic Aps is to enter both 6000' and 23.98 "Hg. If you enter 23.98 "Hg, there is usually a "Pressure is Absolute" toggle that should be checked. You could also enter 23.98 "Hg and zero for the elevation. If you enter both, the resulting density estimate will be far too low and the drops will also be underestimated.

 

[Stanley basin]

Thank you Shaun. I appreciate the info.

 

[dcali]

The difference between high and low pressure at any given spot on earth due to weather changes is going to be small. The difference between going from sea level to 5000' of altitude is not.

Behind the curtains, ballistics calculators have no use for altitude. What they care about is air density, which is determined from absolute pressure, temperature, and to a much lesser degree, humidity.

If there's an altitude input on the calculator, it is using that info to calculate one or more of the above based on standardized assumptions. If you've got the data, it's generally clearest and best to just input temperature, pressure and humidity as measured at the site. Temperature is easy - use a thermometer, pressure can be had with a cheap altimeter, and humidity matters so little you can just guess.

 

[ballisticxlr]

When it goes from what you could call "normal" pressure, aka sunny & warm day to low pressure, aka warm & raining you'll see a reduction of about 1" of Hg at all the elevations I've taken the time to do those measurements at. Air temperature changes over the course of the average day is likely to mess you up more. Of note, 1000ft of elevation is about 1inHg of barometric pressure difference in the altitude range that humans normally choose to live in.

How much 1inHg differences changes your POI varies too much with MV/BC/BP/Temp to give a good number for. It's easy enough to generalize though... Under 500m it's not enough to say grace over. After 600m with the average rifle and high BC bullets it starts being you'll notice but won't probably be a miss. 1000m+ things start to get real and misses will be embarrassingly large.

With my .243AI it was a little over half a minute at 1000.