Barometric Pressure and Ballistics Calculators

[TimberHunter]

While comparing Zeiss’s calculator to Vrotex’s, Nikon and Hornady, I noticed something interesting....

At 5100’ elevation, airport weather report says 30 Inch HG pressure. This location fluctuates approx +-.5 Inch HG through out the year (minimal effect on calculation at 700 yards and under). However, when I input 5100’ elevation into the ballistic apps and click “calculate pressure from elevation” they usually set the pressure around 24 Inch HG.

Well that change from 30 to 24 results in a 4-5” difference in drop at 500 yards. To some that may seem minimal but I like to get as accurate as possible.

For kicks and giggles I looked up San Diego’s and Tamapa Florida pressure and they were essentially 30 inch HG despite being near 0 elevation.

Any idea why the calculators estimate the pressure so low?


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[mcr-85]

Standard pressure is 29.96 at Sea level or something close to that. As you go up in elevation there is less pressure. Weather stations adjust the number so it reads like it does at Sea level. I take off 1" per 1000' elevation if I don't have something to measure. Usually I have my watch that reads baro pressure tho.

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[TimberHunter]

Standard pressure is 29.96 at Sea level or something close to that. As you go up in elevation there is less pressure. Weather stations adjust the number so it reads like it does at Sea level. I take off 1" per 1000' elevation if I don't have something to measure. Usually I have my watch that reads baro pressure tho.

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So the ballistic apps are correct then...?

My real world shooting indicated they were under estimating the actual drop at certain yardages of my ammo. For example at 500 yards, letting the calculators estimate the pressure at 5100’, it says 38” of drop. I got nearly 44” of drop when shooting. Using the airport supplied pressure, the zeiss calculator changed the drop to 43”




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[TimberHunter]

Standard pressure is 29.96 at Sea level or something close to that. As you go up in elevation there is less pressure. Weather stations adjust the number so it reads like it does at Sea level. I take off 1" per 1000' elevation if I don't have something to measure. Usually I have my watch that reads baro pressure tho.

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After some additional thought, I decided to change the BC of the Hornady interlock bullet from .409 to .380 (delta of .029) and with the chrono verifies velocity, the calculated drop is 40” at 500. That is within an inch or two of what I’m seeing as actual drop in the field


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[mcr-85]

After some additional thought, I decided to change the BC of the Hornady interlock bullet from .409 to .380 (delta of .029) and with the chrono verifies velocity, the calculated drop is 40” at 500. That is within an inch or two of what I’m seeing as actual drop in the field


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I use the shooter app. It has a setting where you check "pressure is absolute" and it takes the elevation off as a variable. I only run it with station barometric pressure that I measure with my watch.

As far as drops lining up with range data, I use the velocity calibration tool in the app. I put in my range and the moa adjusted and it will adjust my velocity to match real world drops. I haven't ever adjusted the BC of any of the bullets I shoot.

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[ericF]

While comparing Zeiss’s calculator to Vrotex’s, Nikon and Hornady, I noticed something interesting....

At 5100’ elevation, airport weather report says 30 Inch HG pressure. This location fluctuates approx +-.5 Inch HG through out the year (minimal effect on calculation at 700 yards and under). However, when I input 5100’ elevation into the ballistic apps and click “calculate pressure from elevation” they usually set the pressure around 24 Inch HG.

Well that change from 30 to 24 results in a 4-5” difference in drop at 500 yards. To some that may seem minimal but I like to get as accurate as possible.

For kicks and giggles I looked up San Diego’s and Tamapa Florida pressure and they were essentially 30 inch HG despite being near 0 elevation.

Any idea why the calculators estimate the pressure so low?


Sent from my iPhone using Tapatalk

As mcr-85 mentioned, the barometric pressure that you see is a normalized air pressure against sea level. That isn't the actual air pressure that you would experience there and you need the station pressure which is the actual air pressure that you are experiencing.

http://appliedballisticsllc.com/ufaqs/barometric-pressure-vs-station-pressure/

 

[tdhanses]

I started to use density altitude instead, so far I like it better. I use the weatherflow to measure it.

 

[mcr-85]

This is a pretty good article. Gunwerks explains it better than I do.

https://www.gunwerks.com/blogs/long-range-magazine/shooting-under-pressure

 

[TimberHunter]

I use the shooter app. It has a setting where you check "pressure is absolute" and it takes the elevation off as a variable. I only run it with station barometric pressure that I measure with my watch.

As far as drops lining up with range data, I use the velocity calibration tool in the app. I put in my range and the moa adjusted and it will adjust my velocity to match real world drops. I haven't ever adjusted the BC of any of the bullets I shoot.

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Thanks for the info! So you adjust chronographed verified speeds to match your drop?


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[hrnhntr]

That's where I like the AB on my Kestrel since it is pulling Density Altitude and I feel that that is most accurate.

 

[mcr-85]

Thanks for the info! So you adjust chronographed verified speeds to match your drop?


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Yes, with the velocity calibration tool within the app. It's best if you can shoot multiple ranges and put your actual drop data in. As long as you have your weather conditions accurate and your gun and bullet data correct then use the velocity calibration to get the app to match your actual data.

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[Browninglover1]

Density altitude still needs temperature to be entered manually into the calculator to accurately calculate the Mach number. Absolute/Station pressure also need the temperature entered as well so you can use either DA or Station pressure but just make sure to also enter your temperature with both.

 

[Browninglover1]

Also, if you're using a Magnetospeed or Labradar I wouldn't be adjusting velocities I would verify all other parameters are correct, and then start tweaking BC.

 

[JimCraig]

Agreed, that if you know what DA is, and how to get it, and have the combination of tools to apply it, it is good to go. Regarding baro pressure, don't even mess with the whole sea level corrected, weather station based barometric pressure. You need to use absolute pressure, so you need an app that allows you to enter absolute pressure and you need an accurate weather meter that gives you absolute pressure and accurate temperature.

 

[TimberHunter]

Also, if you're using a Magnetospeed or Labradar I wouldn't be adjusting velocities I would verify all other parameters are correct, and then start tweaking BC.

I’m using a basic Caldwell chrono


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