Long Range Hunting Online Magazine Altitude vs. Barometric pressure
 Home LRH Store Forums Long Range Rifles Articles Reviews Video G7 Ballistics Calculator Rules & FAQ Register Search Today's Posts Mark Forums Read

# Altitude vs. Barometric pressure

#1
06-19-2005, 09:50 AM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,263
Altitude vs. Barometric pressure

Example: RSI Ballistic calculator calculates that at sea level in 59 degree air and 29.53 BP a bullet with a .540 BC at 2700 FPS zeroed at 300 yards will drop 307.9" at 1000 yards.

The same program, same BP, bullet, velocity, zero, temprature, but change the altitude to 5000' and the bullet drop is calculated to be 315.8 with the same barometric pressure imput.

The differance is 7.9"

I always thought that the actual pressure and not altitude changed the bullets perfomance. I know pressure changes with higher/lower altitudes go hand in hand, but thought that altitude in and of itself had no effect on the bullet and just the pressure change with the altitude.

Any ideas?? Is the RSI program correct or is it a glitch?
__________________
Limits are only perceived by what has previously been achieved. Push the limits and good things will happen.

Go the distance or go home

Follow through matters. SEE THE FIRE!!!

August 2013 dall sheep: Passed one legal ram. Yes I regret it....
August 2013 Antelope: SUCCESSFULL!
October2013 Bull elk: Had to burn the tag.
It's been a GREAT year!
#2
06-19-2005, 09:56 AM
 Silver Member Join Date: Dec 2003 Location: Texas Posts: 110
Re: Altitude vs. Barometric pressure

What really effects bullet flight is station pressure,(absolute pressure) at your location.

As an example, a station pressure change from 28.85 to 28.80 means that at 1000 yards my 308 load will need 1 full moa less elevation to strike the same point.

Different programs try to back out corrected sea level based on temp and altitude. Some hit, most don't.
#3
06-19-2005, 12:42 PM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,263
Re: Altitude vs. Barometric pressure

So if I understand correctly, the most accurate way to use RSI is to leave the altitude field at 0' and just use the pressure field to imput the current pressure at my location no matter what the altitude.

Right?

__________________
Limits are only perceived by what has previously been achieved. Push the limits and good things will happen.

Go the distance or go home

Follow through matters. SEE THE FIRE!!!

August 2013 dall sheep: Passed one legal ram. Yes I regret it....
August 2013 Antelope: SUCCESSFULL!
October2013 Bull elk: Had to burn the tag.
It's been a GREAT year!
#4
06-19-2005, 02:57 PM
 Platinum Member Join Date: Dec 2001 Location: Mukilteo, WA Posts: 1,090
Re: Altitude vs. Barometric pressure

If absolute pressure, yes. The confusion comes from "corrected" baro readings for elevation. Say, when the weatherman in Denver says the baro is 29.9, it isn't really absolutely. He's correcting for the elevation, taking the lower reading (24.7 or so) and giving you an equivalent for what it "would be" at sealevel. They do this because the corrected number is more important and useful as far as weather predicting goes. Otherwise it would look like every mountain had its own "low pressure system" surrounding it.

So in short, if you obtain corrected baro numbers for your altitude (29 or something at 5000 ft) you should also enter the elevation into the program. If you obtain absolute baro numbers (much lower for higher altitudes) and use those, you can leave the altitude as zero in the program as the pressure takes care of it.

I hope that makes some sense. I'm not familiar with this particular program but just speaking in general. With any program you sort of have to figure out what input it's looking for so it corrects, but doesn't double correct.
#5
06-19-2005, 06:30 PM
 Bronze Member Join Date: Nov 2003 Location: Woodbine, MD Posts: 34
Re: Altitude vs. Barometric pressure

Couple pretty good articles on the subject here: http://www.riflebarrels.com/articles...erformance.htm

And here: http://www.riflebarrels.com/articles...e_humidity.htm

Helped me get a handle on it.

marc
#6
06-19-2005, 08:18 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 2,826
Re: Altitude vs. Barometric pressure

Your program is assuming pressure at altitude of 29.53, instead of 24.923(std conditions at altitude). Nothing wrong with this if you measured and appropriately entered 29.53 at 5000'. Higher pressure, more drop.
When you enter 29.53 @ 0', this is your pressure at entered altitude. If thats your measured absolute pressure, then your right, you should leave altitude at zero. Even if your standing 5000' up.
#7
06-20-2005, 01:13 AM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,263
Re: Altitude vs. Barometric pressure

That all helps ALAOT!!! Thankx guys. I am actually finishing up a ballistic calculator incorporated into The Reloader's Archive program.

What I have done is inserted two check boxes. One for Altitude and one for pressure. It allows users to choose between which they want to use, but allows only one. When altitude is selected, the pressure field dissapears and is defaulted to 29.92. When pressure is selected, altitude dissapears and is defaulted to 0'. That way the calcs dont get confused. I needed to verify that this was correct before I finish it up and start distributing updated copies.

__________________
Limits are only perceived by what has previously been achieved. Push the limits and good things will happen.

Go the distance or go home

Follow through matters. SEE THE FIRE!!!

August 2013 dall sheep: Passed one legal ram. Yes I regret it....
August 2013 Antelope: SUCCESSFULL!
October2013 Bull elk: Had to burn the tag.
It's been a GREAT year!

 Bookmarks

 Thread Tools Display Modes Linear Mode

 Similar Threads for: Altitude vs. Barometric pressure Thread Thread Starter Forum Replies Last Post tlk The Basics, Starting Out 3 07-10-2010 10:11 AM Tikka Lover Rifles, Bullets, Barrels and Ballistics 7 10-05-2008 11:32 AM Len Backus Long Range Hunting & Shooting 13 08-04-2008 09:05 PM Dave Squires Rifles, Bullets, Barrels and Ballistics 1 07-06-2005 10:16 AM Matt27 Rifles, Bullets, Barrels and Ballistics 2 12-02-2002 11:50 PM

 Current Poll Do you wear hearing protection while hunting? Usually do Usually do not No but I probably will in future
 Current Poll - 2 Do you have a high speed Internet connection? Yes, DSL, satellite, wireless or something else. - 89.17% No, only dialup right now. - 10.83% Total Votes: 3,037You may not vote on this poll.

All times are GMT -5. The time now is 11:14 AM.