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Altitude vs. Barometric pressure

#43
01-12-2007, 07:20 PM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,740
Re: Altitude vs. Barometric pressure

[ QUOTE ]
b.After calculating my holds based on standard pressure and altitude, I run the equation again, only this time I uncheck the “Calculate Standard Pressure” box, and input the current station pressure obtained from my Kestrel 4000, and then run the program again.

[/ QUOTE ]

If youre using altitude along with station pressure, youre hosed.

If youre using station pressure, you should have your altitude set on 0' regardless of your actual altitude.
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Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
#44
01-19-2007, 08:45 AM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 3,974
Re: Altitude vs. Barometric pressure

Looks like Tubb/S&amp;B has generated interest in use of Density Altitude as a single air density input in the field. Might work ok, I'll have to try it.
It is easy to figure and graph for one bullet. More difficult for the rest of the world.
Here we go:

Air Density(Kg/m3) from DA =
((3280.84*44.3308-DA)/(3280.84*42.2665))^(1/0.234969)
Air Density ratio(Ro) to ICAO conditions would equal the air density above divided by 1.225
Local BC would be ICAO BC/Ro

Now Std Metro is less pretty because it's meaningless to the rest of the world.
Std Metro BCs COULD be(and really should be) normalized to ICAO BCs by [StdMetro BC*.982].
Or the density altitude formula above could be biased to account for a zero under StdMetro conditions by adding 625 to DA: ((3280.84*44.3308-(DA+625))/(3280.84*42.2665))^(1/0.234969), adjusting air density.
Then figure Ro = [StdMetro Air Density/1.20275]
And Local BC = [StdMetro BC/Ro]

Some seek a 'rule of thumb' for density altitude and drop table adjustments. But any rule of thumb would be specific to your BC as adjusted above and as washed through ballistic software for affects in the field.
Only then could these affects could be defined with a 'rule' or named something like 'DTAC'.
#45
01-19-2007, 07:28 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 3,974
Re: Altitude vs. Barometric pressure

Above reduced:
Air Density =((145442.26-DA)/(138669.62))^4.26
#46
01-19-2007, 07:41 PM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,740
Re: Altitude vs. Barometric pressure

Obviously, I would need to do more research on how to come up with accurate figures. Right now as I understand them, DA could be entered into the altitude field as long as humidity was set to default at 78%, temperature at 59 degrees and BP at 29.92. Am I safe to assume this is correct?
__________________
__________________
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
#47
01-20-2007, 08:52 AM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 3,974
Re: Altitude vs. Barometric pressure

I don't know what program you're using to get there. I don't know of any rules of thumb for DA, as it doesn't directly correlate to single variable changes. And back to the beginning of this thread; Altitude is still never needed. It's still about pressure, temp, humidity..

For example: given 59deg, 78%rh, and 24.9"Hg(5,000ft pressure altitude), DA = 6,345

DA would equal 5,000ft above if only temp changed to 39.7deg
DA would equal 5,000ft above if only pressure changed to 25.9"hg
DA would equal 5,000ft above if temp was 40deg &amp; Rh was 62%
Or if 59deg, 78%, and altitude was 3908ft(giving a standard pressure at altitude of 25.9")

I'm not a weatherman, I don't fly, and don't even know the history of density altitude right now(I have CRS, ADD, and a new one; self imposed misery -SIM). I researched it in the past, and wrote it all into a spreadsheet I use to calc bullet BC.
If I knew how to post a small spreadsheet here, I would make one just for it.
#48
01-20-2007, 11:46 AM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,740
Re: Altitude vs. Barometric pressure

[ QUOTE ]
I'm not a weatherman, I don't fly, and don't even know the history of density altitude right now(I have CRS, ADD, and a new one; self imposed misery -SIM).

[/ QUOTE ]

LOL!

I can relate. Sometimes these software issues can cause more headaches than they are worth. Someone once said to me, give a man a program and you will give him a headache. Teach a man to program and he will become an alchoholic.

Just trying to learn here. I came up with an example of what I THINK means I understande DA.

Example: shooting a given load in standard conditions and the bullet dropping 383.01" @ 1K. Bump only the temerature up to 110 degrees F. it drops only 353.90" @ 1K. By selecting DA and defaulting temp to 59 and humidity to 78% and BP to 29.92, and using only altitude, if I enter 2433' I come up with 353.90". Is this a correct way of understanding and putting to use DA?

I do understand its all about pressure and temp and that altitude is only valid by way of its effect on pressure which still goes back to the local pressure being the only one of the two that matters, but there are still other ways of arriving to one accurate bullet drop.
__________________
__________________
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
#49
01-20-2007, 01:37 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 3,974
Re: Altitude vs. Barometric pressure

You seem to have 'Density Altitude' and 'Pressure Altitude' mixed up. Different animals all together.

This may APPEAR to work under standard conditions at altitude(in your software), but a Krestel will not likely give you a DA equal to PA. Also, BP won't be 29.92 @ 2433ft(more like 27.4"), and 78% is not in the ICAO std, but instead the Std Metro standard: [59degF/29.53"/78%].

Above, your input of 110deg,78%,29.92"Hg will cause an air density of 1.087Kg/m3 and a DA of 4034Ft
Using 4034'DA with the formulas above I get AD=1.087

On a sidenote, You would have a PA=2433 AND DA=2433 AND Altitude=2433, at standard conditions for that altitude of 50degf &amp; 27.38"Hg &amp; 0%Rh. But air density here is 1.140 and it should have effected BC and your drops.

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