Altitude vs. Barometric pressure

[scfam]

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.

BINGO. Absolute baro pressure, along with humidity and temp pretty much has you handled. If you are using relative, then you need to also use elevation. Good answer.

 

[Bart B]

An increase in altitude decreases the gravitational force. We have to worry about that when we are aiming at flea's eyes at 1000 meters and correcting for the gravitational attraction of the mountain to our side.

This is the first time I've heard of any gravity issue.

Never in my thousands of talks with long range rifle shooters at matches and social events with many of them has this issue came up regarding altitudes of 100 to 6,600 feet above sea level shooting 1000 yard matches. Nor has that of coriolis and spin drift. But such things appear on web sites very often and claimed to be very important.

Exactly how much difference in bullet drop at 1000 is there for a 30 caliber 200 grain bullet with a BC of .560 leaving at 3000 fps at 200 feet and 6600 feet elevation in standard atmospheric conditdions?

Is it greater than a 1% spread in that bullet's BC would cause?

 

[Mikecr]

Gravity is difficult to predict. It changes with geographic density (like mountains) regardless of altitude. Dismissing this, your gravity difference from 6400 feet scenario could lead to .30" change at 1kyd. 1% BC would be 1.2".
So gravity, and given it's unpredictability, is really too small an affect to measure or account for with small arm ballistics(even at distance).

But technically it is a factor, and so is spin drift and vertical and horizontal Coriolis.

 

[Bart B]

One thing few understand is the wind above the line of sight is blowing faster than in the line of sight. Here's a table showing this and two ammo types to see how it effects wind doping:

And the wind closest to the person shooting has more effect on bullet drift than the wind close to the target.

 

[alcesgigas]

Thank you Bart for your comment concerning wind effect: It's a good feeling to have confirmation, scientifically, of what you've reasoned (or logic) is accurate. I've pretty much held that wind effect is greater at the target the closer the interference is to the projectiles origin. My reasoning has been, like a small branch nicked by a bullet at 50' is more likely to miss the target at 300 yards than the same nick--all other things being equal--occurring 50' from the target. It's similar to a .25" miss at 100 yards is most likely to miss by 2.5" at 1000 yards; same for scope adjustments.

In spite of its span in years this is a most informative thread; I'm enjoying it still.

 

[CatShooter]

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.

Thanks again!

When you look at the "External Ballistics" page, there is a "Conditions" tab.
Use that and choose "Standard conditions (Army or ICAO).

Then, there are two small diamonds... "Station" and "Altimeter".
Click "Station".

Then enter your actual barometric pressure from your hand held instrument (not from the radio weather report - because it is "adjusted", and not actual, local pressure).

 

[LastShot300]

One thing few understand is the wind above the line of sight is blowing faster than in the line of sight. Here's a table showing this and two ammo types to see how it effects wind doping:

And the wind closest to the person shooting has more effect on bullet drift than the wind close to the target.

Bart

Do you know, personally or not, if this formula is really accurate? Thanks for sharing it.

 

[Hand Skills]

Supplemental data

 

[Danehunter]

Just buy a Kestrel/AB 5700 and let it calculate all the variables including wind at the FFP and you will have a solution that includes "station pressure" and even coriolis effect if shooting beyond 900 yards.

Eric B.

 

[Trnelson]

View attachment 86702
Supplemental data

welcome to a fellow pilot.

 

[RTK]

Amazing how many fellow pilots hunt.

 

[corey006]

So just a quick question.

You arrive at a hunting location.

Zero your rifle for that elevation and temps are relatively stable.

How does day to day changes barometric pressure affect POI @ 700+ yards IF elevation and temp remain constant?

 

[CatShooter]

So just a quick question.

You arrive at a hunting location.

Zero your rifle for that elevation and temps are relatively stable.

How does day to day changes barometric pressure affect POI @ 700+ yards IF elevation and temp remain constant?

If all else stays the same - pressure goes up, bullet goes down.

 

[wildcat westerner]

Will try to add .02 worth to this discussion. I am way behind the curve as to ballistics programs that can provide the CORRECT answer for a single shot at long distances. Coming from sea level Seattle to 6,500 feet in the Rockies I note my 6.5 x 284, which I have been shooting for 16 years, shoots flatter to the point you can tell on a target at 300 yards. So your statements as to entering 0 for altitude in your programs, really takes some adjustment on my part. I do understand you are speaking of arriving ata right answer and this is the best way to manipulate a ballistics program to achieve a correct conclusion. I have never used my I phone in the field to use a ballistics program, and realize I will also have to carry my anenomoter along also to access the correct data for that location and weather information. Bottom line to me is this: I have been told by a very savvy shooter that if you compare between free programs and very expensive ballistics programs, the actual difference between accuracy at long range is very little. The bottom line for me is which is the very best program i want for my I phone, for the money?
Thanks for your attention in this matter.
Gene S.