Altitude vs. Barometric pressure

[ptosis]

LR hunting accuracy is not a game of probabilities.
It's a game of capabilities.
A matter of constant proof with each and every single shot.

I agree. And that's exactly why probabilities are important. I'd rather know my capabilities beforehand, wherever reasonably possible, rather then discover them by trial and error "with each and every single shot".

 

[eric1115]

Just to add my CHF 0.02 to the old thread. In case you were wondering how pressure measurements affect hit probability, here's a picture:

View attachment 300237
(Lapua Scenar 10 g, cal. 7.62, V0 = 850 m/s, other conditions are assumed excellent)

The scenarios are:
0: ideal
2: Kestrel-style handheld barometer
7: By altitude (std pressure at altitude, ignoring daily and seasonal variations) using map or GPS in mid latitudes of Europe and America. Achtung: this does not apply to deep continental Asia, where seasonal variations are huge.
14: By altitude in Arctic, or anywhere using crappy mechanical pocket barometer
28: Deep continental Asia (e.g. South Siberia) by altitude, or "by feeling" altitude in a mountain relief.

People tend to overestimate the importance of pressure measurements. Most of the time, if you know your altitude and use the standard atmosphere table value, you are very close to the truth (at 1000 m, about 3% loss of hit probability, as compared to the ideal situation). Accurate temperature measurements are WAY more important.

Thank you for putting values to this. I've been comparing station pressure measured by my handheld GPS unit and on my phone, to elevation data using standard pressure for that elevation, and the differences are minor (in the measurements I have taken and the shooting solutions I have applied them to).

It's reassuring to see that I'm not alone in thinking that measuring environmental conditions is not super important at hunting ranges if I have elevation and temp correct.

Also, I had no idea that pressure variations were so much greater in the Arctic and interior Asia. That's super interesting.

 

[ptosis]

just to put some more colourful pictures in the thread:

median error of determining pressure by altitude

but the variations in Asia are seasonal, while Arctic are not; seasonal variations (between Jan and Jul):

(analysis of 10 years data from 11K+ weather stations)

 

[ptosis]

Thank you for putting values to this. I've been comparing station pressure measured by my handheld GPS unit and on my phone, to elevation data using standard pressure for that elevation, and the differences are minor (in the measurements I have taken and the shooting solutions I have applied them to).

It's reassuring to see that I'm not alone in thinking that measuring environmental conditions is not super important at hunting ranges if I have elevation and temp correct.

Yep. Absolutely agree.

In the overall balance, at mid and long range, temperature is the 3rd most important uncertainty factor, behind distance and wind.
At the same time, the error of determining pressure by altitude is an order of magnitude behind these three.

 

[Laguna Freak]

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.

^^^ this!
I prefer the JBM ballistic modeling algos. Most of the calculators needed are in their website. The Ballistic AE app on my phone uses JBM.

 

[TXTrained]

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.

Mike where do you learn all this.I don't know it all.Trying to learn as much as I can.Im in Brazoria TX.
Where are you from.Thanks

 

[Litehiker]

"Mike in Texas",
if you'd like your bullet to drop only 1/2" at 1,000 yards use a 140 gr. 6.5 Creedmoor instead of that old .308.
Better yet get a 6.5 PRC. I have it in my Browning X-Bolt Pro and for western hunting it is great.

 

[TXTrained]

I

"Mike in Texas",
if you'd like your bullet to drop only 1/2" at 1,000 yards use a 140 gr. 6.5 Creedmoor instead of that old .308.
Better yet get a 6.5 PRC. I have it in my Browning X-Bolt Pro and for western hunting it is great.

I am shooting a 6.5 PRC. Thanks.

 

[TXTrained]

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?

Do you think or consider gravity pull has more or less effect on the bullet drop at sea level is the same at 5000 feet above sea level.
At 5000 ft the air is colder and we know temperatures have effect on bullet flight? I don't know..Just curious. THANKS

 

[TXTrained]

"Mike in Texas",
if you'd like your bullet to drop only 1/2" at 1,000 yards use a 140 gr. 6.5 Creedmoor instead of that old .308.
Better yet get a 6.5 PRC. I have it in my Browning X-Bolt Pro and for western hunting it is great.

Texans think alike..lol

 

[RTK]

Planes like low density altitude thin air?

Bullets like high density altitude thick air?

Isn't that backwards?

Nope, it's the opposite of what it sounds like.

 

[PApa Black]

Nope, it's the opposite of what it sounds like.

Gravity is Gravity. Atmospheric drag changed with altitude, temperature, humidity, barometric psi and wind.

 

[RTK]

I am Talking about the word "high" and "low" density altitude, I'm a pilot. "High Density altitude" means thin air opposite of what it sounds like.

 

[Trnelson]

Gravity is Gravity. Atmospheric drag changed with altitude, temperature, humidity, barometric psi and wind.

Gravity is proportional to your elevation. It is decreasing as you move away from the center of the earth. The force of gravity is reduced by 1/R^2 where R represents your distance from the center of the earth.