Advanced Marksmanship Barometric pressure, humidity, temperature?

[Powder_Burns]

Just wondering about how these factors can effect long range trajectories and how they are compensated for.

I'd also like to hear someone's take on barrel harmonics and how they mitigate the effects..I haven't seen any octagon barrels or barrel de-resonators in any pics on this site, curious as to why they are not used.

 

[Lowlight]

Re: Barometric pressure, humidity, temperature?

More molecules in the air make it harder for the bullet to fly.

Short answers follow,

Baro = Bigger the number = harder to fly

Temperature = Lower the number = harder to fly

Humidity = Lower the number = harder to fly

 

[rmfield]

Re: Barometric pressure, humidity, temperature?

Start here .

 

[Powder_Burns]

Re: Barometric pressure, humidity, temperature?

Wow I visited that site like 5 or 6 times last week looking for load data and completely overlooked that section. Thanks for the heads up! Now I have some reading to do..

 

[JRose]

Re: Barometric pressure, humidity, temperature?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">More molecules in the air make it harder for the bullet to fly.

Short answers follow,

Baro = Bigger the number = harder to fly

Temperature = Lower the number = harder to fly

Humidity = Lower the number = harder to fly </div></div>


Simple and concise... Love it.

 

[Suasponte]

Re: Barometric pressure, humidity, temperature?

The 20degree rule - Every 20degrees in temperature change will beget a 1moa change in POI. If you zero at 60deg and shot when it's 80deg the bullet strike should be 1 MOA higher. Bullet impact follow the temperature trend (Temp up/POI UP) (Temp Down/POI down). The opposite follows with the RH (relative humidity), RH goes up bullet impact goes down.

Terry

 

[Lowlight]

Re: Barometric pressure, humidity, temperature?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: suasponte</div><div class="ubbcode-body">The 20degree rule - Every 20degrees in temperature change will beget a 1moa change in POI. If you zero at 60deg and shot when it's 80deg the bullet strike should be 1 MOA higher. Bullet impact follow the temperature trend (Temp up/POI UP) (Temp Down/POI down). The opposite follows with the RH (relative humidity), RH goes up bullet impact goes down.

Terry</div></div>

Oh, ya, at what range does it change 1 MOA, 100 yards, 300 yards, 1000 yards... tell me where (at what distance) the 1 MOA rule works.

and if the humidity goes up, the air density goes down, so how does the impact go down if water vapor is lighter than air. its the reason why we aren't awoken each night to the sound of clouds crashing into the ground.

 

[Graham]

Re: Barometric pressure, humidity, temperature?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">...the reason why we aren't awoken each night to the sound of clouds crashing into the ground.</div></div>It's the 'Chicken Little' theory of bullet performance: ask Bryan Litz, he could probably explain it to you.

Seriously, though, it could be that suasponte got his info from an incomplete reading of Mike lau's book, page 227, the NGSSS deviations from zero chart.

 

[sawhornsoff]

Re: Barometric pressure, humidity, temperature?

read up on density altitude

 

[Graham]

Re: Barometric pressure, humidity, temperature?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sawhornsoff</div><div class="ubbcode-body">read up on density altitude</div></div>Good idea. I can't be bothered to re-set my Kestrel's altitude window, but using DA is an alternative way to derive trajectory information.

 

[Sig685]

Re: Barometric pressure, humidity, temperature?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body"> its the reason why we aren't awoken each night to the sound of clouds crashing into the ground. </div></div>

Next time, could you PLEASE post a warning about reading your message while drinking a soda. Cola shot out of my ears and nose. I now have a mess to clean and it hurt like hell.

 

[Powder_Burns]

Re: Barometric pressure, humidity, temperature?

Lowlight I was under the assumption that higher humidity index = more water molecules in the air = more dense air mass = more air resistance = lower bullet impact.

Can you elaborate on your view? I noticed that what Terry mentioned about the 20 degrees/1 MOA change in POI was mentioned, almost verbatim to what he said, in TC 23-14(us army training circular, distributed as 'sniper training and employment') pg. 3-15. They basically say a +20 degree temperature change can produce a 50fps increase in muzzle velocity, and a -20 degree temperature change reduces muzzle velocity by approximately 50fps, and that roughly constitutes a 1 MOA change in elevation.

 

[sawhornsoff]

Re: Barometric pressure, humidity, temperature?

Higher humidity results in a LOWER air density.

 

[sawhornsoff]

Re: Barometric pressure, humidity, temperature?

Given similar conditions of temperature and barometric pressure there is always the same number of molecules in the air regardless of the ration of components of predominately N2, O2, and water vapor. One molecule of water weighs less than either the N2 or O2. Therefore adding water vapor at the expense of some of the other gasses results in the volume of gas weighing less, thereby being less dense.

 

[Powder_Burns]

Re: Barometric pressure, humidity, temperature?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sawhornsoff</div><div class="ubbcode-body">Given similar conditions of temperature and barometric pressure there is always the same number of molecules in the air regardless of the ration of components of predominately N2, O2, and water vapor. One molecule of water weighs less than either the N2 or O2. Therefore adding water vapor at the expense of some of the other gasses results in the volume of gas weighing less, thereby being less dense. </div></div>

Ok now THAT makes sense to me. So, if I am normally shooting in 80-100% humidity climate, and were to travel someplace that has very low humidity I would have to increase my elevation if a shot of considerable distance were to be taken (bullet drop aside). Is there a ratio or proportion to calculate such effects given variables such as b.c., velocity, temperature, humidity, over distance/time?

 

[Lowlight]

Re: Barometric pressure, humidity, temperature?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Powder Burns</div><div class="ubbcode-body">Lowlight I was under the assumption that higher humidity index = more water molecules in the air = more dense air mass = more air resistance = lower bullet impact.

Can you elaborate on your view? I noticed that what Terry mentioned about the 20 degrees/1 MOA change in POI was mentioned, almost verbatim to what he said, in TC 23-14(us army training circular, distributed as 'sniper training and employment') pg. 3-15. They basically say a +20 degree temperature change can produce a 50fps increase in muzzle velocity, and a -20 degree temperature change reduces muzzle velocity by approximately 50fps, and that roughly constitutes a 1 MOA change in elevation. </div></div>

The books are wrong... wrong about humidity number 1, and wrong in printing a "rule of thumb" while failing to tell you "Where" you need to add 1 MOA... so again, I ask you where does this "rule" apply too, 100 yards, 300 yards, 700 yards, 800 yards, or of the above...

Higher humidity making the air "feel" denser is a human reaction to the water vapor, and has nothing to do with a bullet.

As well, what powder is that temperature sensitive to know it is going to change 50fps in a 20 degree shift, this isn't 1968 and we have better stuff, so to say it is going to change 50fps is assuming a lot.

 

[Lindy]

Re: Barometric pressure, humidity, temperature?

As Lowlight knows, the only way to know how the muzzle velocity of a particular load will change with temperature is to chronograph it at different temperatures.

I have tested loads which changed 0.6 fps per degree F., and I have tested loads which changed almost 5 fps per degree F.

The only way to know is to test. Rules of thumb do not often work for accurate first shots at long distance.

 

[Mo_Zam_Beek]

Re: Barometric pressure, humidity, temperature?

I never knew how little I really know about LR shooting until reading this thread.......


Thank you so much.


Good luck