Bullet drop with Elevation change

Discussion in 'Long Range Hunting & Shooting' started by Niceshot, Aug 20, 2019.


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  1. entoptics

    entoptics Well-Known Member

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    JBM Ballistics is a good online calculator

    If you uncheck the box "Target Relative Drops" near the bottom, it will ignore your zero range, which is helpful to see the true affect of atmosphere conditions. For example, if you have a 300 yd zero input, the bullet will always have zero drop at that distance regardless of conditions, as the software is forcing it there. With "TRD" off, you will see how much actual variation in drop at a given distance for given conditions.

    Also, just to be rigorous, humidity actually decreases air density (H2O is less dense than N2).

    Increasing heat, elevation, and humidity makes the air less dense, and vice versa. Really though, humidity has a very small affect overall, so temperature and station pressure (altitude) are the important variables.

    Just ran some generic numbers for 178 ELDX at roughly 30-06 speeds.

    2% humidity vs 98% = ~1" more drop at 1000 yds
    2000 feet vs 6000 feet = ~24" more drop at 1000 yds
    60° vs 80° = ~8" more drop at 1000 yds
     
    bomberodevil, BAMBAMODA and Niceshot like this.
  2. jshepherd61

    jshepherd61 Well-Known Member

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    What Richard said in the first reply however elevation is not what effects your shot! It is air density (thinner air at altitude) and humidity (high humidity, thicker air/low humidity, thinner air) just for example I load and test in CO at roughly 6,000 ft. I hunt in CO at 9,000 ft to 11,500 ft. Normally in Summer and spring it can be 25% humidity in the Denver area but in the dead of winter in Crested Butte at 10,000 ft the humidity may be 15% during elk hunt providing it stays cold so my muzzle velocity is normally faster at the lower dryer air. So weather is what actually effects your shot/velocity or drop. Density of altitude not true altitude. I’m normally a inch to two inches high at altitude providing weather cooperates. So when using ballistics apps the key importance is the weather conditions!
     
  3. coyotezapper

    coyotezapper Well-Known Member

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    If you go with a 100 yard zero you will not need to change it.

    A RF like the GW BR2 will account for the elevation change for you. If your RF does not have this feature then just buy a simple ballistics app for your phone or go to jbm ballistics website and input your data and elevation and then print it out and take it with you.
     
  4. entoptics

    entoptics Well-Known Member

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    As stated above, this is incorrect. Higher humidity DECREASES the atmospheric density.

    Air is ~78% Nitrogen (N2) which has an atomic weight of 28, and 21% Oxygen (O2) which has an atomic weight of 32. Average atomic weight is therefore about 28.6.

    H2O has an atomic weight of 18.

    Remember, this is not liquid water. It's water vapor (gas).

    Avogadro's Law (and later Ideal Gas Law) states that equal volumes of all gases, at the same temperature and pressure, have the same number of molecules. By increasing H2O vapor (lighter) the proportion of N2+O2 (heavier) decreases. Since you're replacing a heavier substance with a lighter substance, while keeping the same volume, you've lowered the density.

    If you don't believe me, run JBM and see. Keeping all other variables the same, change the humidity from 2% to 98% as I did in my above example, and you will see that the lower the humidity, the more the bullet drops.

    That being said, the effect is minimal, and you can mostly ignore it for anything but extreme long range shooting.
     
  5. I’d agree it’s expensive. I’m interested in hearing what problems You were having.

    ‘1st rd hits at a mile’ that’s legit but I’ve done that with AB as well. Granted tgt was a tank so.....not a lot to brag about there.
     
  6. Larry?
     
  7. GeorgeS

    GeorgeS Well-Known Member

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    Since the issue on drop is time of flight, altitude is not the only variable - the direct measure is air density, which is what affects drag. That means humidity as well. More humid, more drop, less dense, less drop. But it's the drag that affects the time of flight, and time of flight that affects the drop.
     
  8. jshepherd61

    jshepherd61 Well-Known Member

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    Not if you do not take weather conditions into effect! That’s why we carry weather meters along with range finders in the military. Unless you spend the 1,700 for the Sig integrated range finder weather meter. You can be off as much as 8” when going from sea level to 10,000 ft with a temp drop of 30 to 50 degrees.
     
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  9. entoptics

    entoptics Well-Known Member

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    You are correct that air density is proportional to drag (more density = more drag = more drop), but as I've stated twice in this thread already...

    More humid = less dense = less drop.

    I think this myth persists because it "feels" right. Humidity "feels" thick and soupy. Unfortunately physics and "feel" don't always match up.
     
  10. catamountsierra

    catamountsierra Well-Known Member

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    My goof on the humidity effect, though I think overall temperature and pressure (altitude) effects are significantly greater. I've often wondered though how fog or rain would affect bullet flight.
     
  11. Bisbee

    Bisbee Member

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    I had an interesting experience with altitude. I had loaded 150 grain TSXs for elk in my 270, they stabilized just fine at 5000 ft elevation, where I live. When I shot them at 7000+ ft elevation where I was drawn for elk, you could not hit a paper plate at 100 yds.
     
  12. Hand Skills

    Hand Skills Well-Known Member

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    Theoretically, it seems unlikely due the increase in elevation - an increase in elevation represents an increase in gyroscopic stability, because the air is thinner and offers less resistance (all other things equal).

    Temperature has a similar effect.

    For example at 5000ft and 80F air behaves the same, offering the same density as it does at 7000ft and 50F.

    Ballistic computers with built in atmospherics are really nice because they calculate these variables for the user based on the present conditions.

    This is something that demands consideration though if one is using a manual range card, or a BDC reticle/turret. What set of conditions were the holdovers calculated for?

    When the conditions change, the flight changes. Pilots know this well and along with aeronautical engineers, and they use a term 'Density Altitude' to describe the density of air in a given condition. Density Altitude combines temperature, humidity and altitude all into one value.

    This makes prediction a bit easier, but as @Bisbee clearly pointed out, there is no substitute for verifying a load in the conditions one intends to hunt in!