Trajectory vs. Humidity

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by MAX, Jun 24, 2003.

  1. MAX

    MAX Well-Known Member

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    It's of little note at short range, just like temperature, but I'm wondering if any of you folks factor humidity in your calculations for long shots. If so, how; if not, why not? It does change air density after all.

    Thanks
     
  2. Oli

    Oli Member

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    Increased humidity makes the round hit higher. That will be my contribution if that was news to you.
     
  3. MAX

    MAX Well-Known Member

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    Thanks, but I figured that out awhile back [​IMG] Must be someone out there that worked this over... [​IMG]

    Come on guys, humidity helps defy gravity! Gotta be some influence at 2000 yards, don't you think? [​IMG]

    I see nothing in books by Vaughn, McCoy, Rinker or other articles on this subject. It affects the bejeesees out of helicopter performance, bullets cannot be immune.
     
  4. Oli

    Oli Member

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    So you figured that one out. Some military sources (american) that I looked up said that it was the other way around [​IMG]
     
  5. ewallace

    ewallace Well-Known Member

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    With my 338 Yogi and 300g SMK at 3050 fps if shooting at 2000 yards at a temp of 40 degrees 20% humidity BP of 29.92 and elevation of 900 feet above sea level. I come up with 70 MOA come up. If the humidity in increased to 95%. I would need 69.5 MOA. Then if you increase the temp to 100 degree I come up with 60 MOA.
    Crow Mag
     
  6. 700

    700 Well-Known Member

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    Oops. Guys you are completely right. Increased humidity flattens tradjectory. This is due to the molecular weight of water being less than dry air.

    The gif
    [​IMG]

    Shows a drop difference of 5.6 inches at 1000 yards between a .308 fired at 59 Deg. F and 0% Hum, versus 59 Deg. F and 100% Hum, without scope adjustments been made. Plots higher at 100%.

    I believe the USMC marksmanship manuals state humidity makes bullets plot lower, which I think is what initally came to mind.

    Rgd

    700
     
  7. MAX

    MAX Well-Known Member

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    Crow Mag, what was the source of that calculation? JBM? Or a hand held computer?

    700, in jest I say: You quote Uncle Sam's Misguided Children? [​IMG] You are dot on about the molecular weight. FWIW Dept., Rinker said in his book that humid air was denser, Vaughn said dry air is denser...Vaughn wins without debate.

    Guess my question was answered in part by Crow Mag's post, ie. not much effect. On the other hand there is a reason they refer to it as RELATIVE HUMIDITY. The warmer it is, the more effect it must have. Maybe unimportant in cold winter air, BUT, if you're shooting prairie dogs at a mile on a warm summer day...well who knows?

    I do not have the performance charts for this but in tropical environments(S.E. Asia) the effect of humidity was potentially as great as Density Altitude on any given day. DA=pressure altitude adjusted for Temp. 4000' DA's at sea level were routine there. In 1970 or so some bright lad let us in on a little secret: 90-100% humidity on hot days could nearly double the figure above, the significance being that a UH-1 could hover at gross weight out of ground effect with a 4000' DA. Doubling that number cut the payload in half OR ensured an LZ marked with black smoke. [​IMG] The Devil is in the details.
     
  8. Charles A

    Charles A Well-Known Member

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    Yes, not much to do with trajactory. In fact most times less than 2% of your final calculation. [​IMG]
     
  9. 700

    700 Well-Known Member

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    Increasing temperature makes a bullet plot higher.
    Increasing humidity makes a bullet plot lower.

    Rgds

    700
     
  10. moosehunter

    moosehunter Well-Known Member

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    Lets talk 100% humidity. What about shooting 1000yd. in the rain? Ive got a shoot day after tomorrow and the forcast is rain so Im a tad curious.
     
  11. MAX

    MAX Well-Known Member

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    I think in the rain you have to calculate the Relative Air index. Kinda the reverse of my question. [​IMG] Big drops or small? [​IMG]

    On a more serious note I've heard from some folks that rain doesn't affect accuracy...a hypothesis I find vastly amusing. I've read two articles on the subject that were based on both simulated and actual rain. In the Sim. evaluation which had fall rates on the order of 1"/hour it increased dispersion on the order of 3-5 times with a 7mm bullet as I recall. The actual rain test was with light showers to drizzle and and had no measurable influence at 100 yards.
     
  12. Charles A

    Charles A Well-Known Member

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    The only effect I have seen in rain, is accuracy detieration. Really no change in elevation setting. [​IMG]
     
  13. Guest

    Guest Guest

    I would agree with the first point Charles made. I have shot thousands of rounds in rain conditions at long range and have noticed a couple of patterns:

    1) Very little accuracy degradation in very light rain.

    2) Shots do not hit high in medium to heavy rain, like never happens. They tend to hit low by 1/4 to 1/2 MOA at 600 yards.

    3) The vertical component of wind drift is greater on left to right winds in the rain, and less on right to left winds.

    4) The horizontal component of wind drift is minimized some what from what it appears relative to no rain conditions.
     
  14. ewallace

    ewallace Well-Known Member

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    Max
    I use the JBM program. I have shot in the rain and have shot some good groups.
    Crow Mag