How Much Does Barometric Pressure Change?

Discussion in 'Long Range Hunting & Shooting' started by Len Backus, Aug 2, 2008.

  1. Len Backus

    Len Backus Administrator Staff Member

    Messages:
    6,359
    Joined:
    May 2, 2001
    I have paid little attention to pressure as I usually have shot game at distances under 700 yards with a very flat shooting load. However, I am now shooting with a handgun and a much less flat shooting load.

    29.53 is the default used by my Oehler Ballistic Explorer software. Two inches of pressure difference (to 27.53) change my POI by 2.8 inches at 700 yards. In your real world experience, how much variance can there typically be above or below this 29.53 default reading? How common would a 2 inch pressure change be?
     
  2. Ridge Runner

    Ridge Runner Well-Known Member

    Messages:
    987
    Joined:
    Dec 13, 2002
    Len,
    Here in eastern wv, I see swings of 3" pretty often, which at 1K will change POI 36" or close to that.
    RR
     

  3. Len Backus

    Len Backus Administrator Staff Member

    Messages:
    6,359
    Joined:
    May 2, 2001
    RR

    That is a huge POI change, even for 3 inches of pressure difference. What BC, V, are you using to see that much change?
     
  4. Ridge Runner

    Ridge Runner Well-Known Member

    Messages:
    987
    Joined:
    Dec 13, 2002
    Len, thats just an example I use, I ran the numbers once I think it was my 270 wea. firing a 140 gr GKHP @3200 fps, and a 4" swing in BP changed the POI like 50" at 1K, think the BC is .354.
    from 29" to 31.5" of BP swing the BC runs from .357 to .329 drops will change POI at 1K from -318.5" to -349.5. 700 yard drops BP of 29.4 -99.06, 31.5"- -105.81 so that 6" of drop at 700 computes to 41" at 1K
    like stated before an example I use for the shepaerd scope and other BDC guys who make those outlandish claims about 1K shooting with a BDC.

    Just reran the numbers again using load from a disc for windows
    RR
     
    Last edited: Aug 2, 2008
  5. Buffalobob

    Buffalobob Writers Guild

    Messages:
    5,085
    Joined:
    Jun 12, 2001
    Milwaukee monthly highs and lows. Of course there is so much beer involved that it is hard to beleive the data.

    [​IMG]
     
  6. Len Backus

    Len Backus Administrator Staff Member

    Messages:
    6,359
    Joined:
    May 2, 2001
    RR, thanks, yes, those low BC's make accurate pressure readings important, don't they.

    BB, exactly what I needed, thanks. So, the biggest yearly baro spread was about 1.8 inches. That's what I was wondering. So with my current handgun load, POI could shift 2.8 inches at 700 yards. (only 1.4 inches with my former load/rifle combo)
     
  7. Shawn Carlock

    Shawn Carlock Sponsor

    Messages:
    1,897
    Joined:
    Jun 11, 2007
    Len the real problem is when you change locations. If I shoot at my local range the pressure will run 27-28, if I go to my hunting area it runs 23-25. If you are shooting BC challenged bullets or extreme distances it makes a big difference. In the end it is one of the many things to add to the compounding errors list. A little off on BAR, a little off in elevation, cosine of .94 instead of .96, then add in some wind components and scratch your head why you missed.
     
  8. Len Backus

    Len Backus Administrator Staff Member

    Messages:
    6,359
    Joined:
    May 2, 2001
    Shawn, I know! :) I remember a 690 muley I killed in MT. First shot kill but afterward I realized I had made two small errors in my ballistics setup. Luckily they offset each other and I got to eat the venison anyway.

    By the way, I have always adjusted for elevation, just not other pressure changes.
     
    Last edited: Aug 3, 2008
  9. eshell

    eshell Well-Known Member

    Messages:
    177
    Joined:
    Mar 10, 2006
    The use of "Density Altitude" as a ballistic reference takes into account barometric changes, as well as air temperature and humidity (the least influential).

    I use a Kestrel 4000 to give me direct readings, but it can be manually figured if desired.

    Density altitude - Wikipedia, the free encyclopedia
     
  10. JPRITT

    JPRITT Well-Known Member

    Messages:
    186
    Joined:
    Jun 5, 2007
    I'm no weather wizard but the way I understand it is if the Barometer drops so drastically that it effects your shooting, you are probably shooting it a thunderstorm. which most of us arnt shoot/hunting much in anyways. JMO.
     
  11. grit

    grit Well-Known Member

    Messages:
    1,375
    Joined:
    Mar 23, 2005
    I set my Kestrel to read station pressure. I just returned from a scouting trip. The pressure at home before leaving was 25.49 (5889'). The pressure where we were scouting (11,000') was 20.??. Five inches or more pressure change from where i shoot to where I hunt!
     
  12. Shawn Carlock

    Shawn Carlock Sponsor

    Messages:
    1,897
    Joined:
    Jun 11, 2007
    Grit,

    That is the problem. Five inches less pressure will make you shoot quite a bit flatter even at closer distances.
     
  13. Coyboy

    Coyboy Well-Known Member

    Messages:
    2,132
    Joined:
    Jan 30, 2005
    I have a problem with this, that may-be one of you can help me with?

    I'm at roughly 1000 ft with WI muggy conditions. I just started loading for my 300Rum.

    Berger 210 @ 2950fps, I will be hunting at 11,200-12,200 feet in four weeks, CO high country mulies. I'm printing a drop chart for home on JMB calculations.

    I need to print a drop chart for altitude that is reasonably close, I will then shoot and adjust/confirm things when I reach the hunt area.

    What or how should I enter the data to get the info close with the JMB basic balistic calculator?

    Do I need to enable the "standard atmosphere at altitude?"

    I assume I need to keep "corrected pressure" enabled?
     
  14. Ridge Runner

    Ridge Runner Well-Known Member

    Messages:
    987
    Joined:
    Dec 13, 2002
    ok, dumb country hick here, don't know if I do it right or not but here's how I do it

    I have a casio sea-pthfinder watch which after I press the button in 2 seconds it gives me station pressure (bar pressure where I'm standing) I enter this into exbal and enter zero for the altitude.
    My understanding of it is higher altitude lowers the bar pressure, which raises the bullets BC.
    this change in the BC is why altitude effects the bullets flight, so if you know your bar pressure, exbal compensates for it and altitude can be left at "0". if you put in a bar pressure reading and an altitude reading it over compensates, Is this correct?
    RR