Effects of wind?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Coues100, Aug 27, 2005.

  1. Coues100

    Coues100 Member

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    I was wondering how wind, straight on or from straight behind effects trajectory?? How do you calculate for long range shooting? Thanks, Mark
     
  2. Charles A

    Charles A Well-Known Member

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    From the from the front the impact will be higher, from behind the impact will be lower. Depending on caliber, bc, velocity, wind speed and especially range it could be a 1/2minute difference. The problem isnt when the wind is truely from 12 or 6 o'clock, but when its at 1,5,7, or 11 o'clock.
     

  3. 700

    700 Well-Known Member

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    LR7WSM

    This is called "Vertical Wind Component" becĂ use the wind causes your bullet to plot higher or lower, rather than just left or right.

    Give me an email at brXXXian_sinXXXnott@hotXXXmail.com

    remove the XXXs.

    I can then get details of your shooting system and make up some charts showing the Vertical Wind Component at different ranges.

    Rgds

    700
     
  4. Richard338

    Richard338 Well-Known Member

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    A head wind will LOWER the point of impact. A tail wind will RAISE the point of impact.
    Just picture the limiting case of a tail wind equal to muzzle velocity, then you'd have a vacuum trajectory.
    The effect is much less than a cross wind, but I recently shot in a switching head/tail wind that was good for 2MOA up/down.
     
  5. jb1000br

    jb1000br Well-Known Member

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    Well, when this question comes up, you will always get both answers...shown here.

    I have seen more evidence of a HEADWIND RAISING POI AT LONG RANGE.

    But it makes more sense that it would do the reverse at short range...

    WHY?

    On its way to 1000yds the bullet spend more time CLIMBING then FALLING, so why wouldnt a headwind give the bullet LOFT?

    At close range, the trajectory is probably flat enough that it is as simple as the wind adding and subtracting from velocity, meaning a HEADWIND WOULD LOWER POI

    Now how bout 2000yds+ The bullet is much slower further out and angling toward earth, so here, the loft may even out the added drop, or the headwind may end up causing a total effect lowering POI -- i cant say much here because i have not fired in enough varied conditions past 1mile to see a pattern.

    Personally i think the answer depends on the distance you are shooting, and the flatness of trajectory...and surely other things as well.

    The above is based only on a flat fire situation shooting over FLAT ground.

    YMMV,
    JB
     
  6. Richard338

    Richard338 Well-Known Member

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    JB, why do you say it spends more time climbing?
    I ran a trajectory calc on JBM's page using 3000fps and a BC of .7.
    The apogee of a 1000 yard zeroed trajectory is at 540 yards, but that occurs at less than half the time for the 1000 yards trajectory because the bullet is going faster at first.
    I added 20 mph head/tail winds and got predicted lower POI for headwind and raised POI for tailwind.
    I had a clear example of a switching head/tail wind at 1000 yards a couple of weeks ago and definately found this to be the case.
    Target rifle shooters have a memory trick "heads-up, and tails down" (as in aim up in a head wind, and aim lower for a tail wind).
    The effect is small in any case but can get you.
     
  7. jb1000br

    jb1000br Well-Known Member

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    [ QUOTE ]
    JB, why do you say it spends more time climbing?
    I ran a trajectory calc on JBM's page using 3000fps and a BC of .7.
    The apogee of a 1000 yard zeroed trajectory is at 540 yards, but that occurs at less than half the time for the 1000 yards trajectory because the bullet is going faster at first.
    I added 20 mph head/tail winds and got predicted lower POI for headwind and raised POI for tailwind.
    I had a clear example of a switching head/tail wind at 1000 yards a couple of weeks ago and definately found this to be the case.
    Target rifle shooters have a memory trick "heads-up, and tails down" (as in aim up in a head wind, and aim lower for a tail wind).
    The effect is small in any case but can get you.

    [/ QUOTE ]

    Richard -- first, forget computer models for this discussion.

    I guess i should have been more specific, but left out something

    seems TO ME the less flat the traj, the more likely to rise at 1K with a headwind.

    It has been observed by those i trust with a 308 and 260, and myself with a 6br.

    my 6.5-284 has not been noticably affected my any head/tailwinds encountered at 1K thus far.

    IMO 3000fps with a .7BC is FLAT-A$$ traj and many rounds we use cant touch that.

    in my mind, i see a flatter shooting round more likely to drop in a headwind, and a round with more drop getting loft.

    Like i said -- it depends on too many things to make a rule like "heads up whatever down..."

    Also, by "more time" i actually meant distance, and you just showed that was true, even with the flat shooter. 540 compared to 460 -- thats an 80yd diff

    IMHO, and YMMV,
    JB /ubbthreads/images/graemlins/smile.gif
     
  8. Richard338

    Richard338 Well-Known Member

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    JB,
    I noticed it with my 260Rem. I just ran the .7 BC 3000 fps, because I'm waiting for my new 7-270WSM to show up!
    Even with 20mph head and tail wind, the computed difference is only .5MOA.
    Usually you would never get that much difference in one day.
    Where I shoot it is also more complicted because it is open behind us, a dip in the range between 200-400 yards, and there is a hill to the left of us.
    We get all kinds of up/down drafts as well.
    Nevertheless any decent long range cartridge shoots very flat from a geometry point of view.
    I still say that the calcs are correct that a strong head wind will lower the POI.
    The target rifle guys who told me this all shoot .308 with 155 grain bullets.
    Anytime we are seeing enough to be important, there are probably up/down drafts involved as well.
     
  9. jb1000br

    jb1000br Well-Known Member

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    [ QUOTE ]
    JB,
    I noticed it with my 260Rem. I just ran the .7 BC 3000 fps, because I'm waiting for my new 7-270WSM to show up!
    Even with 20mph head and tail wind, the computed difference is only .5MOA.
    Usually you would never get that much difference in one day.
    Where I shoot it is also more complicted because it is open behind us, a dip in the range between 200-400 yards, and there is a hill to the left of us.
    We get all kinds of up/down drafts as well.
    Nevertheless any decent long range cartridge shoots very flat from a geometry point of view.
    I still say that the calcs are correct that a strong head wind will lower the POI.
    The target rifle guys who told me this all shoot .308 with 155 grain bullets.
    Anytime we are seeing enough to be important, there are probably up/down drafts involved as well.

    [/ QUOTE ]

    You definately have some updrafts there then /ubbthreads/images/graemlins/shocked.gif

    the 260 and 308 info above came from a FLAT range, and the 6br was fired at williamsport 1KBR

    the br rose almost 12" with a 15-20MPH head gust.

    the 308 (168SMK) and 260(139scenar) rose 1.5-2.0moa with a seemingly constant 20MPH headwind.

    I'll stick to my thinking, where you cant make up a hard and fast rule for every round and every range /ubbthreads/images/graemlins/smile.gif

    Do update me if you see anything different, and i'll do the same /ubbthreads/images/graemlins/wink.gif

    Cheers,
    JB
     
  10. Richard338

    Richard338 Well-Known Member

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    Sounds good JB. Maybe I'll check McCoy's book tonight.
     
  11. Richard338

    Richard338 Well-Known Member

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    JB, I flipped through McCoy's book and found explanations that might fit both our observations.
    He says that a pure head or tail wind (with no crosswind) lowers or raises (respectively) the trajectory by a small amount.
    This agrees with the calcs I ran.

    What could explain what you saw is this business of the vertical component from a cross wind.
    If a righthand twist barrel is zeroed at in calm conditons, then wind from the left will push the bullets to the right and down.
    Wind from the right will push the bullets left and up. If the wind slowly changed, the string of bullets would form a slanted line (about 20 deg for many bullets, twists etc), going from top left to bottom right.

    So if you had wind from the left when you got zeroed, then the wind switched to a headwind, the new headwind would want to lower the trajectory (only a small amount). But the loss of the vertical component from the crosswind (which switched away) would be a bigger effect, and the net result would be a raised trajectory.
    Were your headwind gusts switching from a crosswind from the left?
     
  12. jb1000br

    jb1000br Well-Known Member

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    Rich -- at williamsport, anything is possible, and that is the prevailing direction, but i honestly dont remember. All i remember was my bud was centered up and down, then a head gust just as he shot and the bullet hit around 10" high

    the crosswind though would have had to have well over 10" in the vert component to use that explaination, and i dont see that being possible.

    JB
     
  13. Richard338

    Richard338 Well-Known Member

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    At 1000 yards a crosswind worth 3-4MOA (horizontal), would have about 10 inches of vertical. The angle of the line of shots on the target for different winds should be 15-20 degrees...
     
  14. Charles A

    Charles A Well-Known Member

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    Alright I guess I should have thought about it and explained myself more. A headwind with the target perfectly(or very near) level with the shooter and the ground level from shooter to target will impact lower. A tailwind, higher. Now if you on high ground and shooting over a depression, valley, ravine, etc., a headwind will raise POI due to updrafts. Tailwind will lower.

    In my expierence shooting 308's and 300win at LR, almost always a headwind will raise POI, a tailwind will lower POI. But at the "targets" I normaly shoot at I dont really worry abou it, as I haven't seen much more then 1/2MOA deflection either way.