Air Pressure and Wind Drift

[6ShotsOr5?]

The basic question is how much does air pressure matter for wind drift, and why? I was just re-reading the thread about the 3871 yd sniper kill and thinking about the wind. Since air density has such an important effect on drag, it seems like a 10 mph crosswind at a lower air pressure such as 20 in Hg should have much less effect on a bullet at a given target range than a 10 mph crosswind at 30 in Hg. I ran a 1000 yd case for my favorite factory .260 Load on a popular ballistics solver with a 10 mph wind from 3 o’clock (full value right to left). The solver is giving my aiming solution as Up 31.2 MOA and left 19.7 MOA at standard atmosphere 59F and 29.92 in Hg. When I change the pressure to 20 in Hg (absolute) and 59 F, the solution is Up 25.5 and left 19.5, almost no change in windage. The elevation solution seems reasonable. If correct, is the low difference in wind drift because the relative velocity is > 2000 ft/s for the forward motion, but the wind speed is only around 15 ft/sec and drag is nonlinear with velocity? Still the calculated difference in wind drift seems very low for such a big change in air pressure.

 

[dcali]

That doesn’t sound right. Try it again. There should be a difference.

 

[6ShotsOr5?]

You are right - the wind drift is down a lot at lower air pressure - huge difference. I was dialing up the wrong dial on the HUD. Thanks.

 

[spclark]

Kinda makes sense... airplanes & birds don’t fly so well when they get too high, air’s too thin to support them. Same reason drift at higher elevations is less than at sea level.

 

[chromatica]

There's a problem with your temperature. In a Standard Atmosphere, temperature drops off with altitude. Pressure of 20 inHg corresponds roughly to an altitude of 11,000 feet and a temperature of 19 degrees F. At this altitude, density of the air is half of what it is at sea level (29.92 inHg, 59 degrees F). Drag is proportional to air density, so drag on a bullet due to a 10mph wind at 11,000 ft is half of what it is at sea level