How does temperature affect windage?

[117D-RTO]

I noticed something interesting tonight that I can't figure out, setting up a new trajectory card for JBM and noticed that at a lower temperature it was calling for less windage which doesn't make sense to me. When the temp drops air becomes denser so I imagined that the wind would have a greater effect on the bullet, JBM is showing me the opposite around 1000yds but is most obvious on my 1238 yard solution. It's showing at 20F and 4000ft Density Altitude windage is 2.2 mils, at 40 and 60F it's 2.3. Switching from Mils to inches shows it's about 2.2" difference between 20F and 60F, so this is likely just an academic thing and not really going to cause me to miss a shot but i'm curious to know what the physics is behind this.

 

[AIAW]

It shouldn't actually. What you describe pertaining to air density is correct. Unless this is something minuscule in the engine accounting for a higher DA being more prone to spin drift effects versus a lower DA.

Try your same calculation comparison with the AB calculator and see if it gives the same output.

http://appliedballisticsllc.com/ballistics/

 

[117D-RTO]

While I couldn't use the same values (no DA or 105gr RDF options in AB) the AB calculator is giving me values that i'd expect, IE decrease in wind hold as temp increases.

 

[AIAW]

While I couldn't use the same values (no DA or 105gr RDF options in AB) the AB calculator is giving me values that i'd expect, IE decrease in wind hold as temp increases.

It's got to be something peculiar with the JBM engine I'd think. Some sort of "proprietary" transonic drift or perhaps Poisson effect calculation.

 

[117D-RTO]

I clicked the 'report a bug' and detailed what I was experiencing, got this response:

Why do you think this is odd? You're holding density constant and
changing the temperature. That changes the mach number (raises it) and
changes where you are on the drag curve.

Brad

 

[AIAW]

Data straight from FFS

1170 Yards - 260 REM - 140g Hornady ELD @ 2740 FPS - RH Twist - 1-10"

Spin Drift Only - MRAD

-3200 DA = 0.34L
2400 DA = 0.32L

With 10 MPH Full Value - MRAD

-3200 DA = 3.3L
2400 DA = 2.62L


Not a shocking conclusion really, at least to me (unless I am doubtfully missing something). Spin drift more affected by denser air. More windage hold required with denser air.

I think it has to be something to do with the drag curve they have for that bullet. Like you said, not a miss on a target most likely but interesting data. I had to enable double-digit turret solutions in FFS to be able to see the difference for the spin drift even.

 

[RidiculousMetacarpus]

I clicked the 'report a bug' and detailed what I was experiencing, got this response:

Why do you think this is odd? You're holding density constant and
changing the temperature. That changes the mach number (raises it) and
changes where you are on the drag curve.

Brad

Brad's reply is oddly worded.

DA (O2 / N2 density of an air mass for the most part) is a function of temp, pressure, and humidity. To say "holding density constant while changing temp" is akin to saying "maintaining an ice cube at below 32f at 29.92inhg whilst we put a blowtorch on it".

One cannot "maintain density" [if we mean O2/N2 desnsity for realistic purposes] while simultaneously changing a variable that partly dictates "what is density".

Perhaps it's as simple as there are not enough data points ( a dense enough cluster) at that specific part (1200yds) of the exponential curve, thus giving you a misrepresentation. Though, i'm not sure how much JBM's tables are based on actual empirical data or instead a set of equations (non-empirical).

<-- pilot

 

[TiroFijo]

Brad is correct... in JBM you can set "Std. Atmosphere at Altitude" or " Pressure is Corrected".

 

[Milo 2.5]

Re-run the table and see what the velocities at distance are doing during this DA curve.