JBM Ballistics Question

[Hondo64d]

My understanding is that the JBM range card function uses density altitude. My understanding is also that the temperature inputs are for transonic calculations. Density altitude is pressure altitude corrected for temperature, and, to a minute extent, humidity. It is a measure of air density. 4000 DA is 4000 DA whether the temperature is 0F or 120F. What is confusing is in a given DA column in JBM, the comeups actually increase with temperature. Only by .2 mils in the most extreme conditions, but there nonetheless. That would indicate that that at higher temps in any given DA, the air density increase. This is completely counter to the definition of DA. Can someone explain this to me?

Thanks,

John

 

[Hondo64d]

Anyone?

 

[delfuego]

JBM online does not use Density Altitude. It uses altitude + corrected pressure or uses station pressure (uncorrected).

 

[Hondo64d]

On their trajectory calculator, yes but on their "trajectory card" function, I don't think so. If it used PA and temp then the come-ups would be less as the temperature increased at any given altitude because air density decreases as temperature increases. The cards do not reflect this.

Trajectory card function: http://www.jbmballistics.com/cgi-bin/jbmcard-5.1.cgi

This article: http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/DA/

...states that the JBM Trajectory Card Function uses DA as well.

John