enviromental effects on trajectory

[Darryl Cassel]

Hello

Most ballistic charts or programs have the humidity built in to the Barometric pressure, temperature and altitude.

Think of your high humid air as a "heavy, wet, thick air" that just hangs there as comparied to a dry thin air of low humidity that will rise when warmed.

The bullet , in my experiance and in different States, will drop more in the high humid air then it will in thin, clear, sunny air with less humidity.
From my understanding, there is less frontal resistance to the bullet in the dry, thin, air. More frontal resistance means the velocity loss will be greater.

Humid air hangs low and dry air rises with heat which, I believe was the comment by the sniper country site about upward air movement.

Upward air movement will cause the bullet to drop less.

For instance, when shooting at Williamsport in a 1000 yard match, lets say you shoot the first relay at 9AM when the temperature is a bit lower like 50 or 60 degrees. Leave all clicks the exact same as when you shot and shoot again at 2PM after the sun has warmed the ground to about 80 degrees and the heat waves are rising. Your bullet will be about 12" to 13" higher on the target then at 9AM.

When the temperture rises 20 or 30 degrees on a clear sunny day is when this happens .
You can run the same test at 500 yards also.

As far as the powder per each degree difference, don't really know as per brand of powder but, that is taken care of in the temperature programing at the start when you put the information in your ballistics program.
The velocity will increase or decrease with the temperature changes.
I don't think there would be that much difference between powders as per one degree temp change once you know your velocity "THAT DAY" and with any powder.

If H4831 is giving you 3000 FPS and H4350 is giving you 3050 FPS (That day) with the same bullet but a different charge, you can change the temp for either velocity and figure what that change is by your actual powder charge. Any good program will do this for you already. You can change your powder charge by one grain, chronagraph the speed and then program for one more degree up or down.

DC

[ 06-07-2002: Message edited by: Darryl Cassel ]

 

[youarenotcrazy]

Ok, from what I understand, there is a formula in the Sierra reloading manual that take environmental effects and modifies your ballistic coefficient so you can simply plug that in to your ballistics program and get your new trajectory. This formaula takes temperature, barometric pressure, and altitude into effect. It does not do humidity. I am in process of writing a ballistics program on my graphing calculator and am trying to get almost every effect taken care of.
Now if humid air is more dense than dry air, your bullet would drop less, correct? There would be less air resistance so there would be a lower drag, right? I went to snipersparadise.com and they said there would be less upward force (

)? So who would be correct, less wind resistance or less "upward force" (whatever the hell that means).
One more thing, please bare with me, what would you all say, on average, the difference in muzzle velocity is with varying temperature with your particular powder (if you could list that powder that would be great)(like 1.5fps/degree F)? I was just trying to map out burn rate trends with different powders so I would have a better idea of how fast I would be starting out under varying conditions.
Hope I didn't lose anybody, sorry for such a long d$%^ post. Thanks everyone!

 

[youarenotcrazy]

Thank you Mr. Cassel, that was quite informative. The only reason I ask such questions is because I am in the process of creating my own ballistics program that can run on my graphing calculator. I suppose that makes sense, there being no rising air but lingering air when conditions are humid. I never took rising thermal air from the earth into consideration before. I appreciate your time thoroughly.

 

[Ivanhoe]

There are 3 atmospheric factors affecting aerodynamics; density, viscosity, and sound speed. Density is literally mass per unit volume, i.e. kilograms per cubic meter (or slugs per cubic foot, slugs being what engineers use for mass in the English system). Viscosity is resistance to shear; even gasses have a little bit of viscosity. Sound speed is just that, the speed of a sound wave thru the gas. I believe density is the primary factor affecting velocity loss, sound speed 2nd, viscosity 3rd.

And velocity is what you've got to worry about, as it is the dominant thing that affects trajectory.

Density is a function of temperature, pressure, and gas composition. Density mainly reduces with increasing altitude due to lower pressure. Believe it or not, humid air is less dense than dry air at the same temperature and pressure. Each water molecule takes up a lot more volume than the nitrogen or oxygen molecule that it replaced. And higher density means higher drag.

As a way to keep this stuff straight, how much aerodynamic drag is there in outer space (i.e. a vacuum with zero density)? Zero drag. How much drag is there on bullets traveling into water (much higher density)? Lots. Therefore increasing density means increasing drag.

Sound speed is dependent on temperature and gas composition, with increasing temperature causing an increase in sound speed. Sound speed affects the strength of the shock waves created by the bullet (and thus the wave drag on the bullet), can't remember offhand if a higher sound speed means higher or lower wave drag.

Dont' forget that higher temperatures generally mean higher bore pressures, which means higher muzzle velocity, which is the dominant effect on downrange trajectory.

Darryl's description if humid air not rising is incorrect. If humid air didn't rise, we would never have thunderstorms or tornadoes. There are circumstances in which there isn't much vertical movement of air, the temperature inversion layers over L.A., Phoenix, and Denver being good examples, but this is not a humidity thing. The effect of natural convection (i.e. the warm ground heating the lowest air layer which then rises) is probably minor compared to changes in muzzle velocity and air density.

 

[Frank in the Laurel Mts]

I agree with Mr Cassel...Being involved with benchrest shooting out to 500 yards I have found that on humid days or cold air (above 65%)my little 67.2 grain bullets will drop an extra 2.5 inches at 500 yards..the same drop also with a frontal wind...on dry days or warm days or going with the wind, they will rise 2.5 inches at 500 yards...the problems occur when I get a combination of problems...cold/dry air or warm/humid air..then you start to add and subtract problems..it drives you crazy! In our form of shooting we have NO SIGHTERS and one shot out of the scoring ring at 500 yards will usually end your day and your chances. I compete in the Long Range Groundhog Matches 100-300-500 yards, no sight-in -- As far as powder changes, stay away from BALL powders, in my experience they are affected the most by weather changes...I use Vitt powders now and also H-extremes and haven't had much of a problem since leaving all ball powders. One other point that you should consider is if you are shooting coated or uncoated bullets... I've had in MY RIFLES better groups and consistency with moly coated bullets at 500 yards and I've noticed they drop less//when compared to the same bullets uncoated//everything else staying the same...I've read lots on this topic and it seems that some people think that it"s because of the edge on the rifling marks on the bullets having less nicks and bumps on them as a results of the molly coating touching the rifling...who knows for sure..they work for me, don't see them used much at the 1000 shoots but they are used at the long range groundhog matches..most of our calibers are below .308 diameters...a few 6.5's, most are variations of the .243's and .224's..this may have something to do with molly use... But Darryl is correct on his comments about environmental effects on bullet performance.. Frank