Wind-Drift-Density

January 23, 2015 2:11 am

This article was published in the 2001-3 issue of VERY HIGH POWER, the publication of the Fifty Caliber Shooters’ Association

A comment a shooter made a few years ago, after a FCSA match at the Whittington Center outside of Raton, New Mexico, has stuck in my mind. Something about it hadn’t seemed quite right and for some reason I’d remembered it again recently. He’d said something to the effect that; “Did you notice how the wind didn’t blow the bullets as far out of the group at this range as it would somewhere else?” On the surface, of course, that statement seemed perfectly reasonable. The 1000-yard range at the Whittington center is nearly 7000’ in elevation. And we all know that the air is thinner up there.

Most dedicated fifty shooters are students of ballistics, and with the proliferation of extremely accurate ballistics software and most everyone owning a personal computer, we all know that the wind drift from a 10 mph crosswind at sea level is noticeably greater than at the 7000’ plus elevation of the range in New Mexico. And we also know that a bullet fired at a given velocity at sea level will drop a lot more at 1000 yards than it will at 7000’.

If using the 750-grain Hornady A-Max, loaded in the 50 BMG case, as an example, my Tioga Engineering ballistics program tells me that the drop is -258” at 1000 yards (with a 100 yard zero) and a 10 mph crosswind will push the Hornady bullet 36.5” at sea level pressure and 59º F. This is using a muzzle velocity of 2700 fps, and C-1 ballistic coefficient of 1.05. Crunching the numbers again, the computer said a 10 mph wind will only move it 26.3” (10 inches less) in the thinner air at Raton and a temperature of 75º F. and that drop is –240”(18” less). So, what’s my problem you say?

If I might, at times, refer to shooting as a vice, I’ll have to admit to a second bad habit as well; airplanes and piloting them. (My wife would agree with both statements, I’m sure.) Part of any pilot’s training relates to the effects of air density on airplane performance. Air density is measured and referred to as Density Altitude. That is, there is a base line standard density, as defined by the ICAO (the International Civil Aviation Organization). That organization states that at sea-level, with an atmospheric pressure of 29.92” and a temperature of 59º F., we have defined the ‘standard density’.

Piston powered airplanes really like that thick sea-level air. The engine produces full horsepower and the airfoils generate maximum lift. The airfoils include both the wings and propeller blades. If the density of the air is reduced, performance will suffer. It is not unusual to have density altitude situations where the ground run of a plane is doubled over what it would be at standard sea-level conditions.

Air density is reduced when temperatures go up, elevations go up, or barometric pressure goes down. Conversely, air density increases when the opposite changes are made. And it is quite possible to have densities thicker than standard sea level with cooler temperatures. I live at about 2500’ elevation. When the temperature is 15º F. outside, we effectively have the same density altitude as standard sea-level conditions at 59º. If it gets colder than that (and it does in Montana) our density altitude goes below sea level.

As an aside, many people think that humidity has an effect on air density, and it does. But its effect is really insignificant (no more than 1%) and in the opposite direction of what is commonly thought. That is, the more water there is in the air, the less dense that parcel of air is. I can already hear a few people saying the only thing dense around here is the author of this silly article. But it is so (the humidity comment that is). The molecular weight of water vapor is less than air, and so humid air is less dense than dry air. It is for that very reason that clouds float and steam rises.

But I diverge too much and so far I seem to have made all the arguments in favor of what the shooter I quoted in the beginning of this article said. And what he said was really true, almost. But it is what wasn’t said that got me to thinking.

When we’re up at the firing line shooting, how do we know how hard the wind is blowing and from what direction it is coming? Of course there are several predominant indicators on any range. And we can often feel the wind on our bodies. We can tell if it is blowing hard or not so hard as well as the general direction. When the trigger is pulled and we see where the bullet punched the paper we know for sure, for that moment, what the wind was doing. But I think for most of us, we get our cue from range flags set out between the benches and the targets.

Some shooters rely more on mirage as a wind indicator, but I’m not one of them and I’ll explain why; if we’re seeing the mirage through our scope, for the most part it is just telling us what the wind is doing at the target. Our scopes are focused for the target distance and so that is where we’re seeing the mirage. And as most good wind dopers will tell us, the closer the wind is to the shooter, the more effect it has on bullet drift. Some savvy mirage shooter will set up a spotting scope and focus it at an intermediate range, just so they can see what the mirage is doing at that distance.

The point I’m trying to make is that we really don’t know, when we’re up to the line shooting, what the actual wind velocity is. But what we do learn to know, is that a certain amount of movement of the flag tail or indicated windmill revolutions, as well as the wind direction, will produce a known amount of movement of the bullet. We’ve learned that through seeing on our target where a bullet is moved by that wind. And we confirm that knowledge over and over, both on the sighter and on the record target. We’ve learned that the relative movement of our flags generates a predetermined movement of the bullet on our target. The better we get at this the smaller our groups become.

There are two schools of thought on accuracy shooting. One holds that; ‘I’ll only shoot when the flags all seem to have the same position and angle as they had for the previous shot’. If conditions are ‘steady’ this is a near foolproof method. The second school of thought is: ‘Well I shot the first two shots in this condition and now the flags have turned and are showing me this condition; I think if I hold the reticle over here, this shot will go where the others went’. And it is a big feeling of satisfaction when the bullet does go where intended. We’ve ‘outsmarted’ the wind at that point.

Regardless though, our decision to shoot is based on what our experience has shown us. We pull the trigger when the flags look right. Kinda like the saying: Good judgment comes from experience and experience comes from bad judgment.

So, why does an airplane take twice as far to take off in some density altitude situations? It takes a given number of air molecules, in a given time period, moving over the airfoils, to produce enough speed and lift to get the airplane flying. When the air is less dense it takes a greater ground speed to generate the lift on the wings and it takes a longer ground roll to get the propeller to pull the plane through the air fast enough. The key words here were ground speed.

All airplanes have an airspeed indicator. Glancing at it, the pilot knows when to pull the stick back and fly the airplane off of the ground. And the required airspeed is always the same regardless of the ground speed or density altitude. If it takes 60 mph for a plane to fly, it will always take an indicated 60 mph on the airspeed indicator. The actual ground speed might be 70 mph or greater, but the airspeed is the same. Always. The airspeed indicator measures the number of air molecules entering the pitot system.

And so it is with our wind flags. It takes the same number of air molecules to raise a flag tail 45º at 7000’ elevation as it does at sea level. And so, once a shooter is tuned into knowing what the correlation is between flag movement and bullet movement, it doesn’t matter what the shooting range elevation or temperature is (ie. air density). The effect will be the same. The actual ground speeds of these two compared winds won’t be the same, but the displacement of the wind flags and the bullet will be equal.

And so, before I get too windy, those are my thoughts and observations on wind drift and making corrections for it.