Reading the wind.
I thought some of you might be interested in this:
"Aim small miss small",
Wind doping and stimating bullet deflection
Shawn Carlock's article points out many of the problems of measuring wind and some practial techniques of estimating wind, but his article doesn't address translating the observed wind into how it affects a bullet as it flys downrange. There's no mention of a bullet's relative sensitivity to wind near the shooter, wind near midrange, and wind near the target. For long range shooting it's the midrange wind which is most important. Thats a bit counter intuitive. It's obvious wind near the target has little effect, but most people don't realize that at long range (800+ yards) the wind at the shooters location is not the most important (though it is important).
In most field conditions wind esitmation is more guessing than science simply because the human eye and brain isn't very well designed for seeing air currents or for calcuating the way a bullet is deflected by winds (in three axes) whch is made more difficult since wind is rarely uniform along the trajectory.
Whats needed is a good scintillation anemometer coupled to a ballistic computer. Until they are available Mr Carlock's methds, the use of sighter shots, and the using wind flags will have to do. The US Amry's Abrams tanks use laser anemometers. It's just a matter of reducing them to affordable pocket sized models. Sounds simple enough. Maybe Bushnell will market one for a couple of hundred bucks in a few years. There was an article about an experimental, though not very portable, laser anemometer in "Very High Power", the FCSA's magazine about ten years ago. It was reported to perform quite well at ranges to 1000 yards.
The equations for determining bullet deflection as a function of wind velocity and direction in three axis along a projectile's path are detailed in Robert F. McCoys book "Modern Exterior Ballistics", Chapter 7 titled "The effect of wind on flat fire trajectories". I can't say that's of much use to the typical shooter because of it's complexity, but if you have a way to measure the wind along the trajectory it's what is required to translate the wind information into the bullets point of impact.
With enough shooting experience some people can estimate wind effects fairly accurately. It's a pattern matching method which doesn't use mathematics at all, somewhat the same way a outfielder can run to the right place to catch a hit baseball without calulating it's trajectory mathematically. Far more shooters just guess and miss. For them knowing a little of the math as presented in Shawn's article may actually be a disadvantage.
Last edited by Louis Boyd; 12-24-2007 at 09:37 AM.
Wow Louis. That's a pretty bold statement. It's not only "counter intuitive", it flys in the face of real-world experience of most long range Benchrest shooters that I know.
Originally Posted by Louis Boyd
This has been discussed and cussed on this and other Forums and I'm not about to get into it again, except to say that I disagree with you.
I probably overstated the situation. When shooting very low drag bullets even at 1000 yards the wind deflection sensitivity could still be hightest at the firing line. But I'd suggest reading Robert McCoy's book which I cited above before you disagree too strongly. Where a bullet doesn't lose a large portion of it's initial velocity before reaching the target the effect of wind is greatest near the shooter. That's condition is intuitive and true for most short and intermediate benchrest shooting. It's also probably true for most heavy caliber 1000 yard bench shooting.
Originally Posted by Cheechako
However, as McCoy uses as an example, for a 30 caliber M2 ball fired at 2800 fps at 1000 yards and reaching the target at 1350 fps the distance of peak wind sensitivity is at 400 to 500yards downrange. The deflection caused by wind at 400 to 500 yards is about 10% greater than by the same wind near the firing point, then falls smoothly to zero at the target. I'm referring to Figure 7.3 in his book. I won't try to explain why here, it takes him a couple of pages of math and text, but it's related to the longer time of exposure to the wind downrage because of the bullet slowing down. The bullet does not slow down linearly over it's flight. It takes about half the time to cover the first 500 yards as the second 500 yards.
The effect is even greater when then ratio of muzzle velocity to target impact velocity is higher. That will likely be the case when shooting at longer distances and for shorter distances too when shooting bullets with lower BCs giving a greater ratio of muzzle velocity to impact velocity.
Last edited by Louis Boyd; 12-24-2007 at 03:34 PM.
As considering my self as a novice if I remember correctly when I went through the 3 day coarse for "High Power" at FT. Benning, Georgia where we trained using the .223 and .308 which are shot out to 1,000 yards. We were told that the wind closest to the rifle causes the most deflection of the bullets. We were also taught to read the mirage in order to calculate the number of clicks on our sight to have the bullet's poi be in the center of the target. This is done with open sights and not with a scope. It must work pretty good for the top shooters, because I saw one of the Army shooting team members put 10 shots smaller than your fist at 600 yards with his .223 M16.
Hope I explained this correctly because I would like to learn more. I am not trying to start an argument, but would like to shoot better at long ranges with a scope. I quit High Power because it was life consuming and I DO have another life.
"Aim small miss small",
Louis, I know exactly what Figure in McCoys book you're talking about, I'm trying to attach a scan of it (a pictures worth 1000 words).
In the scanned figure, the y-axis (fwz) represents the sensitivity of the bullet to wind deflection at the particular range.
Basically, I don't think there is a real disagreement here. The following statements about wind sensitivity are true:
Wind in the first half of the trajectory always is more important than wind in the second half of the trajectory.
Wind in the first 100 yards is always more important than wind in the last 100 yards.
This statement is sometimes true:
The wind in the first 100 yards is more important than any other segment.
The previous statement is true for ranges up to around 600 yards. When going beyond that range (around 1000 yards) the bullet is actually slightly more sensitive to wind around 400-500 yards than the first 100 yards.
In the case of long range (1000 yards), where the wind is more important at 400-500 yards, it's only slightly so as you can see in the scan.
The reason for this behavior in wind sensitivity is the following:
Wind deflection is related to the 'lag time' of the bullet. This is the difference between the actual time of flight and the 'vacuum' time of flight assuming no velocity loss.
For example, the 'vacuum' time of flight for a round fired at 3000 fps to 1000 yards it 1.000 seconds. (3000 fps for 3000 feet). The actual time of flight is closer to 1.500 seconds, meaning the lag time is 0.500 seconds.
Now, if the bullet had a constant drag coefficient for all flight speeds, the lag time and wind sensitivity would be the same for every 100 yard interval. However, since the drag coefficient increases as the bullet slows down, there is more lag time for slower speeds. This results in more sensitivity to wind drift down range.
However, that's not the whole story. Wind deflection that occurs near the muzzle will have more of an effect because the bullet spends more time on the deflected path. This is the intuitive effect that Ray mentioned.
In reality, it is the combination of the above two effects that result in the behavior observed in the attached Figure. At close range (600 yards), there hasn't been enough velocity loss for the difference in lag time to be the 'dominant' factor. At long range the difference in lag time overpowers the 'intuitive' effect of near wind, if only slightly.
In the end, I agree whole-heartedly that coping with wind deflection in shooting matches is far less of a science and more of an art. Louis's example of the outfielder is terrific. The fluid nature of wind just doesn't lend itself to prediction by classical means. It's one of the things that makes our sport interesting!
Here's figure 7.3 from "Modern Exterior Ballistics" by Robert F. McCoy.
The wind 20ft off the ground (max height of bullet) can be twice as much as that at 1 foot off the ground (muzzle height for a prone shooter) or 70% more than 3 feet off the ground (muzzle height for benchrest shooter).
Last edited by steve_podleski; 01-04-2008 at 03:12 PM.
Good point Steve.
This is why flag height has been a topic of discussion at several ranges that I shoot on.
You can get a 'feel' for how the wind is blowing by paying attention to the wind on your skin, hair, etc. But the wind speed aloft can be quite a bit different.
This wind 'gradient' depends a lot on terrain. over a frozen lake, the wind speed 3 feet over the ice is probably not much different that at 30 ft. However, on a range with bumps, bushes, tall grass, saplings, etc, the wind gradient can be much more profound.