In parts l and ll of Reading and Calculating wind, I went over a few wind formulas. These wind formulas were what I learned from Snipers and other long range shooters, as well as the sniper training field manual FM 23-10. These formulas helped to give me an idea of how to calculate wind, but I did see a fault. I saw several. I have been working on a way to come up with something better. I wasn't getting the accuracy of the wind call I was expecting. I would like to take this time to show the errors and give you and updated method to calculating wind.
Errors
The first error I noticed right away when I was being trained was the “Half Value Wind”.
This half value wind is calculated by multiplying by .75. Well, .75 is NOT half. It’s three quarters. The .75 value is also not half way between 12 o'clock (zero value) and 3 o'clock (full value) The correct half value is .707 @ 1:30.
The next error was the fact that 12:30 was considered zero value. I knew this couldn't be correct but I was not about to correct my instructors. All in all, I used the formulas I was taught, but my hit probability for a first round hit at long ranges was low. I just assumed I was no good at reading wind and continued to use formulas that were wrong.
Correcting The Errors
I decided to look at other avenues to learn a better way to calculate wind. I spent a lot of time building charts for wind and using Ballistic calculators down to 1 MPH per 1 yard. I’d compare them to the wind formulas that I was taught and the answers were always different. Sometimes by only a little and sometimes by a lot. I started looking into the competition shooters. The guys shooting King of One Mile and King of Two Miles, PRS and the guys pushing the boundaries shooting over 4,000 yards. What are they doing? PRS (Precision Rifle Shooting) matches are timed. How are the shooting so accurate so fast. How are they calling wind? I got online and started reading everything I could. The answer was right in front of me the whole time. COSINE.
After looking on forums I saw that other people were already using cosines to a greater degree than 0, .75 and 1. The shooters of King of Two Miles as well as others were calculating wind per 1 MPH. So, on to more wind charts and comparisons. I was able to come up with each cosine for the wind angle using the clock method. I continued to research my new found realization and found this was how the top shooters, the best guys in the world were doing it. So, out with the old and in with the new. I made a chart and started comparing it to my old charts and formulas. I then compared my new chart with my Kestrel (it’s fairly new, I never used one before) and the results were just about spot on. No wonder I couldn't make a wind correction. The charts and formulas were wrong. It wasn't even really close for shooting sub MOA sized targets like I was doing.
Let’s look at the correct data.
I use the clock method per 30 Minutes and assign the correct cosine for the angle. I also built a Windrose to match.
12:00 - 0
12:30 - .25
1:00 - .50
1:30 - .707
2:00 - .866
2.30 - .965
3:00 - 1.00
That is a lot more data than the FM 23-10 teaches. Remember this is angler data and will be used for the other hours as well.
12:00 and 6:00 o’clock - 0
12:30, 5:30, 6:30, 11:30 - .25
1:00, 5:00, 7:00, 11:00 - .50
1:30, 4:30, 7:30, 10:30 - .707
2:00, 4:00, 8:00, 10:00 - .866
2:30, 3:30, 8:30, 9:30 - .965
3:00 and 9:00 o’clock - 1.00
Lets use this data with the more accurate cosine compared to what’s shown in FM 23-10.
12 MPH wind from 1 o’clock shooting a 10” plate at 1,000 yards:
For a full value wind I would need to dial 2.28 Mils. Because it’s from 1 o’clock, I need to multiply 2.28 by .5 for my correction.
2.28 x .5 = 1.14 Mils
Lets compare the true 1.14 Mils that I should hold to the formula that I was originally taught.
1 o’clock is shown as a “half value”. The “half value is .75.
2.28 x .75 = 1.71 Mils
The correct 1.14 Mil hold vs. the incorrect 1.71 Mils hold has a difference of .57 mils.
If our target is 10 inches wide at 1,000 yards then it’s only .28 mils wide. You would never get a hit using the old wind formula. In fact you would have missed by over 20 inches.
Lets look at one more example:
5 in target at 500 Yards with a 10 mph wind at 12:30
I would need .8 mils for a full value wind.
At 12:30 I would hold .2 mils
.8 x .25 = .2 mils
Per FM 23-10 that would be a 0 value wind and I would only be off by .2 mils. At 500 yards, with a 5 inch target, the target would measure about .28 Mils. I would likely get a hit but it’s cutting it very close, as I would have been 1.8 inches off. If I was aiming center, my margin of error would be 2.5 Inches leaving me only .7 inches for any kind of shooting error or error within the rifle system. I like to shoot in such a way to maximize my room for error and increases hit probability. This is something I got into the habit of doing because my previous way of calculating wind was less reliable. I would calculate the minimum and maximum wind hold. I would dial in my minimum wind and hold favoring the edge of the target. This would increase my hit probability due to having a larger margin of error.
I will leave the other blogs up but would recommend using the information from this one as I have yet to find anything online that is more accurate.
Start building you own chart. Use a Ballistic calculator and find the correction for 1 MPH per 100 yards. You can see an example of this in my Quick Drop Chart blog. Next, simply multiply that number by your wind speed and that gives you your correction for a full value wind. Using the wind rose above, multiply your full value correction by the given cosine for the wind angle. This will be your wind hold.
The Overwatch
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