Reading and Calculating Wind: Part I

In longrange shooting, there is a calculation or formula for every aspect with an exact or predictable outcome. Bullet drop, temperature change, barometric pressure, coriolis effect, Eotvos effect, and spin drift just to name a few. The hardest part of longrange shooting is reading the wind. Before we get into wind reading and the wind formula, lets do a quick overview.

   Even though we have discussed things that most people in shooting sports don't even know exist, like the Eotvos effect, we need to have our priorities straight. Things like spin drift can easily get lost on a bad wind call, so we need to focus on the more important things first.

   First, we need to know our bullet speed. Just about everything else will be based off of, or relate to, this. We went over this in the Data Collection podcast and the post that goes along with it. We will collect this information using a chronograph at various temperatures.

   Next, we need to determine the distance to the target. This can be done a few different ways. I like to use the reticle first with a formula to keep my skillset sharp. I'll then use a secondary means such as a range finder or G.P.S.

   We can now check our D.O.P.E book or use a ballistic calculator to determine what we need to dial onto our scope for our distance and environmental factors. Don't forget to keep track of everything.

   The final thing we are going to try and determine is the speed and direction of the wind. There will more than likely be wind moving at different speeds and different directions, especially when shooting over a few hundred yards. If you are shooting from one peak to another, over a valley or gully, you will potentially have wind blowing up or down the peaks as well as crosswinds. It's best to have a spotter call your wind for you. If you make your own wind calls like I do, the wind can/will change before you take your shot.

   How do we read the wind? There's a few things we can use to make an educated guess. I like to break up the distance to the target and read the wind in sections, based on direction and speed, and each section should be calculated independently. We can look at the movement of the grass, bushes, trees or any thing else blowing in the wind. This is probably the easiest way to determine wind speed and direction at shorter ranges or in a section. If you can barely feel the wind on your face, this is about 3 - 5 mph, a 5 - 7 mph wind will move the leaves on trees, 8 - 12 mph and the wind is raising dust, 12 - 15 mph sways small trees. Being able to estimate wind speed and direction using this method takes practice.

   The next thing we can use to read the wind is mirage. A very scientific explanation of mirage is, the invisible wavy/blury lines you can see over a heat source like a fire or Bbq pit. You can actually see mirage in the wintertime with snow too, so not just over a heat source. This takes a lot of practice also, and was very difficult for me to learn. Below is a chart to show how wind effects mirage.

   So, now we know how to estimate wind speed and direction. Now we need to know what to do with this information. Lets say we have a 10 mph full value wind (3 or 9 o'clock) and we are shooting a target at 500 yards. There is a military wind formula I like to use but, just a heads-up, it's meant for a 30 caliber round. However, using a ballistic calculator you can "reverse engineer" the formula to get the constants for the equation.

Here is our set of constants per yardage;

0 - 500 Yards, Divide by 15 600 Yards, Divide by 14 700 - 800 Yards, Divide by 13 900 Yards, Divide by 12 1000 Yards, Divide by 11

Distance in 1/100 yards x wind speed in mph ÷ (a) = moa ÷ 3.5 = mils

We are shooting at 500 yards. 1/100 of 500 is 5. So, 5 x our wind speed 10mph.

5 x 10 ÷ 15 = 3.33 MOA

15 is our constant for 0 - 500 yards

If using mils 3.33 ÷ 3.5 = .95 MIL

So we will hold either 3.33 (3.25) MOA or .95 (1) MIL into the wind.

*REMEMBER THIS IS FOR A FULL VALUE WIND*

   Using the same scenario, if we have a 10 mph half value wind (2, 4, 8, or 10 O'clock) (it's really 75% value but many people refer to is as half value) we can use the same formula but multiply by .75 (75%) of our full value.

5 x 10 ÷ 15 = 3.33 moa ÷ 3.5 = .95 mil

3.33 x .75 = 2.49 moa

.95 x .75 = .71 mil

   If you have a 10 mph wind coming from your 2 O'clock, you will hold .71 mils into the wind. You will hold your rifle to the the right. You will be shooting to the righthand side of your target. The wind call will be "hold right .7". I want to over emphasize this because in your reticle it will be left of the crosshair.

   Another tool we can use is a wind meter. I like the idea of using a wind meter in practice. Use your environment and judgment to determine the wind speed and confirming it with a wind meter is a really good way to learn. What I don't like about them is it only gives us the wind speed right where we are standing. And, if it runs out or batteries what are you going to do? I personally like to use my electronics to confirm my work. I want to be the weapon. I want to be able to make hits with only my rifle, bullets, pen and paper. (And a solar powered calculator because I am bad at math)

   Probably the easiest way (but maybe not as accurate) to calculate wind is using a range card with 10 mph full value wind per 50 or 100 yards.

10 mph

100 - .2 150 - .28 200 - .38 250 - .47 300 - .57 350 - .66

Or

100 - .2 200 - .38 300 - .57 400 - .76 500 - .95 600 - 1.22 700 - 1.53 800 - 1.75 900 - 2.14 1000 - 2.59

   The wind formula is pretty straightforward and linear. At 500 yards with a 10 mph wind we hold .95 mils. (A) If we have a 5 mph wind we split .95 in half because 5 is half of ten. (B) If we have a 20 mph wind then we multiply by 2 because 20 is 2 times more that 10. (C) If we have a 15 mph wind we just add half the value.

(A) .95 ÷ 2 = .475 or .5 rounded

(B) .95 x 2 = 1.9

(C) .95 + .475 = 1.425

This also works with .75 value wind. Lets run through a few examples.

10 mph wind from 2 O'clock at 500 yards

.95 x .75 = .71 mils

A 5 mph wind at 2 O'clock at 500 yards

We'll take our full value wind of .95 and divide by 2 because 5 is half 10 (wind speed)

.95 ÷ 2 = .475

Next, because the wind is from 2 O'clock we will multiply by .75 because it's not full value.

.475 x .75 = .35 mils

   This is a fast way to calculate wind. With just a range card with 10 mph full value wind we can do the math in our head for quick wind calculations.

*TIPS*

   When calculating wind, use your focus knob on your scope on different sections of the range to see the mirage at different distances. Also, when using mirage to judge the wind, you need to keep in mind your maximum ordinate and ordinate range. Maximum ordinate is how high the bullets is from the center line of the barrel. Using my data with a target at 1000 yd, the maximum ordinate is 133" or 11' at 568 yards or 57% of the distance to my target. So I am going to look at the mirage 11' above me at 568 yd.

Keep the wind formula and a 10 mph wind card in your dope book and with the rifle for easy access and use.

THE OVERWATCH

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