HUMIDITY

I have talked to many people about long range shooting, casual and formal (but mostly casual). I find that, with many people who are not directly knowledgable about both, there is a disassociation between what it means to be a long range shooter or a sniper. The word sniper is usually followed by an idea of something mystical and almost romantic. I have had people try to explain to me that snipers have to calculate for the spin of the earth and even humidity. My response is normally along the lines of, "It's true; we do calculate and adjust for all of that". However, while you can calculate for humidity, you really can't adjust for it alone because from 0% to 100% humidity the change is only a small fraction of .1 Miliradian, and can really only be adjusted for by compounding fractions from other effects we calculate for. So, let's take a look at calculating for humidity from 0% to 100%. Let's use a .264 with a 147 ELD-M. We have zeroed our rifle and built a dope chart for a specific set of environmental standards. Anything that changes from our environmental standards we need to calculate for. For this example our environmental standard for humidity is zero and we're going to calculate a 100% change in humidity. The target is 1000 yds away. For 1000 yards the correction value is .04 Formula: Measured Humidity (%) x Corrected Value 100% humidity is written as 1 or 1.00 1 x .04 = .04 Mils 50% change in humidity .50 x .04 = .02 Mils 25% change in humidity .25 x .04 = .01 Mils So, as you can see from the formulas above, we can calculate for a change in humidity. But, we can't adjust for humidity alone. It's such a small amount, we can only adjust for it by compounding fractions from other effects we calculate for. So, should you adjust for humidity? Yes. If you can make an adjustment for it then you should calculate for it. With that being said, you are not going to miss a target because you did not adjust for humidity. At least not at 1000 yards. A small error in your wind call, the coriolis effect and even the Eotvos effect have a far greater impact on the bullet than the changes in humidity. Here is how you find your correction value. Use a ballistic calculator (like JBM), and enter you bullets information, your designated standards and environmental information, for when your rifle was zeroed. Change only the humidity to 0% and log the info. Next, change the humidity to 100% and log the info. You can now calculate the difference between the two columns (per 100 yd) and that is your corrected value. Example: 2000 yard drop in inches @ 0% humidity 2095.9 Inches 2000 yard drop in inches @ 100% humidity 2073.8 Inches 2095.9 - 2073.8 = 22.1 Inches 1 MOA @ 2000 Yards = 1.047 x 20 = 20.94 Inches (Inches) ÷ (1 MOA) = (MOA) 22.1 ÷ 20.94 = 1.055 MOA Difference 1.055 ÷ 3.5 = .301 Mils So, your correction value for humidity at 2000 yards is 1.05 MOA or .3 Mils

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