Reading the wind

[old Grump]

There are two primary atmospheric conditions that can affect the point of impact of your fired round. The first, and most obvious, is the wind. The wind pushes your bullet as it flies downrange, changing the point of impact. Mirage on the other hand can cause your target to appear blurry and distorted, or even have it appear to be where it is not, such that firing at the apparent image of your target will result in your bullet hitting somewhere other than the intended point of impact. Compensating for wind is fairly easy, even for novice shooters.

Mirage on the other hand can be a bit tricky. Almost everyone has seen a mirage before. Look out across a blacktop road on a hot summer day and you’ll see the watery mirage caused by hot air rising off of the sun baked asphalt. This same phenomenon can plague shooters who are engaging targets at long-ranges, even on overcast or mild days. Mirage is caused by differing air densities between the shooter and the target. For an easy example of what mirage does, examine a spoon setting in a tall clear glass of water. When you look at the spoon, you will notice that the handle above the water appears to be in a different place than the handle below the water. This is caused by light being bent as it passes through the boundary between the denser water and the less dense air. In much the same fashion, light reflected off of your target is bent as it passes between dense cool air and less dense hot air. Still, mirage can be your friend, as we’ll discuss later you can use the mirage to your advantage by reading it to get very accurate wind speed estimations.

Wind
The first step in negotiating atmospheric conditions is knowing the wind direction and how much value to assign it. Assessing the direction of the wind is a fairly easy task. Wind flags are used at most long range rifle competitions, and are generally a permanent fixture at established rifle ranges. If your range doesn’t have wind flags you can make some easily and inexpensively using some wooden stakes and fluorescent orange engineers tape.

The most basic measurement that a flag is good for is determining actual wind direction. This essential measurement will help you to determine what value to give to the wind; full, three quarters, half, or no value. Wind direction is determined relative to the shooter’s position using the clock face method, or using the angle measured in degrees. When the wind is blowing at 90 degrees (3 o’clock) or 270 degrees (9 o’clock) relative to your shooting position, we assign it a full value of 1. Wind blowing at 45 degrees, 135 degrees, 225 degrees, or 315 degrees relative to your position is given three quarters value. When the wind is blowing at 0 degrees or 180 degrees (12 o’clock or 6 o’clock) relative to your position it is disregarded and given no value. See the diagram to the right for more details on assigning wind value.

Some shooters try to compensate for bullet drop or rise caused by the wind blowing directly away or directly towards the target. In this writer’s opinion, a head or tail wind simply will not affect the bullet flight enough to warrant compensating for. Yes, it is true that a bullet fired into a head wind will drop due to additional aerodynamic drag, but the amount it will drop is almost negligible. At 600 yards, a 150 grain .30-06 bullet will only drop by a half-inch with a 10 mph head wind, a margin of error so small it must be measured in hundredths of a minute of angle (for those doing the math, that’s 1/12th or 0.083 MOA). Only a handful of the most accurate shooters in the world can shoot well enough to be bothered compensating for that small of a drop. If you’re reading this you’re probably not one of them, so don’t worry about it.


Once wind direction and value is determined, it’s time to measure or estimate the wind speed. An anemometer is probably the most accurate device for measuring wind speed, but there are other methods that you can learn. If you find yourself without an anemometer, you can use the guidelines set forth in the Service Rifle Pamphlet produced in 1931 by the US Army Infantry Team. While the information is old, the guideline is as valid today as it was 79 years ago.


0-3 mph Wind hardly felt, but smoke drifts
3-5 mph Wind felt lightly on the face
5-8 mph Leaves are kept in constant movement
8-12 mph Raises dust and loose paper
12-15 mph Causes small trees to sway

http://cheaperthandirt.com/blog/?p=2262

This is primarily for high power shooters and those who have competed are already familiar with it but the same principles hold for the rimfire shooter who wants to shoot beyond 100 yards. I personally have shot a lot of 100 and 200 yard rimfire but not 300 yard and am looking forward to the challenge if I can find a 300 yard range to shoot at.

[Silicon Wolverine]

i use a similar system for long range steel silhouette shooting (out to 1000 yards) but its not infaible. the percentage of wind (how directly it is aimed at you and affecting the bullet) can affect how the wind "feels". i use things like leaves, grass and dust to measure wind that dont require feeling (like the lower MPH speeds are measured above).

SW

[mriddick]

On the old IU range a me and a friend would shoot our 22's at 300yds at a 12 x 18 inch gong, it was alot of fun. If I remember right you had to dope the wind sometimes 10 ft or more to one side and hold 15 ft high, but once dialed in you could hit it with regularity. The best story was when a guy showed up with a 7mm high power rifle and tried to shoot the same gong, he'd fire and miss, then we'd fire wait a few seconds for the ping and so on. I'm not sure he ever hit the gong while we hit it about 70% of the time. At the end he threw the rifle down, muttered something about it not being able to hit anything and left

[old Grump]

What was strange was shooting at a range like Black Canyon where the wind swirled because of the mountain back stop and having to crank on for wind at the line, crank off for the wind 300 yards down range and crank on again at 600 in order to hit at 1000 yards. At El Paso no mid range wind flags so it was learn to read mirage in order to get on target. It was 101 degrees so there was plenty of mirage to read. Us shooters who were shooting with iron sights actually had a slight advantage over some of the rookie shooters with scopes because they didn't know how to compensate.