Wednesday, April 21, 2010

Reading Wind and Mirage

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Last week we discussed how to bore sight and zero your scoped bolt action rifle. In that article, we touched on reading or "doping" the wind, as well as reading mirage. Reading wind and mirage is sometimes mentioned in the same breath as black magic and astrology. But taking cues from the wind and mirage is not so much hocus-pocus. There are some simple techniques for accurately reading the wind and mirage that you can use to determine how these conditions will affect your point of impact.

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.


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

Flags can also be used as a rough estimate of wind speed. When observing a normal rectangular flag, estimate the angle between the flag and the pole and divide that number by 4 to get the approximate wind speed. For example, if a flag is flying straight out at a 90 degree angle, the approximate wind speed is 22.5 mph or greater (90/4). If the flag is limp and flapping in a breeze at a 45 degree angle to the pole, the approximate wind speed is 11 to 12 mph. This same estimation method can also be used for streamers and pennants.

As important as knowing how to read the wind is knowing your cartridge and how your load will be affected by various wind speeds. Many novice shooters simply do not understand, or do not believe, how much of an effect a cross wind can have on even the speediest of bullets. Consider a 55 grain .223 round fired down range at over 3,250 FPS for example. With only a modest 5mph cross wind that little .223 bullet will be pushed over 1/2" off target at only 100 yards. While that might not seem like much, consider that a 10mph wind will result in the same round being pushed more than 1 MOA at any range. Experienced shooters, having been frustrated by wind before, often have the opposite problem and tend to overestimate the effect wind will have on their bullet.

All bullets have a ballistic coefficient that is usually computed by the manufacturer. This number, combined with the flight time of the bullet, can help you determine how much your bullet will be affected by a given wind. By combining the wind direction and value, speed, flight time and the ballistic coefficient of your bullet, you can determine how much to hold over or how much to adjust the windage on your sights. Because of the fact that bullets with differing ballistic coefficients are affected to differing degrees by the wind, there is no hard and fast rule for calculating wind drift. I won't get into the mathematics of computing wind drift using the ballistic coefficient and flight time of your bullet; wind drift charts and calculators are readily available for almost every cartridge load. Use a wind drift chart for your specific load to determine how much holdover or windage adjustment is necessary.

With the information from the appropriate wind drift chart, apply the wind value to determine the actual drift. For example: Our chart shows that M2 match ammunition for an M1 Garand from American Eagle will drift approximately 5.8 inches at 600 yards with a full value wind at 1 mph. If we actually have a 10 mph wind blowing in at a 45 degree angle (1:30 o'clock) we assign it a value of 3/4 and do the math (5.8 inches X 10 mph X .75) to arrive at 43.5 inches of drift. If the wind shifts to be 30 degrees (1 o'clock) we would assign it a value of 1/2, resulting in 29 inches of drift. Doing the math, we correct approximately 5 MOA for wind at 1/2 value and 6.9 MOA for 3/4 value.

Example of a mirage created by a hot blacktop road; image courtesy of BrentDanley licensed under Creative Commons.

Hot air rising up from ground that is warmed by the sun distorts the image of your target, causing it to appear blurry, or even appear to be in a location that it actually is not. This is referred to as mirage. To some degree, heat from the barrel of your rifle can also affect your target image. Eliminating mirage from barrel heat is relatively easy. Many benchrest shooters use extended scope tubes so that the hot air rises around the line of sight, eliminating any blurriness caused by the hot air. Another way to divert the hot air is to tape a light colored piece of cardboard or paper along the top of your barrel.

Mirage caused by hot ground baking in the sun is not possible to eliminate, but it can be understood and worked around. Like the spoon in a glass of water, mirage can cause the image of your target to be higher or lower, but luckily this shift is generally not significant enough to need compensation. For the most part, mirage is only problematic due to the blurriness it imparts to your sight picture. It is in this case that the wind can sometimes be your friend. When looking through your scope across a hot field in calm air the mirage appears to be "boiling" as if peering at your target through a puddle of water. When the wind is blowing however, the mirage will "follow" the wind, in some cases blowing away so that you can get a clear sight picture. Of course, as we mentioned in the section above, you will still need to compensate for the wind. That is where "reading" the mirage comes in. When observing mirage, it often appears as waves running in the direction of the wind. Many people find that reading mirage in this fashion gives a very accurate indication of wind speed. You can actually watch the waves from the mirage as they follow the wind, and estimate the actual wind speed from the speed of the waves.

Reading the mirage in this fashion can be difficult with a headwind or tailwind as those wind conditions can cause the mirage to appear be "boiling" when in actuality it is running with the wind directly away from or towards you. As we stated above however, headwinds and tailwinds generally have only a minimal effect on the overall bullet rise or drop, and for all but the most skilled shooters can be disregarded. Some shooters will even adjust for a boiling mirage in calm conditions as the hot air rising off of the ground can impart a small amount of lift or rise to the bullet. Again, for all but the most skilled shooters this adjustment is not necessary. Any lift from hot air is easily and quickly negated by the force of gravity tugging the bullet downwards at 32 feet per second squared.

When reading mirage to get an idea of wind speed and direction it is important to remember that the mirage you are seeing through your scope is only the first couple of feet in front of your target, as that is the only area that is in focus. The mirage existing the rest of the distance between you and the target is not visible because it is outside the shallow depth of field of your scope. To increase your depth of field, you can narrow the aperture of your scope by placing a lens cover with a tiny hole punched in the middle, effectively stopping down your scope and increasing your depth of field to near infinity. Another method for reducing your aperture size is taping over the objective until there is a small hole between 1/8" and 1/2" in diameter. Increasing your field depth in this manner allows you to see shifting winds indicted by the mirage over the total distance between you and the target.

An alternative to this is to change the focus of your scope so that the middle of the distance between you and the target is in focus. By examining the mirage over the total distance between you and the target, small variations in wind direction and speed can be noted and accommodated. While unusual, it is possible to have eddies and even countervailing winds between your firing position and the target. These variances in wind speed and direction will be easy to pick up with a bit of practice studying the mirage at varying distances between you and your target.

Practice Negotiating Wind and Mirage

It is difficult to explain the visual differences between a boil, a mirage running away, or a mirage running towards you. Wind drift is a simple concept to grasp, but it still takes practice to know just how much your particular load will drift. There is really no substitute for actual time spent on the range practicing. You will need to train and practice in order to properly read wind and mirage. On a hot sunny day when the wind is blowing, observe the effect this has on your mirage. With a rifle and scope that have already been zeroed in optimal conditions, take aim at the center of your target and call your shot. Sketch the target in your shooting log and mark the area where you called your shot. When marking your target sketch, be sure to make a note of the conditions in as much detail as possible. Once the range is cold, check your target and compare the point of impact to the called shot on your sketch. Note the differences between the point of aim and the point of impact that the atmospheric conditions have caused. By examining the conditions and the difference between your point of aim and the actual point of impact, you can learn how to best accommodate those situations.

At this point, do not adjust your scope to compensate for the wind or mirage. Instead, hold over the appropriate amount to bring your point of impact to the bullseye of your target. Changing atmospheric conditions can cause you to "chase the wind", adjusting your scope for conditions that may vary from shot to shot. Take aim at the center of the target. Again, call your shot, mark your target sketch and note where the round actually impacted your target, as well as the observed conditions at the moment of the shot. Repeat this procedure and continue to record information. By taking good notes, you will be able to review your information while not at the range and possibly see things that you might otherwise miss while sitting at the bench.

Repeat this procedure for differing conditions whenever possible. The more information you have, the more you will know how to adjust your point of aim for various conditions.

As with most things in life, there is no replacement for experience when it comes to reading wind and mirage. No amount of explanation can substitute for sitting at a bench and observing how differing atmospheric conditions affect the flight of your bullet. Take what you've learned, head out to the range, and see for yourself how long range rifle shooting is affected by wind and mirage. Every range is different and has its own peculiarities, so talk to other shooters and see what you can learn from them about handling wind and mirage.

Keep an eye out next week for our article on compensating for wind and mirage in rifle competitions, where we'll discuss the tips and tricks used by the pros to keep all of their shots in the X ring under even the most demanding atmospheric conditions.

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