Long Range Wind Effects on Bullets

It is important to note that over long ranges, there can easily be a variety of wind speeds and directions between the shooter and the target. In this case, it is the shooter's job to "integrate" the effects of the various wind velocities to the target, and make the corresponding adjustments.

Part 1 : Estimating Wind Velocity

• Drop a few blades of grass from your shoulder
• After the grass lands on the ground, point at the impact point
• Divide the angle made by your arm and the vertical position of your body by 4
• This is the wind velocity in miles per hour (mph)

• Determine the angle that the mirage waves make from the vertical (straight up)
• Divide this angle by 4
• This is the wind velocity in miles per hour (mph)

Note:

• big, lazy waves = SLOW wind
• small, flat waves = FAST wind

To view mirage, focus your scope as clearly as possible on the target, then back off the point of focus a couple hundred yards towards yourself, so that the target appears blurry when viewed through the scope. The shooter will then be able to observe the mirage as it rises from the ground downrange. The direction and angle of the mirage is a great help in determining what the wind is doing at any point in time. Mirage is a good tool for determining direction and speed of "full force" wind, that is, wind coming from more or less 90 degrees from right or left downrange, but it isn't too much help on determining direction of wind coming straight from behind the shooter, or from directly downrange.

Once the wind has reached about 15 miles per hour, mirage begins to lose its usefulness, as it will appear to run straight across, and variations in wind velocity will not be discernible from a reading of mirage. After about 15 mph of wind is steadily on the range, other methods are more reliable.

A note about wind direction: a less than full value wind will have an effect on elevation as well as windage, albeit minor. (see also "Part 2 : Effects", below)

• (Beaufort) Wind Scale I
• (Beaufort) Wind Scale II

The above exercises provided us with wind speed. Remember that there is another component to velocity besides speed. That is DIRECTION. Obviously, wind direction plays an important role in determining where your bullet will strike. Determine wind direction by observing which way objects lean or are blown. Convert this 'mental direction' to the face of an analog clock, with the direction to the target at the '12:00' position.

Part 2 : Wind Effects

The direction of the wind relative to the bullet's flight path is an important consideration in determining the net effect that the wind will have regarding deflecting the bullet. Think of the shooting situation as if the shooter were sitting on the center of an analog clock face, aiming toward a target at '12' o'clock. If the wind is blowing either directly in the shooter's face (coming from 12 o'clock) or from directly behind ( 6 o'clock) the shooter, the effect of the wind will be minimal. A headwind will slightly lower the bullet impact point, and a tailwind will slightly raise the impact point. Additionally, horizontal deflection is zero, and so does not even enter the discussion. For practical purposes, the effects of 12 and 6 o'clock winds are negligible.

                              12
                         11          1
                               |
                      10       |        2
                               |
                     9         o          3

                       8                4

                          7          5
                               6

However, if the wind is blowing from either 9 o'clock or 3 o'clock, the bullet will experience the FULL EFFECT of the wind (90 degrees). If the wind is blowing from either 10-11 o'clock or 7-8 o'clock, the bullet will be deflected to the right at a fraction of the full value. The same holds true for 1-2 o'clock and 4-5 o'clock winds relative to the FULL (3 o'clock) EFFECT (from 90 degrees right), which will deflect the bullet to the left.

The factors turn out to be :

A fair simplification of the 'clock' is to consider a wind coming from in front or behind you as having a factor of zero, right or left as having a factor of one, and anything in-between as having a factor of 0.5. Multiply the absolute wind velocity by the 'Deflection Factor' to get the effective wind velocity. You will use this wind velocity when you determine the bullet deflection.

Notice that you really only need to remember 3 numbers ( 100% for 90 degree right-left winds really is intuitive). These are 0, 0.5, and 0.9 - with the direction of the effect being the SAME AS the direction of the wind. Also note that the effect for 1 and 5 o'clock are the SAME, for 2 and 4 o'clock are the SAME, and so on. An interesting and useful article to read on this subject is Advanced Marksmanship, Wind Effects (towards the bottom) from the 'Sniper Country' web pages.

Part 3 : Determining Bullet Deflection and Drop

Now you know the 'deflection factor' due to the wind. The next piece of information required is the target range.

An accurate assessment of target range is very important not only for determining the bullet drop (vertically), but is also an important factor in determining the total amount of wind deflection (horizontally). Some scopes have MIL-DOT reticles to aid in determining the range to your target. One problem with using this method is that you need to know the size of the target to accurately determine the range. This is also a shortcoming of stadia rangefinders. Take heart, thanks to modern manufacturing methods and technology, there are very accurate laser and coincidence rangefinders available at very reasonable prices these days. If you need to make accurate shots at small targets at any range or at any size targets at longer ranges, do yourself a favor and buy a coincident (good) or laser (better) rangefinder. Something to beware of are false readings. Sometimes you are better off choosing to range a larger object that is near your target, and then making any necessary (intuitive or objective) adjustments to the reading.

In order to compensate for changes in target range and wind deflection, it is necessary to determine the horizontal and vertical bullet paths of the load you are using, based on target range and wind speed. These will vary as a function of many factors, such as muzzle velocity, bullet mass, bullet BC (ballistic coefficient), the rifle's "zero" point, and scope height. For this reason, these calculations must be performed whenever any one of these factors changes.

Now for some good news - there is software that will do the calculations for you. I highly recommend using external ballistics software to determine not only wind deflection, but also the amount of bullet drop. Many very good software packages are available as freeware or shareware on the internet. Others may be purchased at reasonable prices. PCB and JBM seem to be favorites among the shareware / freeware programs. With these packages, all you need to do is memorize (or write down) the wind deflection for a given wind speed (eg. 1 mph) acting at a right angle to your bullet path in 50 or 100 yard increments. Then adjust for the actual wind speed, and apply the deflection factor based on wind direction.

While wind deflection is NOT linear over range, it IS linear over varying wind speed. (Therefore, you only need to know deflection for 1 given wind speed.) This means that at any given range, twice any given wind speed will move the bullet twice as much. I calculated the bullet path for a 10 mph wind. A 5 mph wind will push my bullet 1/2 as much as my calculations show for 10 mph wind. Deflection is NOT linear over range for a very simple reason - the bullet is NOT travelling at a constant velocity. It is constantly slowing down, so the wind has a disproportionate amount of time to push your bullet at longer ranges. Since deflection and bullet drop are NOT linear over range, it is necessary to learn and either memorize, carry with you, or determine a calculation for, the path of your load at various ranges for both drop and deflection.

A very important component of the calculations performed by the software metioned above is bullet velocity. While using the ammunition manufacturer's data for muzzle velocity and downrange velocites is better than nothing, I highly recommend chronographing your loads out of your rifle. Estimates are OK, but nothing beats actual measured data. One must understand that every firearm is a rule unto itself. This is why the reloading manuals always advise you to start at a low gunpowder weight, and work your way up, always looking for signs of high pressure. Likewise, different rifles will produce different muzzle velocities with the same exact load.

It is also a very good idea to match the performance of your load against what the computer model tells you. Shoot your load in increments of 100 yards as far out as you can, and compare the elevations to the data from the ballistics software. I will then usually adjust the bullet's ballistic coefficient (BC) in the computer model to 'tweak' the model as close as possible to the "real world" conditions observed. In windless conditions (they are rare in most places), note any horizontal offsets as well.

Many people then suggest that you make a chart and tape it to the butt of your riflestock. I recently ran across the suggestion that you tape it inside your rear scope cap. I like this one a lot, although I suppose it assumes you're using a 'flip top' style of scope cover, like Butler Creek^TM, and that your eyesight is good enough to read the necessarily small print.

Part 4 : Adjusting and Compensating

This is the preferred method, as it is inherently more accurate than Method 2 (below). Once you have determined target range and the effects of the wind, you can 'dial in' the adjustment using scope clicks. Obviously, you MUST know the number of 'clicks' per MOA that is built into your scope to use this method. This can be done for both bullet drop and wind deflection.

This method can be used if you know the size of the target or if you know the size of an object that is fixed in your reticle. Examples being (1) MIL-DOT reticles, (2) the length of the fine line in a duplex reticle, or (3) the size of your scope's DOT reticle. For instance, if you determine at the target range that the fine portion in the center of your duplex reticle is 3" on your target at 100 yards at 10 power, this can be used to estimate hold-off for wind deflection and bullet drop. An easy way to determine this measurement for your various scopes is to post a target with 1" grid squares, and just COUNT them. Do this for a couple different magnifications if you're using a variable power scope (just to verify).

Another useful bit of information is your scope's field of view at a given range and magnification. Remember that scope magnification matters for all of the above! Do this for the highest power of your scope, and scale it down appropriately if you are using a lower magnification.

These method works fine for small holdoffs. However, a lot of error may be introduced for larger holdoffs due to the compounding effects of errors. It's easy to hold 1,2, or 3 'DOTS' off the center of your target. Try holding 20 (? does it look like 15?, 23?). Is your target still in your field of view?!

Good luck and good shooting from The Spirit of '76 Pages. I hope this information is useful to you!

Last updated : 13 August 1998