Reading The Wind
Wind impacts the horizontal plane of a bullet’s flight path while air density and gravity impact elevation. In rare topographic cases, such as when shooting up into a narrow canyon, wind may impact the vertical plane. The angle of the wind to the path of the bullet will have either a major impact on the bullet’s path or a minor impact, depending on the wind speed and direction.
The origin or angle of the wind is expressed in values. Full value is a 90-degree wind blowing either right to left or left to right from the shooting position of the rifleman to the target. A half value wind blowing at 45 degrees has half the impact on a bullet than a full value wind, and a 30-degree wind has a third of the value of a full impact wind. As an example, if you have a 10 mph wind blowing across the range of fire at 90 degrees, the windage adjustment will be greater than the same speed wind blowing from a 45 or 30 degrees. If you have a 10 mph full value wind and a range of 500 yards, the bullet will be pushed off course by 18 inches. This requires an adjustment of about 3.4 MOA for a 175-grain bullet while a half value wind will blow the bullet off course by only 9 inches and require a MOA adjustment of 1.7.
There are three basic ways to determine speed and direction, without the expense of a handheld weather station:
- The way the wind is causing foliage to move.
- The direction and angle of the flags on the shooting range.
- By reading the wave patterns contained in a mirage.
To compensate for the wind pushing the bullet left or right (often called “deflecting”), the value of the wind needs to be determined. As stated above, we define wind as a full value, third value or a half value. It is helpful to think in terms of a clock to determine whether a wind is full value or some other value. A full value wind is a wind blowing from 9:00 to 3:00 or vice versa. A third value wind is a wind blowing from 7:00 to 1:00 and a half value is blowing from 2:00 to 8:00.
See the clock diagram:
90 degrees = full value
60 degrees = .85 value
45 degrees = .70 value
30 degrees = .50 value
You can also determine wind speed by minute of angle (MOA). A 1 mph wind at full value is a ½ MOA. If a full value wind is blowing at 8 mph, it is a 4-minute wind. Dividing the target into minutes aids in making the correct windage adjustments.
Experienced shooters make these wind calls and adjustments with relative ease based on years of experience. Winds are rarely at a constant — wind gusts are common followed by a sudden drop in speed or direction. More experienced shooters will adjust to these changing conditions by using the mil-dots etched into the reticle of the scope and then holding over (aiming higher) to compensate for the wind impact.
It is common to find competitors arriving several hours before a match to observe how the wind is flowing over the range and to plot the highs and lows of the wind speed on a graph to determine patterns in wind shifts and direction. It helps the rifleman to think of wind, and how the wind will to react to the peaks and valleys of the topography, more like water moving over the range. By plotting and graphing these effects, it allows the rifleman to see distinct and measurable patterns. These measurements are best plotted on a graph with a timeline across the bottom and wind speed along the top column click reference. These graphs provide a visual representation of shooting conditions, allowing you to predict wind conditions as they start to change and make the correct windage adjustments.
Speed by Reading Foliage
One of the easiest ways to read and determine wind speed is to observe how the wind is impacting foliage. If you are in a grass field, you can observe how the blades of grass are moving; if the blades are moving slowly, then the wind is usually under 5 mph. If the grass is lying flat, a strong wind is blowing and the wind speed may reach 10 to 15 mph, perhaps even more. Look up to the trees. If the leaves are shimmering, the wind is often between 8 to 10 mph. If the branches are bending, it indicates a 15 mph wind. If the tree is swaying back and forward, this is a sign of a 20 mph wind speed. If the branches are lying flat, the wind is 20 mph plus.
Flag Direction and Speed
When reading range flags, it’s useful to use at least two flags: one for direction, which tend to be the windsock-type flags, and one for wind speed, which are the lighter-weight flags. You measure the wind speed by how far the tip of the flag is above or below horizontal. One method is to visualize a quadrant in front of the flag where there are four to six horizontal lines with equal spacing, with the lowest line starting near the base of the flag. As the wind speeds up the flag, the tip will extend further from the pole and climb higher above your imaginary lines. Based on your spacing, you can have a no-wind line, a 3 to 5 mph wind line, a five to 10 mph line, 10 to 15 and 15 to 20.
Another method of reading wind speed is by the angle of the flag. A flag posting at 90 degrees is a strong wind. The speed can be determined by dividing 90 by 4, which means the wind is blowing at approximately 22 mph. If the angle of the flag is 45 degrees, divide by 4 and you have an 11-mph wind. A flag at 30 degrees is a 7.5-mph wind.
The third and most difficult method of reading the wind is by mirage — the heat rising off the surface of the range. It can be read much the same way as a range flag. If you look inside the mirage, you can see the direction of the wind based on the way the mirage is bending the target image. Wind speed is determined by the wave lines within the mirage and how fast those waves are moving.
The most common example of how a mirage impacts shooting is similar to the way a spoon appears bent when placed in a glass of water, an illusion that occurs because of light being refracted off the water. The same effect occurs when looking through your scope: the target sight picture sees the variations of air temperatures between you and the target, which causes a change in the index refraction, causing the image to appear blurred. If the mirage is moving from right to left, it may distort the target to the left and you may have to hold right. If the mirage is running, it may distort the image of the target to appear lower than its actual position. Conversely, if the mirage is boiling, the image may distort upward, requiring you to hold lower.
Reading the wind is key in becoming a successful competitor or hunter. A correct wind call — by predicting speed and direction, then assigning a value and inputting that data into a ballistic calculator — is what will bring you success.