Wind impacts the horizontal plane along the bullet flight path while air density and gravity impacts elevation. In rare topographic cases wind may impact the vertical plane such as when shooting up into a narrow canyon. The first step is to determine the wind direction; the angle of the wind to the path of the bullet will either have a major impact on the travel of the bullet or a minor impact depending on its speed and wind direction. The origin or angle of the wind is expressed in “values.” A full value wind 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 from a 45or 30-degree direction. If you have a 10 MPH full value wind and a range of 500 yards the bullet will push off course by 18 inches and the adjustment needed is about 3.4 MOA for a 175 grain bullet while a half value wind will only blow the bullet off course by 9 inches and the MOA adjustment is 1.7
There are three basic ways to determine speed and direction, without the expense of a handheld weather station:
1. The way the wind is causing foliage to move.
2. The direction and angle of the flags on the shooting range.
3. By reading the wave patterns contain in a mirage.
To compensate for wind pushing (most times called deflecting) the bullet left of right, 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 wind. A 90-degree wind (a wind directly traveling on a straight line right to left or left to right) is a full value wind. A full value wind has the largest impact of how much a bullet will drift. 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 between 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. For example, if a full value wind is blowing at 8 MPH it is a 4-minute wind. This aids the rifleman in making quicker adjustments to the windage turret. 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 the norm. The more experienced shooters will adjust to these changing conditions by holding over or by using the mil dots etched into the reticle of the scope and then holding over to compensate for the wind impact. There are patterns that can be timed and wind, much like water conforms to the shape of the landmass. It is a common event 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 speeds on a graph to determine patterns in wind shifts and direction. It helps the rifleman to think of wind more like water moving over the range, and how the wind will to react to the peaks and valleys of the topography. By plotting and graphing these effects, it allows the rifleman to see distinct and measurable patterns contained in the wind. These measurements are best plotted on a graph with a time line across the bottom and wind speed along the top column. These graphs provide a visual representation of your shooting conditions allowing you to predict wind conditions as they start to change and make the correct windage adjustments as the wind patterns emerge.
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 5MPH; 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 20MPH plus.
Flags Direction and Speed
When reading range flags it is 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 speed of wind by how far the tip of the flag is above or below the horizontal level of the flag. One method is to visualize a quadrant placed in front of the flag where there are 4 to 6 horizontal lines with equal spacing with the lowest line starting near the base of the flag. As wind speeds up the flag, the tip will extent further from the pole and climb higher above your imaginary lines. Based on your spacing you can have a no wind line, a 3-5 MPH wind line, 5 to 10 MPH line, 10 to 15 wind line, and 15 to 20 MPH wind line.
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 degrees by 4, equaling approximately 22 MPH wind. If the angle of the flag is at 45 degrees, again divide by 4 and you have an 11 MPH wind, a flag positioned at 30 degrees is a 7.5 MPH wind.
The third and most difficult method of reading the wind is by mirage. Most long-range shooters will tell you that mirages can help or hurt your windage adjustments. The heat rising off the surface of the range causes a mirage. 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, (by the way the mirage is bending the target image from one side to the other). Wind speed is determined by the wave lines contained within the visual image of the mirage and by 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, this illusion occurs because there is a light refraction coming off the water. The same occurs when viewed 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 then its actual position. Conversely, if the mirage is boiling the target image may distort the image 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 followed by inputting that data into a ballistic calculator is what will bring you success.
This article courtesy of guest writer Ralph Hicks from The Arms Guide.
Ralph Troy Hicks is the author of Fundamentals of Long Distance Shooting and Fundamentals of Long Distance Shooting II. He is a competitive shooter who set out to design and manufacture the finest semi-auto in the world—a rifle that could hit a dime at 100 yards and reach out to 1,400 yards with minimal felt recoil. It took over three years to develop this firearm. His goal was to take the precision of a bolt-action rifle and the speed of a semi-auto and marry them together in a stunningly beautiful firearm platform. It has the power, speed and precision to make any shooter into a champion.