*** If you are using a ballistic calculator to help you compare your math, you must turn off the Coriolis and Spin Drift!***
Applying The Cosine To a Full Value Hold
Determine the wind speed
Ex. 12 MPH
Divide the wind speed by the Q.W. number
Ex. (6 mph gun) 12 / 6 = 2
Multiply the Q.W. dope by the quotient
Ex. 500 yards = .5 Mils x 2 = 1 Mil
Determine the wind angle from the DOF cosine
EX. 1:00 cosine .50
Multiply the product by wind angle cosine
Ex. 1 Mil x .5 = .5 Mil wind hold
Applying The Cosine To The Wind speed
Determine the wind speed
Ex. 12 MPH
Determine the wind angle from the DOF cosine
EX. 1:00 cosine .50
Multiply the wind speed by the cosine
Ex. 12 mph x .5 = 6 mph
Divide the adjusted wind speed by the Q.W. number
Ex. 6 / 6 = 1
Multiply the dope value by the product.
Ex. 500 yards = .5 Mils x 1 = .5 Mil
The difference between the two methods is when to multiply by the cosine. We can multiply the wind speed by our cosine, or DOPE our full value wind, then multiply our DOPE by the cosine. Sometimes doing it one way over the other is a little easier and faster to do the math in your head. It's important to know how to do this both ways.
How to determine the wind speed and direction
In my experience, reading the wind is by far one of the most challenging aspects of long-range shooting. While I can use a ballistic calculator such as a Kestrel with a wind meter, I prefer a combination of methods. To keep my skill set sharp, I'll First face into the wind and give it my best-educated guess. I use a Kestrel wind meter to measure the wind and see how accurate my guess is. If I was incorrect, I'll take a second look at the wind and see what I missed and why. This helps to recalibrate myself. Doing it this way takes a little time to calibrate myself, but it works well, and I never have to rely solely on equipment. I will look at the wind and guess the wind speed throughout the day. The more I do this, the better I get.
Reading Mirage
Reading the Mirage is another tool to help gauge wind speed and direction. In certain situations, the Mirage may be the only source to use when calling wind. I view Mirage as an aid because it can be difficult and tricky to read. Especially if I can't feel the wind. (e.g., if I'm shooting from inside a building or from cover and I can't use a wind meter) Mirage is seen as heat waves, telling me how fast the wind blows and from what direction. Here is an example of how it can be tricky. A boiling mirage is seen as the heat waves straight up and down. A boiling mirage can mean there is no wind. It can also mean the wind is at 12 o'clock or 6 o'clock. In this situation, I will have to use other indicators to give me more information, such as how the trees, grass or a flag is blowing.
In situations where I have no constraints, I will use a combination of methods. This combination will give me confirmation and make sure I didn't miss anything or make a mistake. I start with facing into the wind to get the wind speed and direction, then take an educated guess. I'll confirm the information on my Kestrel. If I have my spotting scope, I'll use it facing my target to get a read on the Mirage. I will then slowly turn it until I see a boiling mirage. Using the clock method will give me the direction of the wind. I'll compare all the information and then start doping the wind.
Another option to get the wind angle is by using a compass. First, I'll shoot an azimuth at my target. Next, I'll shoot an azimuth into the wind. The difference between the two angles is the wind angle. I'll then multiply the wind speed by the cosine of the wind angle, giving me the cross-wind value.
EX.
Target Azmith - 225º
Wind azimuth - 180º
225º - 180º = 45º wind angle
45º sin = .707 (70% wind value rounded)
70% of 12 mph wind = 8.4 mph
(12 x .70 = 8.4)
70% of 10 mph wind = 7 mph
(10 x .7 = 7)
Putting it all together
I'll read the Mirage and use a compass to gather all my data and do some math. I then confirm everything on my Kestrel. I have my wind speed and direction, and now I know I need to adjust for a 7 mph wind. Now what?
You can use a Kestrel or wind cards built from a ballistic calculator. But my favorite method is using Quick Wind. To use quick wind, you first need to find out what mile per hour your gun is. You can do this in two different ways. One way is to use the first number of your G1 B.C. the second way is to use a ballistic calculator. Under wind, set it to 90º (full value). Start with a 5 mph wind. Then check your drop every 100 yards. It should be very close to 1/100 of your distance being your wind hold. If it doesn't match up, change the wind to 6 mph and recheck. When it matches up, this is the gun's wind number/quick wind number. Make sure to turn off the Coriolis and spin drift. If your numbers are not lining up, that is probably why.
EX.
200 - .2 mils
300 - .3 mils
400 - .4 mils
500 - .5 mils
My rifle is a 6 mph gun, so I'll use that as an example.
From above, we know I need to adjust for a 7 mph wind
at 500 yards.
7 / 6 = 1.17
.5 x 1.17 = .585 mils (.6 mils rounded)
Wind speed divided by the gun number
7 mph / 6 (gun number) = 1.17
500 yards = .5
.5 x 1.17 = .585 mils (.6 mils rounded)
EX. 2
10 mph @ 700 yards
10 / 6 = 1.67
.7 x 1.67 = 1.169 mils (1.2 mils rounded)
Let's look at two more examples going through the complete formula using both options of when to apply the cosine.
600 yards with a 15 mph wind from 2 o'clock
The cosine for a 2 o'clock wind or 60º is .86 (86%)
15 mph wind / 6 (gun number) = 2.5
.6 x 2.5 = 1.5 mils
1.5 is the full-value wind hold. Now, I'll apply the cosine
1.5 mils x .86 = 1.29 mils
Multiply the windspeed by the cosine
15 x .86 = 12.9 mph
12.9 / 6 = 2.15
.6 x 2.15 = 1.29
Applying The Cosine To a Full Value Hold
800 yards with 5 mph wind @ 1:30
Determine the wind speed
5 MPH
Divide the wind speed by the Q.W. number
5 / 6 = .83
Multiply the Q.W. dope by the quotient
800 yards = .8 Mils x .83 = .664 Mil
Determine the wind angle from the DOF cosine
1:30 cosine .70
Multiply the product by wind angle cosine
.664 Mil x .70 = .464 Mil wind hold
Applying The Cosine To The Wind speed
Determine the wind speed
5 MPH
Determine the wind angle from the DOF cosine
1:30 cosine .70
Multiply the wind speed by the cosine
5 mph x .70 = 3.5 mph
Divide the adjusted wind speed by the Q.W. number
3.5 / 6 = .583
Multiply the dope value by the product.
500 yards = .8 Mils x 583 = .464 Mil
It gets easier
One of the best things about using the quick wind method is the hundreds of yards that match your gun number is a sweet spot. This is also true for half and double the value. Let me explain.
My rifle is a 6 mph gun.
The wind speed is my hold at 600 yards with a full-value wind.
At 600 yards, a 6 mph wind gives me a .6 mil hold.
A 12 mph wind gives me a 1.2 mil hold.
The only thing needed is if you don't have a 3 or 9 o'clock(full value), you only have to multiply by the cosine.
Your wind hold is half the value at 300 yards (300 is half of 600).
A 6 mph wind gives me a .3 mil hold
A 12 mph wind gives me a .6 mild hold.
I put every detail I could think of into how I explained the formula. It sounds complex, but it's easy once you get the steps down and understand the formula. I can do most of the math in my head. I'm not so good with division, so I'll usually use multiplication and do the problem backward.
Increasing accuracy and distance
One thing with this formula is it does have a limit. But I can increase its maximum effect range by bracketing it with two numbers. On two of my guns, my .308 (5 mph) and 6.5 Creedmoor (6 mph), my wind number is not exactly 5 or 6 mph. When I don't round the numbers to get my gun number, my .308 is a 4.5 mph gun, and my 6.5 is a 6.5 mph gun.
So, what I do is, from 100 to 500 yards; I use 5 on my .308 and 7 on my 6.5. For everything after 500 yards, I'll use 4 (.308) and 6 (6.5 C). Bracketing this way allows me to increase the formula's accuracy and maximum effective range. Give it a try once you get more comfortable using the quick wind formula.
The Overwatch
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