Return of the Wind Masters

[Alaskaman11]

Ok Masters of the wind Riddle me is, I know the formula to calculate wind speed to MOA

Range (Hundreds) devided by 100 velocity (MPH
_______________________________________________
Constant

100 to 500 C = 15
600 C = 14
700-800 C = 13
900 C = 12
1000 C = 11



IE the Target is 800 yards Wind is 15 mph est.

8x14
_______ = 8.6 MOA or 8 3/4 MOA
13

Now I understand that this method is for full value wind. How do you get it for 1/2 value of wind and how accurate is this method? I understand its limited to the estimation of the speed of the wind but over all, is this something that I should add to my data?

 

[codyb]

Re: Return of the Wind Masters

I just half my velocity.

 

[Triggerfifty]

Re: Return of the Wind Masters

That's if you believe that half wind value comes from 45 degrees, it doesn't however.

Trigger

 

[Triggerfifty]

Re: Return of the Wind Masters

That windage formula applies to the old M118 Special Ball and has some inaccuracies in it. There is another one that uses a factor of 14 (I think) for M80 BALL ammo. Works pretty well for .308 173 gr. stuff, not so good for anything else.

Here is a more scientific formula.

Vcw x (T - (X/MV))

Vcw = cross wind velocity in inches per second
T = Flight Time of the bullet
X = Range in feet
MV = Muzzle velocity

To get MOA, divide the result in the above formula by the value of inches per MOA at that range. 1.14" at 100 yards for example.

These two tables show the conversions for cross wind velocity. You can determine the direction your winds are coming from, then simply multiply the corrosponding correction factor against the wind speed you measure or guess at. Note that half wind values come from 30 degrees and 30 degrees relative. 45 degree winds are a value of .707 or 3/4 value.

A 10 mph wind from 30 degrees (say right to left) has a true effective value of 5 mph. A 10 mph wind from 45 degrees has a true effective value of 7 mph.

The last table is a raw detailed data correction table that I use to calculate precise windage corrections.

Enjoy the free stuff,
Trigger

 

[Lindy]

Re: Return of the Wind Masters

The crosswind component of a wind is found simply by multiplying the wind value by the cosine of the wind angle. The cosine of 90 degrees is 1, the cosine of 45 degrees is 0.7, and the cosine of 30 degrees is 0.5.

Personally, I have trouble distinguishing a 30 degree wind from a 45 degree wind, but use whatever you think is best.

Couple that with the fact that all one can reliably tell is the wind angle where one is, which may be very different downrange, as may be the magnitude of the wind, and one can see why shooting in the wind is more of an art than a science.

 

[Alaskaman11]

Re: Return of the Wind Masters

You guys brought up a good point, is the same go with differnt rounds, ie, I shoot a 338 Lapua 250 gr pill with a BC of 0.675. Why I'm asking, I have the Ipod app that does it for you, but it will brake or loose power right before I see a moose or a sheep and need to know how to do it for myself.

What do I do?

 

[The Mechanic]

Re: Return of the Wind Masters

Hey Trig,
where are you figuring in B.C. value for drift? Does this not have an effect on drift based on surface area and mass of projo? Maybe I am just brain farting.

 

[Lindy]

Re: Return of the Wind Masters

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">What do I do?</div></div>

Use the ballistic program to figure the value for a 1 mph full-value crosswind at different ranges. Write those down.

Then when in the field, multiply the appropriate value by the value of the estimated wind, and, if necessary, by the cosine of the wind angle to get the crosswind correction.

Example: your ballistic program says the correction for a 1 mph crosswind at 700 yards is 0.3 mils. You estimate the wind at 10 mph, at a 30 degree angle.

So, 0.3 * 10 = 3 mils.

3 * 0.5 = 1.5 mils. (0.5 is the cosine of 30 degrees.)

So, you use 1.5 mils for your correction.