Scope-Based Wind Speed Estimate

[6ShotsOr5?]

The attached spreadsheet tool will help you estimate the wind speed at some known distance downrange by using a range finder, your scope field of view data, and the time that it takes for something to pass across your reticle. In the case of something unexpected blowing across your field of view, time the event first, and worry about what the range to the object was after that. Mr. obvious alert — this won’t work if you move the rifle while timing the object. Also note that the object doesn’t have to pass perpendicular to the viewing direction, but if you use the average range while the object passes, you will be calculating the wind speed perpendicular to the viewing direction, and that’s all we care about most of the time anyway.

There is also an option to estimate the wind at your target range based on dispersion of a dust cloud that is generated at impact. In that case, for a strong wind the dust may be blown mainly to one side or it may spread out to both sides before being blown clear. The tool gives you a low speed estimate based on the dust being spread out in both directions and also a high speed estimate for the case with the dust cloud spreading in one direction only. The real answer is probably somewhere between.

My apologies to anyone that may have published something similar. I didn't use your work, but the idea is so simple that I would not be surprised if it has been done. At least the price is good. I hope that some of you will check my math and if there’s an error, let’s fix it. Good shooting.

 

[6ShotsOr5?]

This is a reply to questions on the sister thread on the ELR forum, http://forum.accurateshooter.com/threads/scope-based-wind-speed-estimate.3939897/.

Conaso, Thank you for the feedback and questions. The value for Field of View (FoV) is entered in any consistent units. The numeric value doesn't change whether you are entering the data in "yds / 1000 yds" or "m / 1000 m".

If the unit of measure (UoM) value is set to "m", the distance to the ranged object is entered in m, and the wind speed is output in km/hr. If UoM is set to "yds", then the range is entered in yds and the wind speed answer is in miles/hr.

The numeric value for FoV changes when you adjust the magnification ("X"). In that case, you need to enter the FoV at your current magnification setting. Manufacturers often specify FoV at the lowest and highest power levels. If you are somewhere between those and you know the exact magnification, you can interpolate, but knowing the exact magnification might be tough so it will probably work better at max or min power setting. Note the minimum and maximum magnification should be checked in your owner's manual. Sometimes these values are rounded off in the scope name. For example, I have a scope that is called a 2-7x28, but after checking the owner's manual I learned that the real range is 2.4 - 6.7X.

I added a comment in the spreadsheet to explain the FoV input units, so I replaced the original file in the first post. I'm also attaching it again to this reply. The cells requiring input are in white background. The non-input cells are protected. If you use the tab key you will move directly from one input cell to the next. The protection is only to avoid accidental changes to the formulas. There is no password.

I'm also attaching a version using Apple's "Numbers" format. This version is formatted a little narrower to work well with your iPhone. I had to compress it into a zip file so that it would allow me to upload the file.

Because of the questions that were asked, I'm attaching some examples also. Examples 1 and 2 show that when the time for something passing the reticle is the same at different ranges, the wind speed is not the same. Example 3 shows an example using meters and includes the high and low wind speed estimates from raising a dust cloud at the point of impact.

Objects being blown by the wind may be moving quite a bit slower than the wind speed. It all depends on how high the drag is on the moving object. A very aerodynamic shape could be moving much slower than the wind, but a shape with a lot of drag like a feather could be traveling close to the same speed as the wind. The same is true for an object being blown along on the ground -- the wind speed will be higher than the speed the object is moving.

As you can tell from the examples, a stopwatch would be helpful for closer ranges and higher wind speeds! Good luck.