Reading and Correcting for Wind at Long Range

[Bart B.]

Using tree/plant leaves' wiggling to estimate cross winds at target range was mentioned in another thread. It is a waste of time. Cross winds have their greatest effect on bullet drift nearer to the rifle. For example,



The wind above the line of sight's faster than in the line of sight. Here's some examples:



Best way to "read the wind" is with a scope focused about 2/3rds the way to the target, then watch the heat waves wrinkling across the field of view a ways above the target. That's where the bullet spends most of its time. The best learning process is to coach a really good marksman in making sight changes to correct for wind. That way, you'll see the effect of the corrections given the shooter. Many great wind coaches were not so good marksmen, but they sure read those heat waves wrinkling across the field of view good enough to keep the shooter's bullets all in the 20" ten-ring on the NRA 1000 yard target.

 

[taliv]

the key phrase there being "at the target". for much of the day and all of the night, there won't be much mirage to read. of course, my experience shooting high power is that they schedule matches during the middle of the day and conveniently place straight edges on the number boards which make it easy to read as long as the sun is out.

but practical shooting is often quite different. i shoot mostly in the hills and often from near the top of one ridge to near the top of another with a valley between. so max ord for me may be 100-400' above the ground ,even though i'm shooting a much flatter caliber.

just wanted to clarify that while mid range mirage is usually best, sometimes 2nd or 3rd best is all you get. and sometimes 'best' lies to you. yesterday i shot for a couple hours over relatively flat ground and everyone present agreed the mirage was going one way, but the bullets were going the opposite. as they say in kingsville, "believe the bullet"

 

[Walkalong]

90% of Benchrest was reading the wind and mirage. You would see anywhere from three to six flags between the shooter and his target when shooting 100 and 200 yards. Which yardages they were put out at varied a bit by shooter preference and the range its self as well. High Power and other long distance shooters don't have that luxury. You could also put a piece of paper with a pattern of thick straight lines next to your target to help read mirage if you wanted to, or you could just rely on seeing the heat waves in the scope.

Wind is more honest than mirage, or just easier to read perhaps. You know for sure where you are seeing the wind, where it's harder to tell how far out the mirage your seeing is.

Wind moves the bullet, while mirage displaces the targets image in the scope. The target is still in the same physical place, but the scope tells you it is somewhere different, up, down, over.....and the trick is telling which direction, and worse, how much the mirage is moving the image of the target.

 

[Bart B.]

If you've ever fixed a 20X scope on a distant target then watched the target image shift relative to the reticle and wind direction changing all day long, you'll see it's insignificant. The varying winds cause much more bullet drift than mirage (heat waves) displace target image from its actual location.

A wiggling mirage that doesn't have any direction indicates still air. If there's any direction of the heat waves, that's the direction the air is moving. Bullets drift in that direction.

The few seconds of arc errors mirage displace distant images 1/20th MOA is critical for surveyors. It's in the noise level for rifle shooing.

 

[243winxb]

Mirage and barrel heat

Great info guys. For me, to much mirage with my Leupold 36x. I hate dancing targets. Heat coming off the barrel is another problem, but easley fixed. http://bulletin.accurateshooter.com/2007/10/mirage-shields-for-varminters-as-well-as-benchrest-shooters/ I am a wantabe short range Benchrest shooter, so i can wait for a cloudy day, with no wind.

 

[Walkalong]

You will never win a Benchrest match if you ignore the mirage. It is not insignificant. Sometimes it is so little as not to matter, while at other times it will blow your groups out of the water if ignored.

 

[Bart B.]

The mirage that makes the fuzzy target image oval shaped and shimmering on the wind (heat wave) axis is what's not important. Its shape stays near the same size regardless of crosswind speed. If that shimmering target image bothers you, just hold where the target center is at the upwind side of that fuzzy image. That's where it physically is because the heat waves refract light waves from it in the wind's direction. If the crosswind's needs a 5 MOA correction on the sight, that fuzzy target image isn't 5 MOA wide.

I tried mirage bands on a couple of rifles. They helped me none whatsoever in a no-wind condition. Never had elevation shot stringing without one over the barrel.

 

[USSR]

Given a choice between reading wind flags and mirage, I will take wind flags every time. Oh, and while the 2 scopes available to me in 1,000 yard competition had top powers of 20X and 24X, I would always dial down to 16-18X to dial out some of the mirage.

Don

 

[Walkalong]

The mirage that makes the fuzzy target image oval shaped and shimmering on the wind (heat wave) axis is what's not important

Agreed, but that's not what I am talking about. I've seen mirage so bad it moves the targets image at 200 yards an inch or more. When you need to be shooting groups well under .5 that is huge. When the mirage is "running", one better take notice.

 

[Bart B.]

Walkalong, I think we're talking about the same thing. Just using different terms.

I've seen over 1 MOA of horizontal bullseye wiggling in real hot weather during very light winds. That's when the air temperature variables cause the most refracting of light waves as they're barely moving across the line of sight.

In high winds during hot temperatures, there's not near as much bullseye wiggling sideways. The air temperature's more stable in the line of sight in such conditions.

 

[Walkalong]

I've seen over 1 MOA of horizontal bullseye wiggling in real hot weather during very light winds. That's when the air temperature variables cause the most refracting of light waves as they're barely moving across the line of sight.

"Running" Yes, when the wind and mirage don't match up, be very attentive.

In high winds during hot temperatures, there's not near as much bullseye wiggling sideways. The air temperature's more stable in the line of sight in such conditions.

"Shimmering" Pretty stable for shooting.

And I'll add that if the mirage is "boiling" up, regardless of wind, you better adjust for it.