How high off the ground to reduce mirage significantly?

B

Boyd Allen

Active member
I have wondered about this for a while. Mirage comes from "bubbles" of less dense air, that have been heated by the earth coming up through the denser, cooler air above. Differences in refractive indexes of the rising air, and that is is rising through cause target image distortion and displacement. Also, we know that when the wind speed rises to some point, that mirage disappears, as it doe when the ground and the air above are at are the same temperature. I am pretty sure that one increases the height of the target above the ground that mirage will decrease. My question for all of you is have you seen this, and if you have, how high would you think that a target would have to be to see a significant improvement in the effect of mirage? Where this is taking me might be a much taller target frame, that could be disassembled for transport and storage, and quickly assembled on site, and secured so that it did not move side to side or up and down.
 
Boyd Allen said:
I have wondered about this for a while. Mirage comes from "bubbles" of less dense air, that have been heated by the earth coming up through the denser, cooler air above. Differences in refractive indexes of the rising air, and that is is rising through cause target image distortion and displacement. Also, we know that when the wind speed rises to some point, that mirage disappears, as it doe when the ground and the air above are at are the same temperature. I am pretty sure that one increases the height of the target above the ground that mirage will decrease. My question for all of you is have you seen this, and if you have, how high would you think that a target would have to be to see a significant improvement in the effect of mirage? Where this is taking me might be a much taller target frame, that could be disassembled for transport and storage, and quickly assembled on site, and secured so that it did not move side to side or up and down.
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Not sure you can get high enough to ever completely eliminate it. What causes mirage is also what causes turbulence in airplane flight at 35,000 feet. Shooting long distance over 1000 foot deep canyons causes as much mirage as sandy, 100 yard rifle ranges do. I'm afraid as long as there is air and differences is temp, there will always be mirage (and wind!).
 
Perhaps I need some sheets of rigid foam to put on the ground centered on my line of sight...a different kind of mirage shield. It would be interesting to see if 2' wide by 8' long would make a difference. I am going to do some affordable experiments. The first will probably be a lash up that will allow looking at targets from 2' to 8' off of the ground. We can imagine or we can test. When the cost and trouble are not too much, I prefer to test.
 
Try cardboard or even a roll of butchers paper staked down with some wire bent over at the top. Just suggesting a cheaper alternative!!
 
Cardboard sounds interesting. If I look around, I can probably come up with some that encased refrigerators. (way) Back in the day, we used to scrounge that sort of thing for our range, for target frames.
 
Will they allow you to roll out cardboard in front your bench at a match? Just trying to figure out what you could gain from figuring out if up or down would make a difference. Just follow your wind flags with a roll of brown paper!
 
If you are a competitor, why would you want to reduce mirage? It is wind you can see in real time. Learn what it is telling you and embrace it. As Jack Neary said in his tuning seminar " .......people are freaking out about mirage....they're less people you have to worry about, they're going to be behind you."
Also a lower power scope does not reduce the affect of mirage, it's still there making the target appear where it ain't....you just can't observe it.
 
Shooting to practice is one thing. Shooting to test is another. Some of us cannot afford tunnels, and anything that makes testing harder or easier is important...besides, I like to experiment. Some do, some don't.
 
Since mirage is dependent on the temperature differential (delta T) how about monitoring the temperature at various distances above the ground to find your answer. I would think crushed stone would produce the worst mirage, next worst would be bare gravel and lush thick grass would produce the least.
 
I am going for the direct approach and look at targets with a scope of about 42X...as soon as things dry out and the wind lets up a bit. I will do the test on a warm day with little wind, so that the mirage will be maximized.
 
Boyd Allen said:
Shooting to practice is one thing. Shooting to test is another. Some of us cannot afford tunnels, and anything that makes testing harder or easier is important...besides, I like to experiment. Some do, some don't.
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I see what you are getting at now. I just set up a 600 yd. place to shoot on a hard clay road, once the sun comes up good the mirage is terrible no matter what. Gonna regroup and try on a grass road.
 
Boyd, Okie Shooters used to really be a good place to tune at 200 yards before they did all the dirt work trying to level the range to a certain extent. They shoot north and the bullet path was about 20 ' above the ground. At the time they started the matches at two and sometimes finished up under lights. The conditions would really get good there about the time the sun went down. Mirage was practically non existent. Not quite so good anymore since they did the dirt work. Now your bullet path is about 3 or 4 foot or so above ground from about 125 yards to 200 yards shooting basically over a shelf and you get more mirage. Don't need quite as tall flags now as you did previously. I had seen flags there with guy wires.
 
Mirage

Reading Mike's Post reminded me of When I was assigned TDY, to work in New York City. This was back in the Mid 70's. At that time,we had an Office located within walking distance of The Two World Trade Center Towers. I remember,several times, having lunch at the "Windows On The World" Restaurant located at the Top of North Tower,about 110 stories up. There was an Observatory,where you could walk outside on top the Building and look around New York City. The worst Mirage I have ever seen was looking through binoculars from that Tower. The mirage was so bad,you could see it without binoculars on a bright sunny day. Those Towers were over 1,000 feet tall. Based on that experience, I would guess that Mirage extends to the Galaxy and beyond.


Glenn
 
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Yes, I am aware of how mirage is used as an aid to reading conditions. But the reason that I am trying to modify it is that it creates problems with having a precise, shot to shot repeatable aiming point. Besides, there is this fellow named Boyer who has said that he does not use it, and all in all I would have to say that he has done pretty well in the sport.

On the Empire State Building, The exterior of the building was undoubtedly heated by the sun, as was the air in contact with it. It seems to me that this would create a rising envelope of less dense air that would pass by a viewer at the top of the building, and that lack of homogeneity of that less dense air would create considerable optical distortion, especially when viewing through a telescope, but of course, like so many things that we discuss, this is just conjecture.
 
Maybe we should change the name of this site to "Benchrest Conjecture". Seems appropriate quite often.
 
I'll tell you that sometimes mirage matters and sometimes it doesn't. If it matters, you better watch it like a hawk. A couple of sighters will tell the story so it's not that big of a deal. All I know is that if it matters, and you shoot when it's going the wrong way...you won't win.

That's not entirely on the subject but needs said. Certainly you'll listen to Tony instead of me but when you lose a BIG shot, don't say I didn't tell you.
 
Of course the EASY thing to do would test on cloudy days or couple hours before dark.
 
Unfortunately, where I live cloudy days are not all that common, at least of the kind that are useful for mirage reduction, and when we do get them, it is pretty common for the wind to be blowing at a rate that is above what I like to test in. As to testing near dark, the range that I shoot at is about 50 minutes away, and when I make the trip, I try to be there for several hours. This is not a huge issue for me, but I like trying to figure out how to make things work better.
 
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