OzFclass

As a relative newcomer to f class I have found my ability to deal with very heavy mirage a bit lacking...
Things that I have been doing is dialling down magnification, not shooting on a 'boil', trying to eliminate parallax (not easy at long ranges in extreme mirage!) etc.

Any other ideas or strategies out there?

Longranger wrote:...Any other ideas or strategies out there?

You sound like your main strategy is to improve your sighting, and reducing magnification is what helps most people. I will wind down to 10X in heavy mirage, because even at low magnification, the limiting factor for accuracy will still not be the scope, it will be the sighting conditions.

There is however another aspect to mirage which is to use it to assist with wind-reading. Particularly in very light conditions, it can be a better indicator than flags. There are numerous strategies for this e.g. focussing short to see closer mirage, using a spotting scope alongside your F-Class rifle, plus different opinions on where to look for mirage. It can be very difficult to teach mirage reading because different eyes seem to see it differently. The best way to learn is (you guessed it) experience!

Alan

The most recent thing that I've learnt of mirage, which for some reason has eluded me for so long, is the bulk of the reason for not shooting in a boil.

Pockets of hot air rising can cause light vertical deflection of the bullet, but the biggie is that the mirage of a boil, refracts the light?? which moves the picture in the scope higher than the target actually is. So you centre the crosshair on the picture but its actually now a minutes or so high on the target.

If its a boil because you have a strong head or tail wind and the wind is ramping up and down the mounds between you and the target, that's something different again.

If that's not quite right, someone please correct me. But that's my current understanding.

In terms of an aid, I've found that an anti mirage shield over your barrel is a huge help, especially on hot days and days of multiple ranges. Heavy mirage caused by atmospheric can be unpleasant enough with out adding barrel heat mirage to the equation.
I notice that many F-classers don't use anti mirage shields on their barrels.

I've found anti mirage shields to be absolutely critical when load testing and shooting groups at the short range...... or the long.

One thing I haven't tried is a black foam objective reducer. Apparently it increases depth of field as well as producing a better sight picture. How that works eludes me.. I can understand the depth of field bit. Some swear by it though. I did once try a polarized filter on a scope but ruled it out as when rotated, moved the reticle in a circular pattern all over the target. Needless to say, the idea was discarded at that point lol.

At our range WTF moments aren't unusual when mirage is running in the opposite direction of the flags... Not only a trap for less experienced shooters but seasoned ones as well. In my tr days that was a phenomenon associated with the focal point of the spotting scope, but on a correctly parallax adjusted riflescope....?

DenisA wrote:The most recent thing that I've learnt of mirage, which for some reason has eluded me for so long, is the bulk of the reason for not shooting in a boil.

Pockets of hot air rising can cause light vertical deflection of the bullet, but the biggie is that the mirage of a boil, refracts the light?? which moves the picture in the scope higher than the target actually is. So you centre the crosshair on the picture but its actually now a minutes or so high on the target.

If its a boil because you have a strong head or tail wind and the wind is ramping up and down the mounds between you and the target, that's something different again.

If that's not quite right, someone please correct me. But that's my current understanding.
.

Just to confuse things a bit more have a read of this link. In particular read point "9" as it hints at vertical image shift due to the vertical effects of wind on refraction. http://portal.tugraz.at/portal/page/portal/Files/i2720/publications/papers/documents/2011_INGEO_Brunner_Kukuvec.pdf

This may explain why a vertical impact shift will occur when the sun goes behind a cloud and no obvious change in wind direction or strength is picked up. The wind is blowing the more than the bullet! Its actually moving the refraction in a vertical direction.
This can be in both directions depending on the what the heat change has been.
I discussed this last Saturday at the range after seeing the results on the target but it is hard to put into words.

Some more good stuff..........

See figure 1 in part 3.1 and also of interest is figure 2 in 3.2. It gives a hint at some of the errors involved depending on the observed distance.
So this mirage thingy is hard to pin down with 100% accuracy.

http://onlinelibrary.wiley.com/doi/10.1029/2010JD014067/full

Also look at the field results part 6 Results and Discussion. on the 30th August a significant change takes place and is obvious on the chart. On 9th Sept, the sun must have been going in and out of the clouds, the refraction is making obvious rapid changes. If you were shooting during this period you would be getting vertical changes that would match what the chart is doing.

Dennis , Just to confuse the issue more ! I've shot good scores on light winds on a slow boil ! Wondai is an OPM that comes to mind and Muckadilla is another one ! Also 900 yds at Belmont with light winds is another. But got hammered at 1000 yds doing the same with a bit more wind ! I think to wind shear of the 700yd monund . If shooting like at 800 yd on a boil I'll always power down to 12x and also hold done a little ! Haven't been in too good a form lately ! So I don't know if I'm one to give advise !

Regards Graham.

Norm wrote:Some more good stuff..........

See figure 1 in part 3.1 and also of interest is figure 2 in 3.2. It gives a hint at some of the errors involved depending on the observed distance.
So this mirage thingy is hard to pin down with 100% accuracy.

http://onlinelibrary.wiley.com/doi/10.1029/2010JD014067/full

Also look at the field results part 6 Results and Discussion. on the 30th August a significant change takes place and is obvious on the chart. On 9th Sept, the sun must have been going in and out of the clouds, the refraction is making obvious rapid changes. If you were shooting during this period you would be getting vertical changes that would match what the chart is doing.

Norm,

Looking at paragraph [58] in the Conclusions, they found a "range of k between -4 and +16 for sunny summer days" on grassland. I calculate that at 1000yds, this translates to a range of about 10 MOA vertical!!! Obviously it wouldn't change between those extremes suddenly, but looking at the graphs in Fig 5, there are sudden increases of up to 3 in the value of k. That represents 1.5 MOA vertical at 1000yds, which would put you on the upper edge of the 4 ring!

This information will become an important part of my repertoire of excuses .

Alan

True Alan,
Maybe?
Further investigation required.

Well now I know where my 4's come from!

Hi All,
What if we did away with crosshairs and used a laser to find the X ring which would then "move" with the different sighting conditions?
I read somewhere that lasers don't refract in water.... would they also remain true with variations in the atmosphere???
One for the rocket scientists

Norm, thanks for the links, unfortunately I'm not smart enough to understand the first paragraph of the first link. I need catchy little rhymes like light up sight up, light down sight down. When asked why, all I can say is because it rhymes.

I'll put a little more time into it this evening with strong coffee.

DenisA wrote:Norm, thanks for the links, unfortunately I'm not smart enough to understand the first paragraph of the first link. I need catchy little rhymes like light up sight up, light down sight down. When asked why, all I can say is because it rhymes.

I'll put a little more time into it this evening with strong coffee.

Hey I like that! Its as good as anything if it works.
The links and charts were just to show that field research into light refraction over distance, has given some interesting results. It also shows that the magnitude value of the vertical effect of mirage is very hard to accurately predict. However as your saying goes, the direction of vertical change can be predicted with changes in the light conditions. (i.e. it changes the heat gradient above the ground).

Anyone who says that they lost points high or low to mirage has a very good excuse I think. I'm trying to get a handle on mirage effects myself. Some areas of the country are subject to a lot of mirage and the shooters seem to get very good at working with it. In my area it is only a seasonal thing so we get less practice in dealing with it.

As others have said its best to shoot when the mirage is running with a constant flow and avoid a boil. Use it for wind strength when the flags are not telling the whole story when conditions are light.

If we keep this at the basic level there are a few points to make.

Mirage can loosely be equal to a direct crosswind on the Beaufort scale. Four mph or force 1 at say 3 o'clock as a trickle, as it runs, Force 2 or 8 mph. Fast mirage is equal to 12 mph at 3 o'clock. Reading mirage beyond 12 mph is hopeless, use the flags as the apparent aiming mark shifts have been blown away. If its a fishtail with drifting mirage then the lateral movement is equal to 2 mph or a 12.30 wind or quarter of the 3 o'clock allowance.

But by definition the word mirage is a deception and we can be sucked into thinking it is faster than it actually is when it is thicker. A quick glance at the flags will indicate its real speed. On the hot days it looks like the mirage can really change rapidly yet the flags don't show it. What is happening is there are mini climates or wind zones being created by cold air being sucked in from the cooler sides of the range and then rising in convection currents. Depending on what side of the centre of a convection current you are on your wind can be left or right and the guy or gal squadded beside you can be using something different. So at times you are looking at multiple prisms all screwing the apparent aiming mark.

As mirage gets thicker there are more impacts in elevation, so the worst tactic is to shoot on a boil. This is especially so in slow or drifting conditions when it boils because there is upwind with the convection current as well as an apparent shift up of the aiming mark.

The elevation effects are worst with slow mirage and the faster it get the lateral effects are only considered.

The mirage density can change on the day. I tend to go up in 1/2 minute blocks for density and yes Alan's calculations are correct, as the from no mirage condition to heavy mirage, the aggregate changes in elevation can be 1 1/2 minutes or more on some days. This is also linked to the distance you are shooting. The further the distance the greater the light bending.

With any mirage it seems to be stronger from the left than the right despite the picture being identical with say left or right switching mirage. A rule of thumb is the extra 1/2 minute added to the left mirage and taken from the right mirage. This is only a guideline.
This occurs because the mirage shift interacts with the bullet drift.

The elevation will be also effected and counters mirage elevation at times when left wind lowers. Of course a right wind raises and again in slow drifts it tends to push the shot higher along with the upward movement of the apparent aiming mark. There is a lot to take in so just working on the wind changes is a good start.

Reading mirage for the short to mid range is very accurate but it is not so accurate at the long range because the mirage is telling you what is happening at the target. You need to correlate mirage with the flags and wait until they are all singing the same song as changes move down the range. To counter this a bit we focus a spotting scope up range a bit.

Most of the moisture is at the base of the target, so aiming higher in dense mirage with the scope wound back is a good tactic. If your score is lowered so is everyone elses. David.

Thanks for the links Norm. We were looking for papers like that a few years ago - mainly in surveying literature but these are better examples. Reading through Williadas comments I think he has seen some similar effects to the following.

Just a little basics first - I like to think of "mirage" as having two basic elements. "Refraction" - the larger scale bending of light over distance which is not always easily discernible with the eye. and "scintillation" or the shimmering effect I like to call "shooters mirage" - the small scale shimmering and effect we see through the scopes. We use these small scale shimmerings and "waves" to read the wind. In reality both are refractive events but it helps to think of these as having slightly different causes and effects.
Refraction (how far the light is bent) is proportional to the thermal gradient (rate of change in temperature you are looking through and hence angle of change) and distance. It can also be affected by looking at angles through the "thermal blanket" or air density changes (ie into a rise, over the top of a mound etc etc.).
Scintillation can be severely affected by humidity but is also affected by the thermal gradient and results in smaller scale "wanderings" in the target image.

Another little story - a few years ago we set up a concreted pole at one end of a 1000 yard range with a rifle scope attached and time shift video to monitor (careful to avoid differential heating of the pole by sun on one side by shading) We also confirmed the results by using a 32x dumpy level in parallel and from other directions. I had plenty of crazy mirage theories including wind angle effect causing differential refractive conditions on the mirage movement from different directions (angle tail angle head), over mounds (causing waves in the "thermal blanket" if you wish)- etc etc. All this recorded imagery Is on one of the hard drives packed up in my moving gear here somewhere. The files are rather large but I will ask a few people if they still have the smaller email versions (Rod/Linda?, Alan, Peter) of the condensed files I once sent around.

At the time we were particularly concerned with Raton and rumours of severe shift in elevation in the order of 1.5 minutes + with "wafting" winds. Elevation shifts that cannot be explained by aerodynamics/balistics alone. To cut a long story short - over the cane fields here we have measured vertical shifts of 1.5 minutes + (but common in the order of 0.5 minutes) and have a video of severe and fast movement during a cloudy/sunny session where during a single 15 minute shoot you could have had 3 significant shifts. (Denis - just a point to correct - whilst mirage can be inferior OR superior - most of what we see is the mirage driving the image DOWN (not up) and hence the old TR saying (although for a different reason) holds true- lights down sights down, or conversely light up (mirage up) sights up.

BUT - before you think EUREKA!!! -Just be very aware that not all our observed image shift resulted in a similar POI change on the target. We think air density changes due to shifts in the thermal blanket cause significant changes in air density that can cause a similar scale shift in POI to the mirage shift. Thankfully most of this is in the opposite direction to the mirage and thus counters it significantly if not over corrects (check your elevation settings from morning to midday to get an idea).
We spent several months taping shoots through mirage shifts with specialised equipment - the results weren't always as expected!

Now - imagine a situation - increased refraction due to a large thermal gradient (cold air but hot ground such as seen in the high desert country). Is there a time that a cold waft of air comes through clearing the warm air from the thermal solar heating and at long range you are shooting up in this cool air - slowing the bullet significantly, but your sight image is through the lower ground air where the thermal gradient is the steepest. Hot sun but a cold waft of mountain air coming down off the peaks....?????

A lot of work went into this and the results are still as confusing today as ever....

We were watching closely for these phenomenon but luckily through our stay in Raton the air was mainly coming from the high plains to our south west and the resultant conditions were a fairly steady (relatively speaking) mirage and thermal gradient.