I am hesitant to add my two cents here simply because it might be that my experience, much of it based on shooting 45 years ago, might not be considered applicable today, but here it is anyway.
The difference between bright sun and a cloudy day is less a difference in the amount of light than it is a difference in the effect of more light and or heat. Tied into the effects of the light and heat is the accompanying movement of air.
Mirage is the most obvious effect seen on a sunny day and is what makes precise aiming difficult. On the plus side, it also lets you know what the air currents are doing. I have never had any desire to use more than a 36 power scope and have shot in many situations where I thought 24 was better. The reason I felt the 24 was better was because the greater depth of focus allowed me to see the mirage better, between me and the target. This was beneficial if I was to use the mirage to help me to read the windWith that out of the way, how would I aim? In the unlikely event that there was no wind at all, the mirage will cause the aiming point to bounce up and down, with the occasional hop to either side to keep things interesting. If group shooting, I always wanted my first shot to hit in a particular spot so I would hold where I thought would make that happen. In this case that would probably be a 6 oclock average. By 6 oclock average, I mean I would aim at the bottom of the mothball and would largely ignore the lateral hops the image made. It is my belief, in the case of a running or boiling mirage, the displacement of the target will amount to a maximum of 3/8 moa with 1/4 being more likely. If the mirage is boiuling straight up this is all I have to contend with.
If the mirage is running from left to right, I will hold at about 7:30 to allow for defraction plus whatever value I have decided to give the wind. If my rifle shoots well enough, I will, hopefully, learn more as the day progresses and be shooting better as time goes on. All too often, the reality is, I will learn nothing and appear more and more lost as the day goes on! This is how I deal with mirage (or not) for short range BR.
If shooting at long range, things are much the same except the effect of the wind is much greater. Displacement of the target by mirage is still about the same (1/4 to 3/8 moa) and is dealt with by holding off or by dialing in 1/4. In order to use the mirage as a wind indicator, you have to be able to see the mirage where you are trying to read the wind. since the trajectory of the bullet might be 6 to eight feet above the line of sight, at 1000 yds, you will want to try and read the mirage at this elevation at about 650 yards. You can do this with a spotting scope focused short or you can come close by focusing your riflescope a little short in order to see as much as you can without blurring the target too badly.
Today, it is common for competitors in short range BR to shoot a bunch of sighters in order to try and gauge conditions. I never do this and this may be why I am less successful than I was back in the day, or they might just be better than I am.
Maybe later, I'll talk about the effects of a non-running or non-boiling mirage or defraction. The displacement of the target can be much greater and it is much more difficult to read. You may see a beautifully clear image of the target with the only issue being that it is not there! WH
