Mirage is an optical phenomenon where light rays are bent to produce a displaced image of an object. The displacement can been seen because the light rays are actually being refracted creating a false image.
Cold Air is denser than warm air, so it has a greater refractive index. As light travel at a shallow angle along the boundary between air of different temperatures, the light rays bend towards the colder air.
Generally speaking you need approximately a 4 degree difference between the boundary layers. This can be the difference between the ground temperature and the air just above. This is also why the rays will appear to bend up. As the ground heats up, the light rays travel towards the cooler air. This is helpful to know, because it will give you something to focus on. As an example, rocks, the steel frame of targets are a great place to find mirage, especially if they have been out in the sun.
Light Rays that are bent up from the ground to the sky. Often represented by images of heat rising off the road or a desert. The ground has been heated by the sun and the waves travel up.
Is a form of Inferior Mirage, and the one we most deal with. Convection causes the temperature of the air to vary, and the variation between the hot air at the surface of the road and the denser cool air above it creates a gradient in the refractive index of the air. This produces a blurred shimmering effect, which affects the ability to resolve objects, the effect being increased when the image is magnified through a riflescope.
A superior mirage occurs when the air below the line of sight is colder than the air above it. This unusual arrangement is called a temperature inversion, since warm air above cold air is the opposite of the normal temperature gradient of the atmosphere.
A Fata Morgana, the name of which coming from the Italian translation of Morgan le Fay, the fairy shapeshifting half-sister of King Arthur, is a very complex superior mirage. It appears with alternations of compressed and stretched zones, erect images, and inverted images. A Fata Morgana is also a fast-changing mirage. This is the type of mirage you find in the winter and over snow and ice.
Reading the Mirage
Now we know the backstory on Mirage how do we determine it’s effects ?
Mirage tells us two things, wind speed and wind direction. To what degree depends on the conditions.
What we want to look for is the frequency of the waves, the distance between the waves and the direction of the flow. How much the mirage lays over will also give us an indictor of wind speed.
One of the more difficult mirages to shoot in, is the boil. Where we have very, very light winds and heavier displacement of the target image. This is often found with people shooting suppressors as the mirage is directly in front of the scope. It's the number one reason we use a mirage cover for suppressors, we need to break up that effect in front of the scope. Benchrest and F Class shooters will also use a Mirage Band to break up the effect from the barrel itself. This is necessary because they use very high magnification scopes, so they are increasing the effects tremendously. At 55x Competition Scope is going to see all the effects.
When Mirage Helps Us
Mirage is best used in winds under 8 MPH. In light winds, the mirage is a much better indicator of wind speed and direction. Why, because it is effected by the wind before objects around the target like trees and leaves. So in very light wind conditions it is best to focus on the mirage.
Once the wind starts to go over 8 - 10MPH it simply lays over and is really only there to give you wind direction and not so much wind speed. It’s nearly impossible to determine the difference between a 12MPH Mirage and 15 Mirage. They both look the same and just lay over racing. Heavy wind can actually help move the heavy mirage out of the way. As noted a boiling, heavy mirage is the most difficult to shoot through so wind can be our friend.
Where this can help the shooter is with a head wind or tail wind. In these types of wind it is often never coming straight at your or behind you. It’s usually moving and changing direction from a slight left favor or right. It’s never in a straight line. Here the mirage is your best indicator for that constant change in direction.
How do we break through to the actual image and ignore the false image ?
When we see the false image in mirage it is usually dancing between the waves. It appears random and constantly moving as you see it match the frequency. As the image is carried up in the direction of movement, it’s never seen in the same place twice. So look down, where the waves originate and the True Target image will always return to it’s actual location. It’s starting point is fixed.
The false image is carried up, the most displacement is found in a heavy, boiling mirage.
What to do about Mirage
When Reading Mirage we want to focus on the target image and dial back about 1/4” or so. this will give us the mirage in front of the target.
When shooting through mirage the best remedy is to lower the power on the scope. Remember the more magnification you use, the more you enhance the effects. So dial back.
Calibrate what you see in terms of the mirage direction and frequency to you kestrel. You need a base line, so observe the mirage and how it appears. Then match that view to your weather meter to give it value. In the past people broke the wind down to bands, 2-4MPH, 5-7MPH, 8-10MPH, 10-12MPH, and so on. Our goal is to break it down to 1 MPH, so we want to give that value.
When we give our observations an actual value we create a reference point we can use later. This is added to our personal database and becomes our experience. So when we see these conditions again, we know what to call it.
If you look at mirage as tool, and understand when to use this tool, you’ll no longer get intimidated by it. When you need it, use it, when you don’t back the scope off and ignore it.