Mirages are formed by several different processes of optical reflection. Most commonly, changes in temperature cause light rays to bend as they pass through the atmosphere since the `angle of refraction’ is different according to the density, and therefore the temperature, of the air. A similar effect is seen if a pencil is placed in a glass of water. The pencil `bends’ because light rays are refracted at different angles in air and in water.

One may also notice a mirage on a hot, sunny day when one sees what seems to be a pool of water on the road ahead. In fact this is refracted light from the sky caused by the air just above the road surface being much warmer than the surrounding air. It is possible for a meteorological effect known as Temperature Inversion to occur. This involves a similar process, but in the sky itself. What this can do is to refract a moving or stationary ground light into the air (e.g. car headlights) providing the illusion of a UAP streaking across the sky. It can also cause stars to move appreciably as they pass through an inversion layer.It is a good idea to use an astronomical program to check to determine whether any of the stars and/or planets cited earlier in this chapter were present near (or just below), the horizon – even in cases where such a cause seems unlikely. Refractive effects can only occur within fourteen degrees of the horizon, and full-scale mirages within one or two degrees above (and below) it. Flat landscape conditions are desirable, plus little atmospheric turbulence. An inversion layer can commence suddenly, and an image so caused can thus appear and disappear with great rapidity.

For example, movement of either the witness or the primary light source could lead to this. If both remain stationary then the inversion image might remain visible for protracted periods, if the conditions remain stable in the atmosphere. Your meteorological office will, if you check fast enough, be able to advise on possible inversion layers that might have existed. Of course it has to be said that such layers are more common during periods of warm, stable weather, and that they are a relatively rare source of UAP reports.

Telltale signs: Proximity of image to the horizon line, ability of image to appear and
disappear if witness alters his location, possible `scintillation’ effects (like stars wavering or changing colour on the horizon line). Weather reports mentioning the presence of inversions and/or a weather front moving over the locus around time of sighting may also be significant.

Resources:
An Introduction to Mirages:
http://mintaka.sdsu.edu/GF/mirages/mirintro.html