Note:  Do not rely on this information. It is very old.

Mirage

Mirage, an optical illusion produced by refraction yvhen adjacent layers of air have very different densities, and are consequently very different refracting mediums. On flat deserts, when the air nearer the ground is rarer than the air above, such excessive refraction produces the appearance of objects reflected in water, the actual objects being often quite hidden below the horizon. The phenomenon occurs at sea as well as on dry, level plains. Objects may appear not only displaced, doubled, or simply inverted," but also distorted and turned over from a vertical to a horizontal position. Inferior mirage furnishes a visible object with a reflection as if it were in or near water.