A Mirage is an optical illusion that can be sometimes observed on hot days. When air near ground level is heated strongly by contact with the hot ground, it becomes less dense. This is because the air near the heated ground becomes considerably hotter than the air above, causing refraction of light rays from the sky, since the refractive index of air depends on its density and therefore on its temperature. As a result, the relationship between objects and the horizon becomes distorted. In fact often a patch of sky is mirrored in the hot air giving the shimmering appearance of a pool of water, where there is none.

There are two main formes of mirage, classed according to whether the image of a distant object appears lower or higher that would normally expected. An inferior mirage occurs when the ground surface is strongly heated and the air near the ground is much warmer than the air above. Everyone might know the this of mirage where pools of water appear to be lying on a hot road.

In a superior mirage, the opposite conditions occur: the air close to the ground surface is much colder than the air above, which is known as temperature inversion. Light is bent downwards from the object towards the viewer so that it appears to be elevated or floating in the air. Superior mirages are less frequent than inferior ones and a more common over larger water bodies, which are sometimes much colder than the air above it, e.g. in spring. Superior mirages are also frequent in high-latitude regions, such as Island and over glaciers.

A famous superior mirage is the Fata Morgana , most frequently seen in the Strait of Messina between Italy and Sicily. However, Fata Morgana's are also frequent in deserts, after night time radiation has cooled down the sand to temperatures lower than the air above. Distant objects appear extremely elongated, giving the impression of buildings and towns in the distance. This phenomenon is also known as 'castles in the air'.