A Fata Morgana is an optical phenomenon which results from a temperature inversion with warmer air above cooler air. The illusions that are created by Fata Morgana show alternations of compressed and stretched image zones.
Fata Morgana can be observed from any altitude within the Earth’s atmosphere including airplanes and mountain tops. For Fata Morgana to be observed, temperature inversions have to be strong enough that the light ray curvature within the inversion is stronger than the curve of the Earth. The rays will bend and create arcs.
In calm weather, the undisturbed interface between warm air over cold dense air near the surface of the ground may act as a refracting lens, producing inverted and erect images. Fata Morgana is the most common in polar regions, especially over large sheets of ice with a uniform low temperature. While in polar regions Fata Morgana is observed on cold days, in deserts and over the oceans and lakes Fata Morgana is observed on hot days.
Sixteen images of the mirage of Farallon Islands, which cannot be seen from the sea level at all under normal conditions because they are located below the horizon, were photographed one hour apart on the same day. The first fourteen frames have elements of Fata Morgana displaying the alternations of compressed and stretched zones. The last two frames were photographed before sunset. The air was cooler while the ocean was a little bit warmer, which made temperature inversion lower. The mirage was still present, but it was not as complex as few hours before sunset, and corresponded not to Fata Morgana, but rather to a different effect called superior mirage display.
Fata Morgana is still playing jokes with unexperienced observers. It is sometimes mistaken for a UFO and can display objects that are located below astronomical horizon as hovering in the sky. Fata Morgana might also magnify these objects and make them look completely unrecognizable.