Fata Morgana

Air density, and therefore its refractive properties, changes with temperature. When light travels through layers of air at different temperatures, its path curves. A superior mirage occurs in an inversion in which colder air lies beneath warmer air; light is bent downward and an observer may see an image apparently above the object's normal position. A Fata Morgana is a particularly complex form of superior mirage, associated with a strong inversion and an atmospheric duct that guides light over long distances. An inversion is necessary for the duct, but not every inversion produces a Fata Morgana.

A Fata Morgana may combine upright and inverted images of a distant object, stacked above one another and interspersed with compressed or stretched regions. A ship, island or coastline can look elongated, elevated, broken into layers, or difficult to recognize. Because the atmospheric temperature structure changes, the image can alter quickly, sometimes shifting into a simpler superior mirage. The apparent forms are refracted images of real distant objects, not objects suspended in the sky.

The phenomenon is commonly associated with polar regions and may also occur over seas, lakes, deserts or land elsewhere. It is observed within the atmosphere at different elevations when the observer is positioned in or below a suitable duct and has a line of sight to the refracted image. Stories of apparent castles or distant shores are historically linked to the Strait of Messina, reflected in the Italian name Fata Morgana, associated with the legendary enchantress Morgan le Fay.

An ordinary inferior mirage, familiar as a shimmering apparent pool on a hot road, forms under a different vertical temperature arrangement and appears below the real object. A superior mirage places images above their actual position, and can make objects below the geometric horizon visible. Fata Morgana belongs to the superior-mirage family but is distinguished by its complex, layered distortion; the name is sometimes used loosely for other mirages, though that usage is broader than the strict definition.