fata morgana

   Fata morgana is Italian for mirage. The eponym is derived from Morgan le Fay, the name of King Arthur's half-sister, a fairy and shape shifter who features in the Legend of the Grail.The term fata morgana is used to denote a complex type of what is commonly called a * superior mirage, i.e. a complex * physical illusion appearing above the horizon or above a distant object. Like other superior mirages, fata morganas are attributed to the refraction and reflection of light due to the presence of relatively hot air over a cold surface, a condition known as atmospheric temperature inversion. This temperature inversion may in turn be due to the radiative cooling of the earth during the night, or to the presence of warm air over a mass of cold water. The particularly complex mirage images that may occur, consisting of spikes and shoots reminiscent of architectural structures or 'castles in the air', are attributed to discrete temperature inversions in the air. Fata morganas are sometimes observed in the morning following a cold night. Especially in calm weather, the interface between warm and cold air near the surface of the ground or water may act as a refracting lens, producing an inverted image over which the distant direct image appears to hover. The turbulence of the air may promote the appearance of elongations and so-called 'dancing spikes', which may cause distant objects to appear elongated and elevated like fairy tale castles. As the British surgeon Walter Cooper Dendy (1794-1871) wrote in 1847, "During this splendid illusion, gigantic columns, and cloud-capped towers, and gorgeous palaces, and solemn temples are floating on the verge of the horizon, and sometimes beneath this picture of a city, on the very bosom of the water, a fainter spectrum may be seen, which is a reflected image of the other. These spectra are usually colourless, but if certain watery vapours are floating in the air, they are beautifully fringed with the three primitive colours of the prism." It has been suggested that the * hafgerdingar or 'sea hedges', described in a thirteenth-century Norse manuscript, were in fact a superior mirage or fata morgana. The notion of fata morgana should not be confused with the notion offata morgana of the visual sphere,which is used to denote a visual distortion called * visual allachaesthesia. Nor should it be confused with the * desert hallucination, which has a propensity to occur during the night.

   References

   Dendy, W.C. (1847). The philosophy of mystery. New York, NY: Harper & Brothers. Flammarion, C. (1873). The atmosphere.Translated by Pitman, C.B. Edited by Glaisher, J. London: Sampson Low, Marston, Low, & Searle.

   Lynch, D.K., Livingston, W. (1995). Color and light in nature. Cambridge: Cambridge University Press.