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A Fata Morgana () is an unusual and complex form of superior mirage that is seen in a narrow band right above the horizon. It is the Italian name for the Arthurian sorceress Morgan le Fay, from a belief that these mirages, often seen in the Strait of Messina, were fairy castles in the air or false land created by her witchcraft to lure sailors to their deaths. Although the term Fata Morgana is sometimes applied to other, more common kinds of mirages, the true Fata Morgana is not the same as an ordinary superior mirage, nor is it the same as an inferior mirage. Fata Morgana mirages significantly distort the object or objects on which they are based, often such that the object is completely unrecognizable. A Fata Morgana can be seen on land or at sea, in polar regions or in deserts. This kind of mirage can involve almost any kind of distant object, including boats, islands and the coastline. A Fata Morgana is often rapidly changing. The mirage comprises several inverted (upside down) and erect (right side up) images that are stacked on top of one another. Fata Morgana mirages also show alternating compressed and stretched zones.〔(An Introduction to Mirages ) by Andy Young〕 This optical phenomenon occurs because rays of light are bent when they pass through air layers of different temperatures in a steep thermal inversion where an atmospheric duct has formed.〔 (A thermal inversion is an atmospheric condition where warmer air exists in a well-defined layer above a layer of significantly cooler air. This temperature inversion is the opposite of what is normally the case; air is usually warmer close to the surface, and cooler higher up.) In calm weather, a layer of significantly warmer air can rest over colder dense air, forming an atmospheric duct which acts like a refracting lens, producing a series of both inverted and erect images. A Fata Morgana requires a duct to be present; thermal inversion alone is not enough to produce this kind of mirage. While a thermal inversion often takes place without there being an atmospheric duct, an atmospheric duct cannot exist without there first being a thermal inversion. ==Observing a Fata Morgana== A Fata Morgana is most commonly seen in polar regions, especially over large sheets of ice which have a uniform low temperature. It can, however, be observed in almost any area. In polar regions the Fata Morgana phenomenon is observed on relatively cold days. However, in deserts, over oceans, and over lakes, a Fata Morgana can be observed on hot days. To generate the Fata Morgana phenomenon, the thermal inversion has to be strong enough that the curvature of the light rays within the inversion layer is stronger than the curvature of the Earth.〔 Under these conditions, the rays bend and create arcs. An observer needs to be within or below an atmospheric duct in order to be able to see a Fata Morgana.〔(Atmospheric Optics Glossary ) by Andy Young〕 A Fata Morgana can be observed from any altitude within the Earth's atmosphere, from sea level up to mountaintops, and even including the view from airplanes. A Fata Morgana can be described as a very complex superior mirage with more than three distorted erect and inverted images.〔 Because of the constantly changing conditions of the atmosphere, a Fata Morgana can change in various ways within just a few seconds of time, including changing to become a straightforward superior mirage. The sequential image here shows sixteen photographic frames of a mirage of the Farallon Islands as seen from San Francisco; the images were all taken on the same day. In the first fourteen frames, elements of the Fata Morgana mirage display alternations of compressed and stretched zones.〔 The last two frames were photographed few hours later, around sunset time. At that point in time, the air was cooler while the ocean was probably a little bit warmer, which caused the thermal inversion to be not as extreme as it was few hours before. A mirage was still present at that point, but it was not as complex as few hours before sunset: the mirage was no longer a Fata Morgana, but instead had become a simple superior mirage. Fata Morgana mirages are visible to the naked eye, but in order to be able to see the detail within them, it is best to view them through binoculars, a telescope, or as is the case in the images here, through a telephoto lens. Gabriel Gruber (1740–1805) and Tobias Gruber (1744–1806), who observed Fata Morgana above Lake Cerknica, were the first to study it in a laboratory setting. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fata Morgana (mirage)」の詳細全文を読む スポンサード リンク
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