superior mirage

   Also known as arctic mirage. Both terms are indebted to the French verb se mirer,which means to reflect or to be reflected. The designation superior refers to the position of the phenomenon relative to the horizon or a distant object. The terms superior and arctic mirage areusedtodenotea * physical illusion which has a relative position above the horizon, or a perceived distant object. The resulting image is attributed to the presence of relatively hot air over a cold surface, a condition known as atmospheric temperature inversion. This temperature inversion may 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. Superior mirages may present in the form of erect or inverted images, or a mixture of the two, depending on the distance of the perceived object and the temperature gradients involved. Temperature gradients of a relatively high complexity may yield complex distortions. These may be vibrating, vertically extended (i.e. 'towering'), flattened (i.e. 'stooping'), etc. A special type of superior mirage, known as the hillingar effect, is characterized by a flat or slightly concave upwards appearance of the earth and horizon. It is attributed to mild, uniform, and widespread temperature gradients. The terms * fata morgana and * hafgerdingar are reserved for particularly complex superior mirages, in the former case consisting of spikes and shoots reminiscent of architectural structures or 'castles in the air' and in the latter case imitating a circular tidal wave of an impressive height. The vertical spires offata mor-ganas are attributed to discrete temperature inversions in the air. Two additional types of superior mirage are the Novaya Zemlya, which consists of an impressively distorted, multiple-imaged Sun seen above the horizon after sunset, and the fata bromosa or fairy fog, which consists of a horizontal white band that appears over a water or snow mass in arctic regions. The term superior mirage is used in opposition to the terms * inferior mirage, * double mirage, and * lateral mirage.

   References

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