halo

   Also known as optical halo and corona. The term halo comes from the Greek noun halos,which means area. In meteorology and physics the terms halo, optical halo, and corona are used to denote a * physical illusion consisting of a luminous or coloured circle, arc, spot, pillar, or cross seen in cirrus clouds and ice fogs. The best known is the circular halo surrounding heavenly bodies such as the Sun or Moon when these are viewed through a mist or thin clouds, and sometimes surrounding other light sources such as street lights, especially in foggy weather. From the middle to the periphery, the colours of circular halos are white, blue, green, yellow, and red. When multiple halos are present these colour sequences may be repeated. The term halo is used for a vast number of atmospheric phenomena. The circular halos portrayed above make up no more than a fraction of these. The French astronomer and author Nicolas Camille Flammarion (1842-1925) gives the following examples in his work The Atmosphere of 1872. "Under the name of halo... is designated a brilliant circle which, under certain atmospheric conditions, surrounds the Sun at a distance of 22° or 46°, while, under the name ofparhelia, ormock suns... are designated luminous circular spaces, generally of a red, yellow, or greenish colour, which appear both to the right and to the left of the Sun, at the same distance (viz. about 22°), bearing a sort of rough resemblance to the Sun itself. The same appearances may be seen about the Moon; and it is, indeed, easier to observe them, as the diminished brilliancy of the Moon's light renders an examination of the area around it less difficult. These luminous spaces are called paraselenes.. .,or mock moons" The mediation of halos around heavenly bodies is associated primarily with the refraction and reflection of light by ice crystals present in cold cirrus clouds located in the upper troposphere. The mediation of halos seen around street lights and other mundane light sources is attributed to a similar mechanism in ice fogs. The variation in the phe-nomenological appearance of halos is attributed to the particular shape and orientation of the crystals within ice clouds. The terms parhelion and sun dog are used interchangeably to denote an optical halo taking the shape of bright, multicoloured patches of light on both sides of the Sun. The name * Buddha's halo refers to a * physical illusion sometimes seen in mountainous regions. In biomedicine, the terms halo and * visual halo are used to denote a luminous or coloured circle mediated by the optical system itself. Etio-logically, this type of halo is associated primarily with ocular conditions such as a cataract and glaucoma. As a consequence, it tends to be classified as an *entoptic phenomenon. In the past the term halo has also been used to denote what is currently known as a * corona phenomenon, i.e. a *visual illusion consisting of an extra edge around objects, which has been described in the context of * migraine aurae.

   References

   Allen, R.J., Saleh, G.M., Litwin, A.S., Sciscio, A., Beckingsale, A.B., Fitzke, F.W. (2008). Glare and halo with refractive correction. Clinical & Experimental Optometry, 91, 156-160.

   Flammarion, C. (1873). The atmosphere.Trans-lated 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.

   Sussmann, R. (1997). Optical properties of contrail-induced cirrus: Discussion of unusual halo phenomena. Applied Optics, 36,4195-4201.