Halo (redirected from Halo (meteorology))

halo, in art

halo, in art: see nimbus.

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halo, in meteorology

halo, in meteorology, short-lived circles or arcs, and less commonly spikes and crosses, of colored or whitish light surrounding the moon or sun or in clouds as seen from above. A halo occurs when the light from the sun or the moon is refracted and reflected by ice crystals in the atmosphere, usually in a thin layer of high cirrostratus clouds. Under certain circumstances a second, or outer, halo appears, which is fainter than the inner one. At times white or colored luminous arcs are also seen lying somewhat parallel to the horizon and passing through the source of light, called mock suns, parhelia, or sun dogs for the sun, and paraselenae for the moon. A single mock sun, the anthelion, directly opposite the sun, may be added. In general a white halo results from the reflection of light by ice crystals, while one which appears as colored rings results from the refraction of light by ice crystals. Halos are more brilliant and complex near the poles than in other parts of the world. The theory attributing their formation to the presence of ice crystals was first suggested by the 17th cent. French philosopher Descartes. Similar to a halo and sometimes confused with it is the sun's corona. In X-ray electron diffraction, the term halos refers to the broad rings that appear on a photographic film as a result of the diffraction of a monoenergetic beam of X rays or electrons from a crystalline powder located at the center of the camera.

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halo

1. a disc or ring of light around the head of an angel, saint, etc., as in painting or sculpture

2. a circle of light around the sun or moon, caused by the refraction of light by particles of ice

3. Astronomy a spherical cloud of stars surrounding the Galaxy and other spiral galaxies

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halo [′hā·lō]

(astronomy)

A type of ray system in which many short, filamentary streaks form a complex network of bright matter surrounding the lunar crater. Also known as nimbus.

(electronics)

An undesirable bright or dark ring surronding an image on the fluorescent screen of a television cathode-ray tube; generally due to overloading or maladjustment of the camera tube.

(geology)

A ring or crescent surrounding an area of opposite sign; it is a diffusion of a high concentration of the sought mineral into surrounding ground or rock; it is encountered in mineral prospecting and in magnetic and geochemical surveys.

(meteorology)

Any one of a large class of atmospheric optical phenomena which appear as colored or whitish rings and arcs about the sun or moon when seen through an ice crystal cloud or in a sky filled with falling ice crystals.

(optics)

A ring around the photographic image of a bright source caused by light scattering in any one of a number of possible ways.

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Halo 

several optical phenomena in the atmosphere that are due to the refraction and reflection of light by ice crystals forming cirrus clouds and fogs.

There is a great variety of halo phenomena: they take the form of iridescent (for refraction) and white (for reflection) bands, spots, arcs, and circles in the firmament. The most common forms are iridescent circles around the disk of the sun or moon at an angular radius of either 22° or 46°; parhelia, or “mock suns,” which are bright iridescent spots to the right and left of the sun or moon at distances of 22°, rarely at 46°; a circumzenithal arc, which is a segment of an iridescent arc touching the upper point of a 46-degree circle that turns its convex side toward the sun; a parhelic circle, which is a white horizontal circle passing across the disk of the luminary; a column, which is part of a white vertical circle passing across the luminary’s disk; and a white cross formed by the combination of the parhelic circle with the column. Halos differ from coronas, which, though outwardly alike, have other origins related to diffraction.

In order for certain halos to occur, ice crystals, which have the shape of six-sided prisms, must be oriented the same way or predominantly the same way with respect to the vertical. The theory of halos has been developed in detail. Thus, the 22-degree parhelia occur because of the refraction of rays in the vertically oriented crystals when a ray passes through the faces forming angles of 60°; the 46-degree circle is created by refraction when the faces form angles of 90°; and the vertical and horizontal circles are obtained as a result of reflection from horizontal and vertical faces of the crystals.

REFERENCE

Minnaert, M. Svet i tsvet v prirode. Moscow, 1958. (Translated from English.)

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Halo 

(in optics), the light background around the image of a source of optical radiation that can be observed by the eye or recorded by a light detector. Halos are caused by light scattering at small angles in the medium through which the light is passing.

The size, color, and brightness of a halo depend on the dimensions and physical nature of the particles of the medium and on the optical thickness of the medium. Scattering at small angles, resulting in the formation of halos, is especially strong in media of small optical thickness with particles whose dimensions are greater than the wavelength λ of the radiation (the Mie effect). If the dimensions of the particles greatly exceed λ, the intensity of scattering is independent of λ. This explains, for example, the “white” color of the halo surrounding the solar disk (the combination of rays with different λ gives white light). Halos significantly affect the resolving power of photographic materials and luminescent screens and, consequently, the quality of the images produced by them. The character of a halo is taken into account in measuring the transparency of scattering media; in particular, the change in the brightness and spectral distribution of light in the solar halo is a criterion of atmospheric purity and transparency.

L. N. KAPORSKII