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halo, in meteorology
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.
REFERENCEMinnaert, M. Svet i tsvet v prirode. Moscow, 1958. (Translated from English.)
(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
What does it mean when you dream about a halo?
A dream of oneself with a halo may signify that perfection is a goal for the dreamer. Alternatively, it may represent an exaggerated “holier than thou” attitude.
ii. A high-altitude, low-opening (HALO) paradrop system. The paratroopers jump from aircraft at very high altitudes and open their parachutes at very low heights. This technique ensures rapid infiltration and tactical surprise. The latter is achieved as the paratroopers can jump from the aircraft, which is flying some distance away from the drop zone.
iii. The reflection of cockpit instruments seen in a canopy at night.
iv. The colored ring seen on clouds in the direction away from the sun (i.e., with the aircraft's shadow at the center). Also called a pilot's halo.
v. A bright ring around the spot produced by a beam of electrons striking the fluorescent coating in a cathode-ray tube.