circumhorizontal arc


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circumhorizontal arc

[¦sər·kəm‚här·ə′zänt·əl ′ärk]
(optics)
A halo phenomenon consisting of a colored arc, red on its upper margin; it extends for about 90° parallel to the horizon and lies about 46° below the sun.
References in periodicals archive ?
Technically they are known as circumhorizontal arc, an ice halo formed by hexagonal, plate-shaped ice crystals in high level cirrus clouds.
Apart from circumhorizontal arcs, the right combination of the sun and the cirrus clouds can also create other kinds of optical illusions that resemble rainbows, such as cloud iridescence (a diffraction phenomenon caused by small ice crystals individually scattering sunlight), infralateral arcs (formed when sun light enters horizontally oriented, rod-shaped hexagonal ice crystals through a hexagonal base and exits through one of the prism sides), and circumzenithal arcs (located at a considerable distance above the sun, they usually form a quarter of a circle centered on the zenith).
An 'upside down rainbow' or circumhorizontal arc, is an optical phenomenon formed by the sun shining on and reflecting off ice crystals in high level cirrus clouds to form a halo.
Scientists say a circumhorizontal arc is an optical phenomenon - an ice-halo formed by plate-shaped ice crystals in high level cirrus clouds.
But the circumhorizontal arc can occur only when the Sun is at least 58 [degrees] high.