circumhorizontal arc

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An occasionally spectacular halo phenomenon that consists of a brilliantly coloured arc lying parallel to the horizon and sometimes extending for as much as 120°. It arises from refraction through ice-crystal faces with a 90° included angle, when light enters through a vertical face and exits from a horizontal one (compare circumzenithal arc). It may be observed only when the solar elevation exceeds 58°, and is thus most readily visible from lower latitudes. If the 46° halo is present the arc normally lies slightly below its lowest point, becoming tangential at a solar elevation of 68°.

Subjects: Meteorology and Climatology.

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