An arc parallel to the horizon
and found at least 46° below the sun (or moon).
It is produced by the refraction
through the 90° prisms of ice crystals
, having entered through the vertical sides and passing out through the horizontal bases. Normally, the crystals are large, oriented, hexagonal plates. The circumhorizontal arc is the low sky counterpart of circumzenithal arc
. The circumhorizontal arc is low in the sky when the sun is high (above 58° elevation
); the circumzenithal arc is high in the sky when the sun is low (below 32° elevation). The most colorful circumhorizontal arcs occur when the refraction
is close to the minimum angle of deviation, and this corresponds to a solar elevation angle
of about 68°. In the midlatitudes, the sun only climbs to that elevation for a few hours mid day around the summer solstice
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