The circumzenith arc is caused by sunlight being refracted by ice-crystal plates floating horizontally in the atmosphere. The light enters the top face of the plate and exits a sideface. As the crystals have freedom of rotation along their vertical axis, an arc is produced with constant elevation, thus a part of a circle with the zenith at its centre. Only the quarter circle towards the sun is visible. In rare cases, the circle has a larger extend, and in extremely rare cases, it may be completed by an arc on its other side, being referred to as Kern's arc.
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