The supralateral arc is the 46 deg. variant of the 22 deg. upper tangent arc. It is caused by refraction of light by pencil-shaped horizontally oriented ice crystals. Light enters one of the side faces of the pencil crystal, and exits via a vertical end face. The supralateral arc is easily misinterpreted as being part of the 46 deg. halo, as it follows about the same curvature. Any doubts can be ruled out when taking into account other halos present, for example, when only circular halos are present, this is an indication that only randomly oriented ice crystals are present, and the 46 deg. arc is actually a 46 deg halo.
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