An infralateral arc (or lower lateral tangent arc) is a rare halo, an optical phenomenon appearing similar to a rainbow under a white parhelic circle. Together with the supralateral arc they are always located outside the seldom observable 46° halo, but in contrast to supralateral arcs, infralateral arcs are always located below the parhelic circle.
The shape of an infralateral arc varies with the elevation of the sun. Between sunrise and before the sun reaches about 50° over the horizon, two infralateral arcs are located on either side (e.g. lateral) of the 46° halo, their convex apexes lying tangent to the 46° halo. As the sun reaches above 68° the two arcs unite to a single concave arc tangent to the 46° halo vertically under the sun.
Infralateral arcs form when sun light enters horizontally oriented, rod-shaped hexagonal ice crystals through a hexagonal base and exits through one of the prism sides. Infralateral arcs occur about once a year. They are often observed together with circumscribed halos and upper tangent arcs.
- Atmospheric Optics - Supralateral & infralateral arcs - including HaloSim computer simulations and crystal illustrations.
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