The 46° Halo and Its Arcs

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Science  09 Nov 1979:
Vol. 206, Issue 4419, pp. 643-649
DOI: 10.1126/science.206.4419.643


Ice crystals in the form of right hexagonal prisms have faces that form 90° prisms. Light rays were traced through these prism faces by computer calculation, and the light patterns that would be produced in the sky for a particular distribution of crystal orientations were simulated. Crystals with random orientations produce a 46° halo. Hexagonal plate crystals with nearly horizontal end faces produce circumzenithal and circumhorizontal arcs. Hexagonal column crystals with horizontal axes produce supralateral and infralateral arcs. Plate crystals spinning about a horizontal axis that is a face diagonal of the crystal produce a series of arcs touching the 46° halo. Each of these effects was simulated for several elevations of the sun.