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Cake day: July 9th, 2023

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  • The circumzenithal arc, also called the circumzenith arc (CZA), the upside-down rainbow, and the Bravais arc,[1] is an optical phenomenon similar in appearance to a rainbow, but belonging to the family of halosarising from refraction of sunlight through ice crystals, generally in cirrus or cirrostratusclouds, rather than from raindrops. The arc is located a considerable distance (approximately 46°) above the observed Sun and at most forms a quarter of a circle centered on the zenith. It has been called “a smile in the sky”, its first impression being that of an upside-down rainbow. The CZA is one of the brightest and most colorful members of the halo family. Its colors, ranging from violet on top to red at the bottom, are purer than those of a rainbow because there is much less overlap in their formation.