What is a white arc or circle in the sky attending frosty weather and formed by reflection of sunlight from ice crystals floating in the air; the parhelic circle whose center is at the zenith called?

frostbow — A white arc or circle in the sky attending frosty weather and formed by reflection of sunlight from ice crystals floating in the air; the parhelic circle whose center is at the zenith. (Webster's 1913.)

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