Upper Tangent Arc & Apricot Tree at Zsámbék, Hungary imaged by Tam�s �brah�m on a cold and windy day in February.
©Tam�s �brah�m, shown with permission.
Column crystals created the tangent arc. As they drift slowly downwards relative to the surrounding cloud air, drag forces tend to align them so that their long axes are horizontal.
Ice refracts red light more strongly than blue. The inner edge of the red halo is therefore closer to the sun than the blue one. All but red is overlapped by other colours because the halo is a minimum deviation phenomenon like sundogs, the 22° halo and the rainbow.
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