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Can snow form "snowbows" similar to rainbows?

William Seip, Baltimore
Rainbow-formation
Spherical raindrops are required to form rainbows. A snowflake has a more complex structure and thus cannot create a "snowbow." Astronomy: Roen Kelly, after Les Cowley
Snowflakes are beautiful, incredibly complex, six-sided branched crystals; each one is different. They cannot form a “snowbow” — a rainbow seen while snow is falling — because rainbows need spherical raindrops. Sunlight enters a drop, refraction changes the light’s direction, and it bounces off the sphere’s opposite side before leaving the drop. The entrance and exit refractions split the light into rainbow colors. We see a rainbow only when the raindrops are near perfectly spherical, and a snowflake with all its icy intricate surfaces and angles just cannot make one.

People occasionally mistake some ice halos for rainbows, though. Sunlight passing through tiny hexagonal ice crystals in the form of plates and long columns creates colorful halos, sundogs, circumzenithal arcs (sometimes called “upside-down rainbows”), 22-degree halos, and so on. But again, these halos need simple and optically perfect crystals. Snowflakes can sometimes produce a sun pillar, but no other halos.

Even so, we might see a rainbow or ice halo during a snowstorm. When temperatures are not too low, small raindrops occasionally accompany snow and could form a rainbow that shines through the snow. Also, under the right conditions, some tiny crystals perfect enough to form halos might mix in with the snow.
 
Snow can form a rainbow-colored halo on the ground rather than in the sky. Snow crystals thaw and recrystallize slightly during daily temperature changes and morph into larger, simple crystals that glint brightly in sunlight to make a ground halo. Look for one the next time you’re walking in sunlit snow. — Les Cowley, Atmospheric Optics (www.atoptics.co.uk), England
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