Have a Merry and Prismatic Christmas!

A partial solar halo made of "diamond dust" brackets the sun on Christmas Eve morning from Duluth, Minn. A faint sun pillar (vertical glow above the tree) is also visible and forms when light reflects off the bottoms of ice crystals floating parallel to the ground. Bob King

Last night's winter storm and plummeting temperatures left us with piles of snow and dazzling sunshine this morning. As I heaved shovelfuls of the stuff to clear a path to the driveway, I caught sight of an arc of rainbow light between the trees. It turned out to be part of a rare diamond dust halo. Ordinarily, a halo forms when minute, hexagonal ice crystals in cirrostratus clouds refract sunlight or moonlight into a circle with a radius of 22°. 

But no such clouds streaked the sky this morning. Instead, nearby but invisible ice crystals suspended in the -8° F air took it upon themselves to do the refracting. The halo was partial — twin, arc-shaped stubs — with a hint of the complete semicircle wrapping around the sun, but the colors were bright and intense. Around 10 a.m. the sight quickly faded away as clouds thickened near the sun. Darn! Over so soon? Not quite.

Delicate cirrocumulus clouds (magnified here) diffract sunlight to create iridescent colors in the clouds on Thursday morning. Bob King

Blocking the sun with my fist I could see hints of vividly colorful iridescence in the gill-like patches of cirrocumulus. Once the translucent clouds fully shrouded the sun, multiple rings of iridescence, called a corona enveloped it like nested Christmas wreaths. What a sight! This meteorological corona is unrelated to the sun's atmosphere, which bears the same name.

Multi-ringed coronas often form when sunlight is diffracted or scattered by extremely tiny water droplets in high-altitude cirrocumulus clouds. As light waves squeeze between the droplets they overlap and interfere with each other much the same way two different sets of waves on a pond do. Where the crests of two waves combine, a bigger wave results. When a crest and trough meet, the wave is canceled. Together these cancellations and reinforcements create a series of colored fringes resembling a bullseye around the sun. 

A multi-ringed corona ornaments the sun during the early afternoon on Dec. 24 from Duluth, Minn. Bob King

Simple coronas a few degrees across form from droplets of varying sizes. Truly awesome examples like this morning's are made of extremely tiny droplets that have nearly the identical size. Large coronae can reach up to 15° in diameter and often change shape as the clouds move past and droplet size changes.

Nature gives gifts every day of the year. How appropriate that on the eve of Christmas it should tack wreaths of color on the blue dome above.

Merry Christmas and a wonderful holiday to you all. I wish you a 2021 filled with sky adventures plus the return of normal life and the social connections we miss so much.

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