Rare Upside-down Rainbow

Photograph by Andrew G. Saffas

This circumzenithal arc, photographed in Concord, looks like an upside-down rainbow but is formed differently. Photo by Andrew G. Saffas, special to the Chronicle

When sunlight hits a hexagonal ice crystal about 5 miles above the earth, each crystal bends the light and breaks it into all the colors of the rainbow.

Combined, the millions of crystals form what atmospheric scientists call a circumzenithal arc, but the band of colors in the arc is reversed from the way it appears in regular rainbows.

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