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An Upside Down Rainbow In The Sky

Circumzenithal arcNo, this isn't an upside-down rainbow, and the photographer hasn't faked the picture. It's an unusual phenomenon caused by sunlight shining through a thin, invisible screen of tiny ice crystals high in the sky and has nothing at all to do with the rain.

Andrew G. Saffas, a Concord artist and photographer, saw the colorful arc at 3:51 p.m. on a beautiful day recently when a slight rain had fallen in the morning. He thought it was a rainbow, created by raindrops refracting sunlight the way glass prisms refract any bright beam of light.

Instead, what Saffas saw was what scientists call a circumzenithal arc, according to physicist Joe Jordan. The flat, six-sided ice crystals that cause the arcs are no larger than salt grains and usually form in the cold haze of wispy cirrus clouds about 5 miles (8 km) up, said Jordan.

When the sun is low in the afternoon sky on a hazy day -- even though the sky appears bright blue -- sunlight can hit the flat face of the ice particles at a slant. Then the rays bend within each crystal and emerge with the colors appearing separated into all the rainbow colors of red, orange, yellow, green, blue, indigo and violet.

But, said Jordan, the colors in those arcs appear in the opposite position from the colors in rainbows: In zenithal arcs, as in Saffas' image, the red hues are on the bottom and the blue and violet are on the top. The arcs appear to terminate where the millions of ice crystals end, he said.

Link & Image: SFGate
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