No, 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, a former NASA space scientist at the Ames Research Center in Mountain View, who is now director of the Sky Power Institute in Santa Cruz, which promotes solar power and other alternative fuels.
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 up, said Jordan, who viewed the image shot by Saffas. In the far north, zenithal arcs are more common than rainbows, but here in the Bay Area's more moderate climate they are rare, even in winter.
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.
"The arcs are like a Cheshire cat grin," Jordan said, "and they vanish just the same way."
But the one Saffas saw Jan. 13 lasted at least an hour, he said -- plenty of time for him grab his Nikon D70 camera and shoot away.
"We were having a huge family party, and everyone had stepped out on our deck," Saffas said, "when I saw this strange arc right overhead in the sky. It was a beautiful day, the sun was sinking behind the two tall redwoods in our yard, and the colors in what I thought was a rainbow upside down were absolutely brilliant."
Alexandre Andronikos of Portland, who was at the party, vividly recalled "the fantastic sight."
"Oh, my God, look up at the sky!" she remembers calling to everyone. "I never saw anything like it in my life, and we all ran into the house to grab our cameras. Mine is a Sony digital, and I got some wonderful pictures, too."
Saffas, who is 85 and a retired bank loan officer, has always been devoted to his painting and sculpture. More than 50 years ago, he said, he was hired to paint murals during the renovation of the fabled Egyptian Theatre in Hollywood, now the home of the American Cinematheque. He still paints and sculpts, he said, but he's also an avid photographer.
Les Cowley, a retired physicist in England, maintains a Web site on atmospheric optics filled with incredible images and explanations of displays in the sky that most of us have never seen: supernumerary arcs, halos, sundogs, fogbows, glories, holy lights, green flashes and more. The Web site is www.atoptics.co.uk/
E-mail David Perlman at firstname.lastname@example.org.
This article appeared on page A - 1 of the San Francisco Chronicle