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December 11, 2012

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Rare upside-down 'rainbow' cheers city

AN upside-down "rainbow" was spotted over downtown Xujiahui around 8:20am yesterday.

The optical phenomenon turned out to be what is sometimes called a "smile in the sky," a circumzenithal arc or Bravais' arc, caused by refraction of sunlight through horizontally oriented ice crystals rather than raindrops.

Pictures of the multicolored smile were widely circulated on the Internet, cheering up local commuters on the chilly morning.

The arc is rarely observed in Shanghai because of its relatively warm climate, said Zhang Ruiyi, chief meteorologist of the Shanghai Meteorological Bureau.

The temperature at 6,000 to 8,000 meters, however, is lower than 30 degrees Celsius below zero, and the optical reflection occurs when high-level clouds composed of tiny ice crystals are struck by sunlight on the flat face of the ice crystals. That causes a splash of the colors of the spectrum, explained Zhang.

Bravais' arc is more commonly seen in polar regions, Zhang said.

The smiley "rainbow" is rarely noticed because it occurs so high in the sky and usually in frigid areas. But it was not the first time that it appeared in the city, Zhang said.


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