By Jess Thomson
Science Reporter
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A photographer has snapped a picture of a strange and rare aerial phenomenon in the foggy San Francisco skies.
Known as a fogbow, as well as a white rainbow, this ethereal apparition is a much less common sight than its rainy counterpart, the rainbow.
"[The picture] was taken at Hawk Hill in the Marin Headlands [peninsula]," photographer Stuart Berman, who lives in San Francisco's Russian Hill neighborhood, told Newsweek. "I saw it for about 15-20 minutes, but it was still there when I left"
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Like rainbows, fogbows are caused by a similar physical phenomenon, with light being refracted by fog droplets as opposed to rain droplets. Rainbows occur as light from the sun passes through raindrops in the air and reflects, refracts and disperses, forming a multicolored arc of light in the sky. For a rainbow to be seen, the observer must be positioned at a 42-degree angle to the light source.
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The same refraction occurs when a fogbow is formed, but it appears colorless because of the difference in the size of the fog water droplets. Those droplets are generally smaller than 0.05 millimeters (0.0020 inches), compared with rain droplets, which are between 10 and 1,000 times larger.
The small size of the droplets causes diffraction to be the dominant effect on the light, rather than refraction, according to the U.K.s Meteorological Office. This means that instead of the light being split into its rainbow spectrum, it's just scattered and smeared into a white haze.
In the comments to Berman's tweet sharing the picture, others shared images of fogbows they had captured.
"I caught one over Bodega Bay earlier this year! I'd never even heard of them, but I knew exactly what it was!" wrote one user.
"The photographer was at just the right vantage point where the sun was behind them," Brooke Bingaman, a meteorologist for the National Weather Service, told the SFGate news site. "If you look more toward the right side of the image, you can kind of see the reddish tint on the outside. It's got a little bit of the color, but the prisms aren't as effective as regular raindrops are."
Fogbows aren't actually that rare, but catching one on camera is, Bingaman said. "What's rare is that someone would be around to capture it."
The best time to spot a fogbow is during a thin fog when the sun is bright, according to EarthSky.com.
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