Mysterious “fogbow” weather phenomenon occurs over California

A photographer snapped a picture of a strange, rare aerial phenomenon in San Francisco’s foggy skies.

Known as both a mistbow and a white rainbow, this ethereal apparition is a much rarer sight than its rainy counterpart, the rainbow.

“[The picture] was recorded at Hawk Hill in the Marin Headlands [peninsula]said photographer Stuart Berman, who lives in San Francisco’s Russian Hill neighborhood news week. “I watched it for about 15-20 minutes but it was still there when I left”

Fogbow Rainbow over San Francisco
A fog arch in San Francisco photographed by Stu Berman. Fog bows are caused by the same phenomenon as rainbows, except fog water droplets are smaller.
Stu Berman/@stuinsf/

Like rainbows, fog arcs are caused by a similar physical phenomenon where light is refracted by fog droplets as opposed to rain droplets. Rainbows form when light from the sun passes through raindrops in the air and reflects, refracting and scattering, forming a multicolored arc in the sky. For a rainbow to appear, the viewer must be at a 42-degree angle to the light source.

The same refraction occurs when a fog arc forms, but it appears colorless due to the different sizes of the fog water droplets. These droplets are generally smaller than 0.05 millimeters (0.0020 in), compared to raindrops, which are between 10 and 1,000 times larger.

The small size of the droplets means that diffraction is the dominant effect on light, rather than refraction, according to the UK’s Meteorological Office. This means that the light is not split into its rainbow spectrum, just scattered and smeared into a white haze.

In the comments to Berman’s tweet sharing the image, others shared images of fog arches they took.

“I caught one over Bodega Bay earlier this year! I had never heard of them but I knew exactly what it was!” wrote one user.

“The photographer was at just the right vantage point with the sun behind them,” Brooke Bingaman, meteorologist for the National Weather Service, told news site SFGate. “If you look more to the right of the image, you can see the reddish tint on the outside. It has a bit of the color, but the prisms aren’t as effective as regular raindrops.”

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.”

According to, the best time to spot a fogbow is during a thin fog when the sun is bright. Mysterious “fogbow” weather phenomenon occurs over California

Rick Schindler

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