A photographer snapped a picture of a strange and rare aerial phenomenon in the hazy skies of San Francisco.
Known as the Fog Arc, as well as the 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]“said photographer Stuart Berman, who lives in the Russian Hill neighborhood of San Francisco. Pleasemynews. “I saw him for about 15-20 minutes, but he was still there when I left”
Like rainbows, fog arcs are caused by a similar physical phenomenon, with light being refracted by fog droplets as opposed to rain droplets. Rainbows occur when sunlight passes through raindrops in the air and reflects, refracts and scatters, forming an arc of multicolored light across the sky. For a rainbow to be visible, the observer must be positioned 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 difference in size of the fog water droplets. These droplets are typically smaller than 0.05 millimeters (0.0020 inches), compared to rain droplets, which are between 10 and 1,000 times larger.
The small droplet size causes diffraction to be the dominant effect on light, rather than refraction, according to the UK Met Office. This means that instead of the light being split into its rainbow spectrum, it is simply dispersed and coated in a white mist.
In the comments to Berman’s tweet sharing the photo, others shared images of arcs of mist they had captured.
“I caught one over Bodega Bay earlier this year! I had never even heard of it, but I knew exactly what it was! wrote one user.
“The photographer was just in the right spot where the sun was behind them,” Brooke Bingaman, a National Weather Service meteorologist, told the SFGate news site. “If you look more towards the right side of the image, you can see a sort of reddish tint to the outside. There is some color, but the prisms aren’t as effective as regular raindrops. “
Fog arcs aren’t exactly uncommon, but catching one on camera is, Bingaman said. “What is rare is that someone is there to capture it.”
According to EarthSky.com, the best time to spot a fog bow is during light fog when the sun is bright.