Rare White Rainbow Spotted Over World's Largest Freshwater Lake in Siberia

The colorful rainbow that we all know and love has a ghostly cousin—the fogbow. And one of these ethereal aerial arches has been spotted above Lake Baikal—the world's largest freshwater lake.

Images of the unusual phenomenon taken in Buryatia, a Russian Republic in south-eastern Siberia on the border of Mongolia, were shared by the Federal State Institution, Zapovednoe Podlemorye.

"Our photographer Aleksey Svatov took a picture of a rare natural phenomenon on the Ushkany Islands—a white rainbow," the institution said in a post on August 1 on the Russian social media platform VK.

Svatov is then quoted, describing the experience.

"I've seen this a couple of times before today, but I couldn't photograph it," he said. "Here we were lucky. Fog was on the islands for almost two days, seals were not really visible, but the rainbow appeared just opposite the checkpoint. You cook dinner, raise your eyes, wait out of habit for impenetrable fog, and there is such a miracle."

Fogbows, also known as ghost bows or white rainbows, are much rarer than their colorful counterparts and only appear when conditions are just right.

Ultimately, both occur when light interacts with droplets of moisture in the air.

Light travels more slowly through water than it does through air, so when it enters a water droplet, it slows down and bends. This is called refraction. The light is then reflected off the inside of the droplet and shoots back the way it came, bending again as it leaves the droplet.

The light that we see is made up of a spectrum of different colors and some of these bend more than others. So, when the light is refracted, these colors separate out, with violet bending the least and red bending the most.

But why does this separation not happen in fogbows? The difference comes down to the size of the water droplets. Raindrops tend to fall between 0.5 and 2 millimeters (0.02 to 0.08 inches) in length, while the water droplets in fog are generally much smaller, at around 0.05 millimeters.

" A white rainbow can be seen if the water particles in the fog are less than 25 microns in diameter," Zapovednoe Podlemorye said. "A little more size and we already see the usual, familiar color rainbow."

Because of the smaller size of water droplets, the color separation in a fogbow is weaker and more smudgy, meaning it appears mostly white. This smearing can also make fogbows appear wider than most rainbows.

EarthSky.com says that the best time to catch a fogbow is during a thin fog when the sun is bright.