It’s also called a “fog bow” or “moonbow.”
Earlier this month, UK photographer Melvin Nicholson captured a photo of an all-white rainbow in Rannoch Moor, Scotland. Since he first posted it on his Facebook page, the stunning image has grabbed the attention of stargazers and astronomers around the world. But what causes this type of rainbow to appear in the sky?
Though it lacks the colors of a traditional rainbow, a fog bow is basically the same optical phenomena as a rainbow, according to Jonathan Kemp, a telescope specialist at Middlebury College Observatory. “They both usually involve light encountering moisture in the atmosphere and being bent or changing directions in such a way as to spread out or separate the light into its constituent colors,” he says.
With a rainbow, water passes through large water droplets, and each droplet acts as a prism that refracts and reflects light. The light is then distributed in the colors of the optical spectrum (RGBYV). But when light passes through smaller droplets of water, the light is diffracted (or diffused), resulting in an effect that is not as colorful or saturated as a rainbow. This phenomenon is what is known as a fog bow.
Fog drops are much smaller than raindrops, which is where the name fog bow comes into play. “Little fog drops only scatter the light without ‘breaking it up’ into its colors,” says Jason Kendall, who is on the board of the Amateur Astronomers Association of New York. “Fog bows are therefore fuzzier than rainbows, because while the interactions (between the light and water) are shorter, there are more of them, so the light gets bounced around more.”
And here’s a fun fact: The sun isn’t the only source of light that can cause these phenomena. “The moon can also be the source of light,” Kemp says. “In this case, the more white or less colorful appearance is due to moon light being less intense than sun light.” These types of fog bows are also called moonbows.