This rare weather phenomenon is the cousin of rainbows

fogbow-tip-of-kayak-on-lake Photo by Francisco Blanco/Shutterstock

What the heck is a fogbow?

Ever see a colourless rainbow? Look closely—that’s a fogbow. (Legit weather term. We can’t make this stuff up.) Like its more colourful cousin, a fogbow forms through the interaction of sunlight and moisture—in this case, the moisture found in fog or mist. The water droplets in fog are much tinier—up to 1,000 times tinier—than raindrops. And size matters when it comes to water drops and rainbow formation: drop size has an effect on the reflection, refraction, and light dispersion that generates this meteorological phenomenon.

In the case of rainbows, “the process of refraction causes the different colours in the reflected beam of light—the sunlight—to be separated, whereas for fogbows, the process of diffraction broadens the reflected beam of light and smears out the colours,” says Gerald Cheng, a meteorologist with Environment and Climate Change Canada. This results in a white, or nearly colourless, ghostly-looking rainbow.

Fogbows are rare in the sense that to see one, you have to be in the right position. “You have to have the sun behind you and fog before you,” says Cheng.

This leaves only one unanswered question: is there a pot of colourless gold at the end of a colourless rainbow?

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