White rainbows are real—but very rare
Unlike ordinary rainbows, fogbows only appear in humid conditions. Here’s the science behind them—and where you have the best chance of seeing one.
When you think of a rainbow, you think of the ROYGBIV rainbow painting a colorful arc across the sky. But a lesser-known white rainbow, commonly called a “fog bow,” sometimes develops in foggy conditions.
The first recorded sighting of a fog bow took place in modern-day Ecuador sometime between August 1737 and July 1739 by Spanish scientist and explorer Antonio de Ulloa. Ulloa described “boundless astonishment” at seeing three glories, or optical halos, which he referred to as rainbows and “a fourth Arch of a whitish hue"—what we know today as a fog bow.
Here’s what you need to know about white rainbows, including how to spot them in the wild.
How fog bows differ from rainbows
Fog bows and rainbows are both created when sunlight hits droplets of water. “It's the same principle, just with a different size droplet, which then impacts what colors you see or don't see,” says North Dakota's KX News chief meteorologist Kenny Miller.
A typical rainbow forms when sunlight refracts off of large raindrops—meaning it’s “both bending through the water droplets and bouncing off the back of them,” explains Bailey Nordin, a weather observer at New Hampshire’s Mount Washington Observatory. Those traditional colors—red, orange, yellow, green, blue, indigo, and violet—come from “light being bent at different angles.”
(24 brilliant pictures of rainbows around the world.)
A fog bow forms when sunlight refracts and diffracts off cloud droplets. That diffraction is key to a fog bow’s white color, says Miller. But first, you need fog.
Fog can form year-round, but it’s more common during extreme temperature changes, such as when snow begins to melt as temperatures warm in spring. Fog bows are like seeing your breath on a cold day, says Miller. “You're basically creating a cloud or creating a fog because of the condensation effect of the warm air leaving your mouth, interacting with the cold air and the environment around it.”
To see a regular rainbow, water droplets need to measure about 2 millimeters because the droplet is spherical at this size, which allows for a more even distribution of color.
Meanwhile a fog bow only requires 0.05-millimeter cloud droplets, Miller says.
You still get some refraction with the smaller cloud droplets, “but you're also going to get diffraction, which is going to spread those colors in multiple directions,” Miller explains. Diffraction occurs when light bends around the water droplet, whereas refraction happens when the light bends as it moves from the air into the water droplet. “The combination of that smears all those colors together, and that's why you get white, as opposed to different colors,” Miller says.
Fog bows aren’t always entirely white. Occasionally there’s also a little red or violet, Miller says. “When that happens, it's like the refraction is kind of winning out over the diffraction in terms of the spread of the colors.”
Researchers study fog bows to better understand atmospheric optics and the behavior of light as it interacts with water droplets in the air. By adjusting the size and shape of the droplets in simulations, computer scientists have been able to recreate many types of optical phenomena, including fog bows, double rainbows, and twinned rainbows where one rainbow splits into two.
How to see a fog bow
Fog bows aren’t always easy to spot, and even if there is one in your area, you need to be in just the right position to see it. Fog must be present to even have the chance of seeing a fog bow, but fog reduces visibility—meaning you might not see it if you’re directly in the fog. “You have to have either the sun or the moon behind you illuminating the fog,” says Nordin.
Atop the tallest peak in the northeastern United States, the Mount Washington Observatory sits at an elevation of 6,288 feet. Nordin and the rest of the team spend 60 percent of their time in fog. “If we are completely on the inside of a cloud, which is what's happening to us fairly often, you're not going to get to see [a fog bow],” says Nordin.
Nordin says fog bows are rare globally except for in places known for fog, like mountaintops or the Arctic.
It’s exciting to see a fog bow and appreciate the science behind it, but they don’t hold any other special significance, Miller says. “It's just one of those cool things that if you're a science geek like me, you're like, ‘Oh, wow. Look at that. Look at what's happening out there. Do you appreciate that this is extremely rare?’ So it's just one thing to geek out at.”
