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Are white rainbows for real? Meet the cloud bow

 Fog bow or cloud bow appears over Kalskag in November. Lena Stewart says it was the first time she'd ever seen a white rainbow in her Kuskokwim River community. (Photo edited to better show cloud bow)
Fog bow or cloud bow appears over Kalskag in November. Lena Stewart says it was the first time she'd ever seen a white rainbow in her Kuskokwim River community. (Photo edited to better show cloud bow) (KTUU)
Published: Dec. 16, 2019 at 1:54 PM AKST
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White rainbows might sound like appearances from the land of make-believe, but Lena Stewart in Kalskag saw one with her own eyes.

It was the first she’d seen of what she called a 'snow bow' and she sent a photo of it in to Channel 2.

Was it for real?

The answer is yes, and they're technically called a "fog bow" or "cloud bow." Professor Emeritus at UAF and celestial optical phenomenon enthusiast Walt Tape says that they’re fairly common in Alaska - if you know where to look.

“Any time you can get some light source falling on the fog, you're apt to see a fog bow. You can make these things with your auto headlights,” he said.

That doesn’t mean everyone has seen them. For one, people often don’t notice things they don’t expect. They also require very specific conditions.

While any child can tell you that rainbows are caused when rays of light reflect through a layer of precipitation, fog bows, as the name implies, require fog. The difference - at least optically - is in the size of the water droplets. While the rain that usually causes precipitation can be large enough to say, measure on a ruler, the size of the water droplets that cause fog are minuscule-- less than 20 micrometers, according to NASA. That’s about the width of a thin piece of human hair.

That causes the light to bend in different ways. Instead of rainbows in which “ray tracing” separates the light into different colors, giving a traditional rainbow its characteristic color palette, cloud bows are formed by a different optical effect.

The smaller particles contained in the fog cause diffraction of light, which smears the normal rainbow colors, rendering them white except for an occasional trace of red or blue. Tape says that in fact, the bows can theoretically have any amount of color in them.

“You get a full spectrum between the fully white rainbow and an ordinary-colored rainbow,” he said. It all just depends on the amount of fog and precipitation.

While these elusive rainbows can theoretically occur anywhere, Alaska has some characteristics that make it a good place to find them. For one, both rainbows and cloud bows require that the sun be shining at below 42 degrees above the horizon. Since many areas of Arctic Alaska are at such oblique angles to the sun, that means that the higher latitude one travels, the more time per year is spent with the sun below 42 degrees.

Then, all a cloud bow needs is fog. One common way that fog is formed is when relatively cool air from land passes over open water, essentially sucking the moisture from the river or sea into the air and creating a veil of fog.

And in a twist, global warming might actually be a boon for cloud bow viewing. Since shorter winters and warmer temperatures mean longer periods of open water in rivers or lakes, the time period in which that sort of fog can appear is likewise greater.

A cloud bow appears in the sky above Manokotak, Alaska (Photo courtesy of Kuli Itumulria)

“When that moisture hits that cooler air along the coast or along the riverbank, it has nothing to do except condense and form a cloud,” said Brian Brettschneider, climatologist with the International Arctic Research Center. That's because cold water holds very little water vapor before it becomes saturated and the vapor forms into clouds.

"The number of days of fog blowing in from the open ocean are gonna increase along the North and West Coast of Alaska," because of global warming, he said. That could mean more cloud bows.

So either people like Stewart, who sent us the photo, didn’t know where to look for cloud bows, or maybe climate change helped create the conditions along her community on the Kuskokwim River that allowed her to see her very first one.

Copyright 2019 KTUU. All rights reserved.

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