Video: Uncommon unrainbow brings dash of color to sky

Video courtesy of AOS Systems Programmer Pete Pokrandt

 

It’s a rainbow. It’s a sun dog. No, it’s a fire rainbow, which isn’t actually a rainbow at all.

This morning, the University of Wisconsin–Madison Registrar’s office tweeted a photo of a colorful, wispy smear in the sky. The tweet read: Must be a finals good luck sign?! #studystrong #rainbowovermadison.

But as it happens, the rainbow imposter is a different kind of atmospheric phenomenon known to science as a circumhorizontal arc. The case of mistaken identity is understandable; fire rainbows don’t occur all that often.

“They’re not all that common but they’re not completely rare, either,” says Steve Ackerman, director of the UW–Madison Cooperative Institute for Meteorological Satellite Studies. “I’ve only seen a half-dozen in my lifetime here in Madison.”

Unlike rainbows, which are formed when sunlight reflects off of water droplets in the atmosphere, circumhorizontal arcs require somewhat precise conditions in order to form.

“You need ice crystals and the ice crystals need to be a certain shape,” Ackerman says. “You can’t have too many in the cloud.”

As sunlight passes through a cloud laden with ice crystals, such as we had this chilly spring morning, the light bends through the crystals and separates out into the vivid colors of the spectrum.

The colors separate out horizontally because of how the cloud is oriented. Other phenomena, such as halos and sun dogs (parahelion), are also due to ice crystals in clouds, but what we see depends on the position of the cloud, and our position on Earth.

Fire rainbows occur only when the sun is 58 degrees or higher above the horizon, which means they are primarily only visible in the U.S. in the six weeks before or after the summer solstice. It also means the weather has to be somewhat decent; low-lying storm clouds won’t do. A good fire rainbow requires good clouds.

“The wispy, wavy appearance is part of the cloud,” says Ackerman, also associate dean of physical sciences at the UW–Madison Graduate School. “A lot of times you see the wispy, wavy appearance without the optical phenomenon – mare’s tails. They’re caused by ice crystals in the sky that are being moved by the wind.”

Most of us know these as cirrus clouds.

In case you missed the circumhorizontal arc this morning, fret not. The Department of Atmospheric and Oceanic Sciences (AOS) and Space Science and Engineering Center (SSEC) roof camera captured the fire rainbow. The video, courtesy of AOS Systems Programmer Pete Pokrandt, can be found on YouTube.