After a gusty thunderstorm on Saturday afternoon, the residents of Boise were treated to an unusual spectacle: a supernumerary double rainbow.
Saturday’s storm approached Boise from the northwest shortly before 3 p.m., according to radar posted on the National Weather Service’s Boise Twitter account. At 3:22 p.m., the Boise Airport recorded wind gusts of 62 mph and a 13-degree temperature drop in 15 minutes, according to the NWS.
In the foothills, gusts of 68 mph were recorded.
Around 7:30 p.m., the sky cleared for a sun shower, where the rain falls while the sun is shining.
“After the main storm passed, we were able to put the sun in the right place just because of the time of day,” said Stefanie Henry, meteorologist with the National Weather Service in Boise. The setting sun, free from the cloud cover common in many storms, directly hit the falling rain.
During a standard rainbow, light is refracted and reflected by water droplets in the air, according to National Geographic. Each color visible in the electromagnetic spectrum is a different wavelength, and refraction, or bending, through water causes light to separate into different colors, according to Henry. Slight variations in the size of the water droplets can cause changes in the way colors are projected.
Saturday’s rainbow was supernumerary, meaning viewers could see not only the full spectrum of visible light, but also the repetition of other colors below the rainbow. The supernumerary effect occurs when competing rays of light interfere with each other during reflection, experts say. The phenomenon is common, according to Henry, but it is often too faint to be seen.
“What has surrounded the supernumerary rainbow is that it basically has a repeated effect in the rainbow itself,” Henry said. “Water acts like a prism, then it reflects light just enough that all the colors in the spectrum are visible, and if it’s strong enough, you can see more and more rows of colors.”
On top of that, the angle of the sun caused the light to reflect twice, revealing a second rainbow higher in the sky.
The rainbow received a lot of attention on social media, where residents of Boise venturing out after the heavy rain were greeted with a colorful display.
A Twitter user captured a video of the colorful bow:
“There was a lot of stuff in the right place at the right time,” Henry said.