After a gusty thunderstorm on Saturday afternoon, Boise residents were treated to an unusual display: a supernumerary, double rainbow.
The storm on Saturday approached Boise from the northwest shortly before 3 p.m., according to radar posted to the National Weather Service’s Boise Twitter account. By 3:22 p.m., the Boise Airport clocked 62 mph wind gusts and a temperature drop of 13 degrees in 15 minutes, according to the NWS.
In the Foothills, gusts of 68 mph were recorded.
At around 7:30 p.m., the sky cleared for a sun shower, wherein rain falls while the sun is shining.
“Once the main storm went through, we were able to get the sun just in the right spot just because of the time of day,” said Stefanie Henry, a meteorologist at the National Weather Service in Boise. The setting sun, free of the cloud cover common during many storms, hit directly onto the falling rain.
During a standard rainbow, light is refracted and reflected through water droplets in the air, according to National Geographic. Each visible color in the electromagnetic spectrum is a different wavelength, and the refraction, or bending, through water causes the light to separate into different colors, according to Henry. Slight variations in water droplet sizes can cause shifts in how the colors are projected.
Saturday’s rainbow was supernumerary, meaning that 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, according to experts. The phenomenon is common, according to Henry, but it’s often too faint to see.
“What cased the supernumerary rainbow was that it basically has a repeat effect in the rainbow itself,” Henry said. “The water acts as a prism and then it reflects the light just enough to have all the colors of the spectrum visible, and if it’s strong enough, you’re able to see more and more of the rows of colors.”
On top of that, the angle of the sun caused the light to be reflected twice, revealing a second rainbow higher in the sky.
The rainbow received great attention on social media, where Boise residents venturing out after the heavy rain were greeted with the colorful display.
One Twitter user captured a video of the colorful bow:
“It was a lot of things in the right place at the right time,” Henry said.