What is a 'fire rainbow?' Hint: it's not a rainbow, and it's not caused by fire

The appearance of a 'fire rainbow' in South Carolina over the weekend drew spectators in person and on Twitter, but the name of the phenomenon might be misleading.

Tracey Smith via WCSC-TV
Tracey Smith captured this photo of Sunday's "fire rainbow" in lower South Carolina, saying the phenomenon reminded her of a friend who had passed away. Circumhorizontal arcs – how fire rainbows are known to meteorologists – form when the sun shines through ice crystals in cirrus clouds at the perfect angle, Live 5 News reported.

Residents of South Carolina’s Lowcountry witnessed an unusual natural light show on Sunday. A “fire rainbow” appeared in the sky and drew comparisons to “a multi-colored angel” for about an hour before disappearing, WCSC-TV in Charleston reported.

Images of the iridescent clouds went viral, but scientists say “fire rainbow” is a misnomer: the phenomenon is not a true rainbow, and it has nothing to do with fire – instead of rain or flame, the phenomenon is caused by ice.

Meteorologist Justin Lock told Live 5 News several variables must coincide for the spectacle to appear. The sun has to be at least 58 degrees above the horizon, and it has to shine through high-level cirrus clouds, which are made of little ice crystals.

"To produce the rainbow colors the sun’s rays must enter the ice crystals at a precise angle to give the prism effect of the color spectrum," Mr. Lock said.

Lock also said the same phenomenon can explain colorful sunsets: when the sun is low, the light reflects and refracts through crystals in high-level cirrus clouds, giving off the bright, warm shades of red and purple we see.

The term “fire rainbow” was apparently coined by a journalist in Spokane, Wash. in 2006. Circumhorizontal arcs – the real, if less catchy, name – can many times be seen during the spring and summer months in middle-latitude locations, the Weather Channel reported, saying that the spectacle looks  “as if wispy cirrus clouds take on a rainbow palette.”

Perhaps the most memorable time a “rainbow” – real or false – went viral was in 2010, when a YouTube video of a “double rainbow” spotted near Yosemite National Park was posted and called possibly the “funniest video in the world” by late night talk show host Jimmy Kimmel.

AccuWeather meteorologists say a when light bounces off the back of raindrops twice, a second, fainter rainbow may appear behind the “primary” rainbow, causing the double rainbow effect. The order of the second rainbow’s colors gets inverted, so blue is on the outside and red on the inside, rather than vice versa.

Tracey Smith saw Sunday’s fire rainbow from Isle of Palms, S.C. and captured the photo that went viral, in which the rainbow seems to resemble angel wings, WCSC-TV reported. She said it reminded her of a friend who had passed away.

"I'm sure she came to visit us on the beach we all love! We miss you Leslie!" she wrote.

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