A stunning weather phenomenon was spotted over parts of Virginia this week – a so-called “rainbow cloud,” which looks like a bright prism of color behind soft clouds. Several people in Hamilton, Virginia, sent in photos of phenomenon to WUSA-TV, the CBS affiliate in Washington, D.C.
While many were calling it a “rainbow cloud,” it is more accurately an atmospheric optic called cloud iridescence. But what exactly is that?
“Cloud iridescence forms when the water droplets or ice crystals in the cloud diffract the light around the outside of the droplet, as opposed to bending the light through it,” The Weather Channel meteorologist Jen Carfagno told CBS News.
“The colors of the spectrum are not as neat and organized in iridescence as in a rainbow,” she added. “Cloud iridescence reminds me of pixie dust or unicorn sprinkles.”
So-called rainbow clouds are fairly rare, said Carfango, who co-hosts “America’s Morning Headquarters.” They are often spotted near the polar regions, or in mountain areas in the winter. The clouds need to be near the sun.
The clouds must be very thin and made of ice crystals or water droplets of uniform size. So cloud iridescence usually happens in lenticular or alto-cumulus, cirrus and cirrocumulus clouds.
“In the case from this week in Northern Virginia, what happened was that a pileus cloud, AKA a ‘cap cloud,’ formed on top of a late afternoon cumulonimbus cloud,” Carfagno said. “This cap cloud is a common feature on top of strong thunderstorms.”
A rainbow cloud is a similar rainbow, halo, sundog, circumhorizontal arc, because it forms due to the diffraction or refraction of light.
In 2018, a similar “fire rainbow” was spotted in Vermont. This phenomenon, technically called a circumhorizontal arc, occurs when the sun is very high in the sky – higher than 58 degrees above the horizon – and its light is refracted by ice crystals suspended in the atmosphere, according to National Weather Service (NWS).
This phenomenon looks like a very large halo and is only visible in cirrus clouds, which are made out of ice crystals and look wispy. When the cirrus clouds are lit with color, they can be mistaken for iridescence, according to the NWS.
Rainbow clouds and fire rainbows are different, and the colors in a fire rainbow are much more vivid than in a rainbow cloud, Carfagno explained.
Carfagno said the “rainbow cloud” spotted over Virginia has been described by some as psychedelic in color, which is a good description because it gives a clue about the cloud type.
“Cloud iridescence is not as uniform in color as is a regular rainbow or a halo or a circumhorizontal arc,” she said.