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A Rare Look at an Iridescent Cloud

We’ve been told that every cloud has a silver lining—but did you know some clouds wear a rainbow cap?

Lara Sorokanich

National Geographic

Published July 18, 2013

A halo of multicolored mist floats over an ominous storm. At first glance it looks like an angelic mural or even extraterrestrial activity. But this breathtaking photo is neither manipulated nor paranormal. It’s an iridescent cloud, a phenomenon occurring right in our own atmosphere.

This photo was submitted to National Geographic by V. Harish, a university student and amateur photographer from Noida, India. It was captured in mid-July, shortly after a summer rainstorm, an ideal condition for rainbow clouds.

“I decided to take some shots of the after-shower scenery,” said Harish. “As I was working on a shot of a dewdrop, my friend spotted an exuberant colored patch peeking above a cloud.”

Iridescent clouds, known as “fire rainbows” or “rainbow clouds,” occur when sunlight diffracts off water droplets in the atmosphere. And the recipe for these heavenly sights is actually pretty simple.

Like common cloud-to-ground rainbows, iridescent clouds usually accompany thunderstorms. According to atmospheric phenomena expert Les Cowley, they often appear in the late afternoon, on very hot and humid days. This stems from the fact that most rainbow clouds form on top of cumulus clouds—the fluffy cotton-ball-shaped clouds we often see in children’s drawings.

“What happens is that the cumulus cloud, boiling upwards, pushes the air layers above it higher and higher,” Cowley explained. “As the air gets pushed upwards, it expands and cools. And sometimes moisture in that air suddenly condenses into tiny droplets to form a cap cloud.”

This “cap"—which scientists call a "pileus"—is the source of the brilliant spectacle.

"The droplets in the cap cloud scatter sunlight to form the gorgeous colors,” Cowley said.

Though the ingredients for rainbow clouds seem simple, they’re not spotted often, and are even less frequently photographed.

“For a moment we thought it was a portal opening for an alien species to come to Earth,” said Harish, who had never seen a rainbow cloud before. “But the beauty of it really moved me, so I just took as many shots of it as I could.”

A good call, according to Cowley, who says the rainbow clouds aren’t a common occurrence.

“Not all pileus caps show iridescence,” he said. “I usually get images of them from Florida, Southeast Asia, equatorial Africa.”

“I felt very lucky to have seen this in India. It’s a very rare sight,” Harish said. “Me and the guy who accompanied still joke that it’s an alien invasion, and share a laugh about it!”

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Iridescent clouds over Malaysia 

A relatively rare phenomenon known as iridescent clouds can show unusual colors vividly or a whole spectrum of colors simultaneously. These clouds are formed of small water droplets of nearly uniform size. When the Sun is in the right position and mostly hidden by thick clouds, these thinner clouds significantly diffract sunlight in a nearly coherent manner, with different colors being deflected by different amounts. Therefore, different colors will come to the observer from slightly different directions. Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too far from the Sun to exhibit striking colors. 

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Spectacular video of an iridescent cloud recorded in the skies over Costa Rica last week. Will have a more detailed post on this soon.

This was the peak of the solar eclipse in Tromsø at 11:09 am, with 95% of the sun covered by the moon. Got the added bonus of the altocumulus clouds turning iridiscent and shimmering pastel rainbow hues at the peak of the eclipse. Alongside the crisp blue skies and the soft snowflakes gently drifting down, I think this might be have been one of the most beautiful things I’ve ever seen in my life. ;u;