Why would a cloud appear to be different colors? 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. The above iridescent cloud was photographed in 2009 from the Himalayan Mountains in Nepal, behind the 6,600-meter peak named Thamserku.
this atmospheric phenomenon is known as a circumhorizontal arc, which occurs when the sun is at least 58° above the horizon and the hexagonal ice crystals which form cirrus clouds become horizontally aligned.
Nacreous clouds, sometimes called mother-of-pearl clouds, are rare and unbelievably bright with vivid and slowly shifting iridescent colours. They are mostly visible within two hours after sunset or before dawn.
They are seen mostly during winter at high latitudes like Scandinavia, Iceland, Alaska and Northern Canada. Sometimes, however, they occur as far south as England. They can be less rare downwind of mountain ranges. Elsewhere their appearance is often associated with severe tropospheric winds and storms.
Nacreous clouds far outshine and have much more vivid colours than ordinary iridescent clouds which are very much poor relations and seen frequently all over the world. (Source)
as a cumulus cloud containing warm air convects upwards and condenses, a thin layer of humid air containing water vapour is thrust above the cloud, cooling from the lower pressure into droplets, which, when small enough and uniform in size, diffract light from the sun when it’s at least fifty eight degrees above the horizon.
Noctilucent clouds are actually crystals of ice that hang around 80 kilometres (50 miles) high in the atmosphere. The ice crystals catch the light of the Sun long after it has set on the horizon. The uppermost parts of the cloud in this image are iridescent (nacreous), which gives them the appearance of mother-of- pearl. Natural nacreous clouds occur at altitudes of 20-25 kilometres. The lower parts of the cloud have a redder colour due to the large amount of dust and water in the lower atmosphere scattering blue light. The cloud’s shape is due to differing wind speed at differing altitudes.
The cloud in this image formed from the exhaust of a missile launched from a distant firing range. Same basic process - ice crystals in the atmosphere, just a different source.
Sitting atop of a hill in the village of Oshino near Mt. Fuji, Japanese CG artist and photographer Kagaya found himself capturing a magical moment as a rare phenomenon known as cloud iridescence lit up the sky and a planes contrails with a pastel rainbow glow.
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!”
Rainbows are a fan favorite in Yosemite National Park, commonly seen along the park’s flowing waterfalls on sunny days. Halos and coronas around the sun or moon are also a typical sight. Recently, a lucky visitor was able to spot and capture this rare occurrence –cloud iridescence – over Glacier Point. This optical phenomenon is caused when sunlight diffracts off tiny ice crystals or water droplets and creates a rainbow effect. Iridescent clouds typically occur in late afternoons in hot and humid weather, so keep your eye out!