I've only been lucky enough to see this once (so far) in my life, and I remember it like it was yesterday. I stared up at the rainbow-colored cloud in the middle of the sky on a warm day and wondered what in the world it was. I thought perhaps it was pollution in the air, the way puddles in a parking lot have an iridescent sheen from the oils coating the surface. But I was wrong. Turns out what I was witnessing was a very rare phenomenon.
Iridescent clouds are also called fire rainbows or rainbow clouds. They form when cumulus clouds (those associated with thunderstorms) start rolling upward and push the layers of air above it higher into the atmosphere. The air getting pushed up expands and cools, sometimes condensing into tiny water droplets, called a cap cloud, which scatters sunlight and creates the rainbow-colored spectacle.
According to atmospheric phenomena expert Les Cowley, "The usually delicate colours can be in almost random patches or bands at cloud edges. They are only organised into coronal rings when the droplet size is uniform right across the cloud. The bands and colours change or come and go as the cloud evolves. They occur most often in altocumulus, cirrocumulus and especially in lenticular clouds. Iridescence is seen mostly when part of a cloud is forming because then all the droplets have a similar history and consequently have a similar size."
You're most likely to see iridescent clouds on hot and humid days, especially when thunderstorms are forming; however, they are rarely seen. National Geographic writes of this phenomenon, "Though the ingredients for rainbow clouds seem simple, they're not spotted often, and are even less frequently photographed."
So if you want to be lucky enough to spot these otherworldly clouds, now you know when and where to keep your eyes peeled.
* * *