This photo of a rainbow cloud was taken by a National Geographic reader. What is a rainbow cloud? Nat Geo asked atmospheric phenomena expert Les Cowley:
"What happens is that the cumulus cloud, boiling upwards, pushes the air layers above it higher and higher. 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."
Anyone ever seen one in person? The photo above was taken in India.