A cartoon cloud filled with rainbow colors.

You've probably seen a rainbow before. When sunlight passes through raindrops in the sky, the light is split into the colors the rainbow. If you have the right angle, you can see the full band of rainbow stretch across the sky. Sometimes you can even see the mirror image of the band, or the full double rainbow.


A cloud reflecting rainbow colors in the blue sky behind a snowy mountain.

Credit: Giorgio (CC BY 2.0)


But have you ever seen a rainbow cloud?

This can happen because of something called cloud iridescence. It usually happens in altocumulus, cirrocumulus, lenticular and cirrus clouds. Iridescent clouds happen because of diffraction – a phenomenon that occurs when small water droplets or small ice crystals scatter the sun's light.

Why doesn't this happen more often?