Sky watchers sometimes report seeing rainbow colors within clouds. These colorful clouds are called iridescent clouds. When you see a cloud like this, you know there are especially tiny ice crystals or water droplets in the air. Larger ice crystals produce solar or lunar halos, but tiny ice crystals or water droplets cause light to be diffracted – spread out – creating this rainbow-like effect in the clouds.
The phenomenon is called cloud iridescence or irisation. The term comes from Iris, the Greek personification of the rainbow.
By the way, it’s easy to confuse circumhorizon arcs with iridescent clouds. Here’s how to tell the difference.
Bottom line: You might on occasion see a rainbow-like cloud. They are fairly rare, but people do spot them, and we sometimes receive photos of them. They’re caused by the presence of very tiny ice crystals or water droplets in the air, which cause light to be diffracted or spread out.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.