Sky watchers on occasion report seeing colors in the clouds, and sometimes our friends send photos of these rainbow colors in clouds. You’ll find some examples below. These colorful clouds are called iridescent clouds, and the phenomenon is called cloud iridescence or irisation. The term comes from Iris, the Greek personification of the rainbow. It’s similar to the colors you might see when oil lies on the surface of a puddle of water. 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. See the images below, mostly via EarthSky friends on Facebook and Google+, for more about iridescent clouds.
Iridescent clouds seen by EarthSky Facebook friend Mike O’Neal in Oklahoma on May 27, 2013. Thank you, Mike.
Iridescent cloud seen by EarthSky G+ friend Dave Walker on April 17, 2013. View larger. He wrote, “Through gaps in the fast moving lower clouds I spotted these beautifully coloured high clouds as the sun headed for the horizon. Added a little vibrance and contrast enhancement but otherwise the colors here are as I saw them.”
Our friend Dave Walker in the UK caught another iridescent cloud on April 25, 2013. He wrote, “There’s been a lot of very high cloud recently, always a cue for me to look out for more atmospheric optics.”
The best way to see an iridescent cloud is to place the sun itself behind some foreground object, a building or mountain, for example. Other aids are dark glasses, or observing the sky reflected in a convex mirror or in a pool of water. EarthSky Facebook friend Duke Marsh captured this image on September 9, 2012 in New Albany, Indiana.
A beautiful example of cloud iridescence from ConcreteAbstractions via Wikimedia Commons
Rosanne Harter Haaland in Johnson City, Tennessee captured this beautiful iridescent cloud on January 24, 2013. View larger.
The colors in an iridescent cloud tend to be subtle and are usually pastel, but in some cases they can be vivid. Here is cloud iridescence captured by George Quiroga in Boynton Beach, Florida on July 31, 2012. Thank you, George. Image via Wikimedia Commons.
Read more about iridescent clouds on Les Cowley’s great website Atmospheric Optics
Bottom line: You might on occasion see a rainbow-like cloud. They are fairly rare. They’re caused by the presence of tiny ice crystals or water droplets in the air, which cause light to be diffracted or spread out.