On a walk along the Merced River today, we were treated to an unusual atmospheric phenomena—a cloud iridescence. As I gazed up at the sky, a high cirrus unicus (mare’s tails) cloud stretched its tendrils over our heads. A rainbow of color decorated the bottom of the cloud, and while we watched, it slowly spread upward, like paper absorbing ink. The colors evolved as the cloud formed; it was like watching the birth of a rainbow. Shad, my partner, definitely regretted not bringing his camera!

An iridescence is caused by a difraction of light on small water droplets, and is related to the corona, another optical formation. The term iridescence comes from the word irisation, relating to the Greek deity Iris, who represented the rainbow. Not wanting to miss any portion of the show, I waited until the cloud had disappeared behind the hills (much to the disappointment of my dogs who could not understand why I stood still during our walk), delighted that the sky had painted so delightful a picture for me.