Cloud iridescence or irisation is a peculiar colorful optical phenomenon. It occurs when clouds are in of the Sun or Moon. The colors resemble those seen in soap bubbles or oil on a water surface. It is a type of photometeor. This phenomenon is most often observed in altocumulus, cirrocumulus, lenticular, and cirrus clouds. This relatively rare phenomenon in clouds can bring up unusual colors vividly or even a whole spectrum of colors simultaneously. The colors are usually pastel, but can be very vivid or mingled together, sometimes similar to mother-of-pearl. When appearing near the Sun, the effect can be difficult to spot as it is drowned in the Sun’s glare.
These clouds are formed of small water droplets of nearly uniform size or small ice crystals individually scattering light. Larger ice crystals do not produce iridescence, but can cause halos, a different phenomenon. When the Sun is in the right position and, typically, hidden from direct view, these thin clouds can be seen significantly diffracting sunlight in a nearly coherent manner, with different colors being deflected by different amounts.
In the cover image the iridescent Clouds over Sweden (Image copyright by Goran Strand) are reflected by the water below tha sky.
The term “iridescent” has classical orgins: irisations are named after the Greek goddess Iris, goddess of rainbows.
If you have seen this phenomenon, you’re absolutely lucky!
References:
https://apod.nasa.gov/apod/ap240707.html
In the painting: Iris Carrying the Water of the River Styx to Olympus for the Gods to Swear By, Guy Head, c. 1793 – Nelson-Atkins Museum of Art
