An iridescence is the occurrence of colors in a cloud similar to those seen in oil films on puddles, located in the general vicinity of the sun or moon. It is a fairly common phenomenon, most often observed in altocumulus, cirrocumulus, cirrus and lenticular clouds.

Iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light. Larger ice crystals do not produce iridescence, but can cause halos, a different phenomenon.

When the particles in a thin cloud are very similar in size over a large extent, the iridescence takes on the structured form of a corona, a circular bright disk around the sun or moon, surrounded by one or more colored rings.