Cloud iridescence

This fairly common phenomenon is most often observed in altocumulus,[1] cirrocumulus, lenticular,[2] and cirrus clouds.

Other aids are dark glasses, or observing the sky reflected in a convex mirror or in a pool of water.

Larger ice crystals do not produce iridescence, but can cause halos, a different phenomenon.

[11] If parts of clouds contain small water droplets or ice crystals of similar size, their cumulative effect is seen as colors.

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 bright circular disk around the Sun or Moon surrounded by one or more colored rings.