Cloud iridescence is an optical phenomenon producing pastel bands of pink, green, and blue at the edges or thin sections of clouds near the sun. It is most commonly seen in altocumulus, cirrocumulus, and pileus.
Optical mechanism
It is caused by diffraction of sunlight passing between water droplets or ice crystals of very uniform size (typically 10–30 μm). Unlike a solar corona, which forms regular concentric rings, iridescence appears as irregular colour patches. The most vivid colours occur in young, thin clouds with uniform droplets and are best seen through polarised sunglasses.