Cloud Iridescence

Cloud iridescence, also called irisation, is a colorful optical effect that appears in a cloud near the Sun or Moon, with colors resembling those seen in soap bubbles or oil films on water; it is most often seen in altocumulus, cirrocumulus, and lenticular clouds, and occasionally in the rare polar stratospheric clouds. It is a diffraction effect caused by very uniform small water droplets or ice crystals individually scattering light, producing color within about 10 degrees of the Sun through diffraction and, farther out, through first-order interference that can extend to about 40 degrees away. The cloud must be optically thin enough that light passes through only a single droplet layer, which is why the effect shows up most vividly at the edges of clouds or in newly forming, still-thin clouds.