Iridescent clouds

From Glossary of Meteorology

iridescent clouds

Ice-crystal clouds that exhibit brilliant spots or borders of colors, usually red and green, observed up to about 30° from the sun.

This irisation results from an optical diffraction phenomenon, usually of several orders. The cloud particles that occasion this phenomenon are very small (a few micrometers), and locally all are of nearly the same size; they result from local near adiabatic lifting and condensation in moist air often in a lenticular (wave) cloud, in pileus above a developing cumulus, or occasionally in irregular patches of uniform color in a region of shallow convection (in this case sometimes called mother-of-pearl cloud).