Definition: Irisation or iridescence: Colours appearing on clouds, sometimes mingled and sometimes in the form of bands nearly parallel to the margin of the clouds. Green and pink occur most often, and in pastel shades.
The irisation or iridescent colours are often brilliant, and resemble the colours observed on mother of pearl. Within about 10° from the Sun, diffraction is the main cause of irisation or iridescence. Beyond about 10°, however, interference is usually the predominant factor. Irisation or iridescence can extend to angles exceeding 40° from the Sun, and even at this angular distance, the colours may be brilliant.