Iridescence is caused by diffraction of light by uniform cloud droplets or ice crystals. Iridescence is a remnant of very large coronal diffraction rings around the moon or sun, much distorted and incomplete. It looks like an oily water surface, or petrol stains, pastel colors, with pink and light green appearing most prominent. Cloud types which often show iridescence are altocumulus lenticularis, cirrocumulus and occasionally fractocumulus.
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