This is a rare phenomenon called iridescent clouds. It occurs in the clouds with ...

volcanic dusts
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sulphuric acid
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detergent vapors
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small droplets or ice crystals
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The effect is similar to irisation. Iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light. Larger ice crystals do not produce iridescence, but can cause halos, a different phenomenon.
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