The phenomenon was observed early morning on October 18, 2009 on the path to Khumjung in the Himalayas. The mountain pictured is Thamserku (6623m). Iridescent clouds are a diffraction phenomenon cause by small water droplets or small ice crystals individually scattering light. Larger ice crystals produce halos, which are a refraction phenomena rather than iridescence. Iridescence should similarly be distinguished from the refraction in larger raindrops that makes a rainbow. If parts of clouds have small droplets or crystals of similar size, their cumulative effect is seen as colors. The cloud must be optically thin, so that most rays encounter only a single droplet. Iridescence is therefore mostly seen at cloud edges or in semi-transparent clouds, and newly forming clouds produce the brightest and most colorful iridescence.
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