Daily Mail brings a story about Oleg Bartunov, 51, who caught the optical spectacle on camera during a Himalayas expedition in Nepal. His two images show almost the whole spectrum of the rainbow in a natural event rarely recorded at Mount Everest.
The phenomenon is caused by light reflecting off tiny ice crystals inside the body of the cloud’s water vapour. This optic phenomena is known as cloud iridescence. Iridescent clouds are a diffraction phenomenon. Small water droplets or even small ice crystals in clouds individually scatter light. Large ice crystals produce halos, which are refraction phenomena rather than iridescence. Iridescence should similarly be distinguished from the refraction in larger raindrops that gives a rainbow. Cloud iridescence is the occurrence of colors in a cloud not dissimilar to those seen in oil films on puddles, and is similar to irisation. It is a fairly uncommon phenomenon and is usually observed in altocumulus, cirrocumulus and lenticular clouds. When parts of clouds are thin and have similar size droplets, diffraction can make them shine with colours like a corona. In fact, the colours are essentially corona fragments. Very much rarer iridescence is that of nacreous or mother-of-pearl clouds. They can glow very brightly and are far higher than ordinary tropospheric clouds. Nacreous clouds far outshine and have much more vivid colours than ordinary iridescent clouds which are very much poor relations and seen frequently all over the world. Iridescence is also seen in rocket exhaust trails.
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