An Iridescent (Rainbow) Cloud in Himalaya

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.

Credit: Oleg Bartunov