Alaska Iridescence captured by Doug Short."These clouds were changing fast and within 10 minutes were overtaken by a solid layer of clouds. The cloud layer showed some iridescence on its leading edge for a few minutes as it merged with the first clouds."  ©Doug Short

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Think of iridescence as a fractured and jumbled corona.

Diffraction by a cloud of uniform sized droplets gives multiple coloured rings around the sun - a corona.

When the droplet size varies across a cloud the rings and colours are disordered, changing from place to place - iridescence.