It's Grim up North

A raven poised to land on Wigan Town Hall chimney (Lancashire, England) oblivious of the sky scattered with iridescent clouds. Image by Dave Walker (Flickr).   ©Dave Walker, shown with permission.
Cloud iridescence is closely akin to a corona around the sun or moon. Both are produced by light scattered by small things, be they tiny water droplets, ice crystals or even pollen grains.

Imagine (at left) plane waves of blue sunlight scattering from a cloud water droplet. Outgoing spherical waves radiate from every point on the drop’s rim. For simplicity the diagram shows waves from only two points. In some directions the outgoing waves are in-phase and radiate blue light – in others they are out of phase, cancel, and there is no light. The process is interference or diffraction – the two terms are more or less synonymous.

The same droplet also scatters longer wavelength red light. There are different directions for the in-phase and out of phase conditions compared to those for blue light; different directions for brightness and darkness. A cloud of similar size droplets will appear red in some directions and other colours elsewhere.

Viewing angles, sun direction and local variations in cloud droplet size all influence the colours. Uniform droplets across a cloud will give an orderly corona; variations in droplet size from place to place give more disorderly iridescent colours.
Atmospheric
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