Corona & Iridescence

Ruthanne Fenske imaged this corona merging into more chaotic iridescence near Ely, Minnesota, USA. Thicker cloud prevented the corona's lower part from shining.

©Ruthanne Fenske, shown with permission.
Atmospheric
Optics
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Coronae and iridescence have the same causes. Small water droplets in thin cloud scatter sunlight predominantly from their surfaces. The outgoing spherical scattered waves interfere. They cancel giving darkness where the wave crests are out of phase and brightness where wave crests coincide. The result at some distance from the droplet is a diffraction pattern.

When cloud droplets are all of similar size the diffraction patterns from each individual drop are the same and we see a circular corona. When the droplets are of different size across the cloud their diffraction patterns combine (not interfere) to give more disorderly colours - iridescence.
Iridescence from droplets of varying size across thin cloud
A monochromatic light corona simulated by IRIS. Blue coronae are smaller than red ones.   The different sizes are the source of the white light corona's many hues.
Cloud droplets are 1-100 micron diameter, Some 10 - 1000X smaller than raindrops. The droplets are widely separated, Relative to their diameters their distances are similar to those of the Earth and Moon.