Corona & Wave Clouds
A corona and filmy iridescence light shifting föhn wave clouds. Hermann Scheer (site) took this series of evocative images from the Sonnblick Observatory at an altitude of 3100m in the Austrian Alps.
  ©Hermann Scheer, shown with permission.

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Here we see a remarkably perfect four ringed corona at times apparently overlain by see-through iridescent films of cloud.

A four ringed corona signals diffraction by water droplets with exceptionally uniform sizes. Wave clouds are the source of the droplets.

The atmospheric wave motion, likely induced by the mountains, causes air pockets to rise and fall over a period often of only a few seconds. Rising air expands and cools. If the air is sufficiently moist condensation occurs into small droplets. The air pocket then descends again and the droplets evaporate.

In any one section of cloud the droplets all have the same short history and are uniform sized. Diffraction from them produces the wonderful colours imaged here.