Greenland Ice Cap Optics

Days are shortening rapidly at Summit Station high on the Greenland Ice Cap. While they last Ed Stockard (photostream) is experiencing an amazing variety of optical phenomena - halos, fogbows, glories and more. Here an astonishing four ringed corona surrounds the sun shaded by the station wind turbine.

The corona was generated by a thin fog of supercooled water droplets (the air temperature was -33C/-27F). Small ice crystals generate coronae but they would be highly unlikely to be of so uniform a size and regularity to generate so many rings.

The fog is visible by the shadows of the turbine blades spread by perspective as they stream back towards the camera.

©Ed Stockard, shown with permission.
Atmospheric
Optics

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Coronae are a diffraction phenomena.

Light incident on an individual droplet is scattered from its surface. The scattered light propagates outwards as spherical waves.

The waves overlap and interfere. In directions where overlapping wave crests coincide there is light. Where crests coincide with troughs of the other wave there is destructive interference - darkness.

The result in the far field is a circular diffraction pattern of a bright central aureole surrounded by ever fainter rings.