Light scattering by airborne ice crystals – An inventory of atmospheric halos
Jarmo Moilanen, Maria Gritsevich
Abstract
Atmospheric halos are a light scattering phenomenon caused by airborne ice crystals in the atmosphere. Halos can be seen by the naked eye. They provide the observer the information on the kinds of ice crystals present in the sky during a halo display. A combination of ice crystals’ shape, their orientation, and light ray paths through the crystals dictates what halos will be observable. All well-known halos are produced by the interaction of light with crystals of hexagonal water ice. However, some of the documented halos cannot be explained using common hexagonal ice crystals. It might be required to consider such factors as abnormal crystal shape, crystals of cubic water ice, or airborne crystals of other minerals to explain the nature of some exotic halos. Halos can be also observed in the atmospheres of other planets or their moons, which provides information on airborne ice crystals or crystals of other minerals. Ice crystal halos have been already photographed in the Martian atmosphere. In this article, we summarize the current knowledge of atmospheric halos and show which halo forms cannot be explained by ordinary hexagonal ice crystals. We list here 119 different identifiable halo forms known by today.