St Petersburg Halos ~ Late afternoon St Petersburg, Russia Thursday 18th March. A sundog, parhelic circle and a trace 22° halo glow over the still frozen Gulf of Finland. An image by Nikita Kulanov. ©Nikita Kulanov, shown with permission.

The sundog and in this instance probably also the trace of the parhelic circle are formed by plate shaped ice crystals drifting with their large hexagonal (basal) faces nearly horizontal.

Sundog rays are usually shown as entering a side face and leaving from another inclined 60° to the first. It is not always that simple for, unless the sun is very low, the rays enter at a skew. They then internally reflect up and down between the large basal faces. The fraction of sundog rays that are so reflected increases as the sun climbs but even when the sun is low many undergo these reflections.


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