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Circumhorizon Arc ~ Marc Salomon captured this summer ice halo blazing in high cirrus cloud over California's Long Valley.     ©Marc Salomon, shown with permission

The circumhorizon arc needs a sun at least 58° high and is therefore at most latitudes it is a summer only halo.

It's a frequent halo in the US but rare in northern Europe. London has a two month visibility slot for an hour or two around noon. Copenhagen and points further north never see it.

Do not believe cameras with their lens distortions. The halo is parallel to the horizon.

Red is uppermost. That, and its ordered spectral colours, help distinguish it from iridescent cloud.