Last Arc of Summer? - Fragment of a circumhorizon arc captured over New Jersey in early August by Lisa Gonnelli. ŠLisa Gonnelli, shown with permission.

If you are in the northern parts of the US make the most the next week or so to see it. It is already almost too late in Seattle. LA will see it until late September. In Europe it is not now visible anywhere north of Paris.

The sun must be higher than 58° to form the arc from plate shaped ice crystals in high cirrus cloud. In most of the US with high summer sun and clear skies it is a relatively common halo seen maybe five or more times each summer. Europe is not so favoured and there it is a rarity.

Atmospheric
Optics

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  Circumhorizon Arcs
      
  When to see it