Circumhorizon Arc imaged by Caroline Tucker at Port Townsend, Washington July '08. Image ©Caroline Tucker, shown with permission.
This circumhorizon arc is remarkable in the way that it completely lights each fibrous tuft of high cirrus cloud. The halo is the collective glints of spectral sunlight from millions of small plate shaped ice crystals. When the cirrus layer is continuous the crystals glint along a huge band parallel to the horizon. Discontinuous clouds of ice crystals appear instead to be individually filled with colour.

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