Circumhorizon Arc imaged near Hooper, Colorado on August 10,'08 by Steve Bardwell.
Image ©Steve Bardwell, shown with permission.
Circumhorizon arcs are summer visitors. the sun needs to be at least 58° high for plate shaped ice crystals in high cirrus cloud to form the halo. Under those conditions this huge halo is not uncommon. It is thus frequently seen in the USA but is rare in mid to north Europe.
Rays pass between faces inclined 90° to each other and this produces the wide colour separation. The minimum deviation angle through a 90° ice prism is about 46° and for that reason - plus the required plate orientation of the crystals - the halo's technical name is a 'Lower 46° plate arc'.
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