Sign of Summer - Circumhorizon arc imaged by Dave Williams. McDowell Sonoran Preserve, Scottsdale, Arizona looking south from the Overlook Spur Trail above Windgate Pass on April 16, 2009 at approximately 1:00 pm. ©Dave Williams, shown with permission.

Circumhorizon arcs, huge colourful halos far beneath the sun and running parallel to the horizon are heralds of summer. The sun must be higher than 58° for them to form.

The arc's rays enter a near vertical side face of plate crystals in high cirrus clouds and exit through the lower near horizontal face. When the sun is lower than 58 degrees the rays cannot leave the lower face but are instead totally internally reflected.



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