Circumzenithal Arc Over London, London England

April 11, 2008

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Provided by: Brenda Schumpert
Summary authors & editors: Brenda Schumpert, Jim Foster 

The photo above showing a zesty circumzenithal arc over London, England, was taken just before noon on the cold, autumn day of November 12, 2007. These brightly colored arcs are formed when sunlight passes through the uppermost basal faces of similarly oriented hexagonal plate crystals and then exits through vertical side faces. In order to be able to observe circumzenithal arcs, the Sun's altitude must be less than about 32 degrees above the horizon. Even at mid-day in the south of England, during November, the Sun doesn't attain this bearing. At the time when this photo was recorded, the Sun's altitude was approximately 21 degrees.