Saturday, February 23, 2013


Circumzenithal arcs, aka CZA or the Bravais' arc, are considered the most beautiful of the haloes, often described as an ‘upside down rainbow’ by people viewing them for the first time. The CZA is never a complete circle around the zenith; the centre of the bow is always sunwards and red is on the outside. The zenith is a circle around the point in the sky directly above the viewer.

Oriented plate crystals produce the CZA. These are the same crystals that form sundogs. Downcoming sunrays enter the uppermost horizontal face and then leave through a vertical side face. The result is the refraction of rays that produce very pure and well-separated prismatic colours.
Image: A CZA lights threads of early morning cirrus. Imaged by Eva Seidenfaden (Atmospheric Optics) in April 2003

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