Circumzenithal Arc Above Ritzville, Washington

June 17, 2012

Circumzenithalarc_plumb (2)

: Michelle Plumb
Summary Authors: Michelle Plumb; Jim Foster

The photo above showing a cheery circumzenithal arc (CZA) was captured over Ritzville, Washington, on May 30, 2012. The day before I saw this beauty, I was pleased to have spotted a faint circumhorizontal arc in a similar sky. When I noticed the impressive mare's tail seen here, I looked high overhead, spotted this arc and then grabbed my camera. CZAs result from refraction of sunlight in hexagonal ice crystals that make up cirrus clouds. When the Sun lies less than about 32 degrees above the horizon CZAs can form when light shines through the upper horizontal basal face of horizontally aligned ice crystals and then exits through one of the side faces. Note that the Sun is hidden here by the grain elevator at bottom.

Photo Details: Camera: NIKON D300; Focal Length: 18.0mm (35mm equivalent: 27mm); Aperture: f/7.1; Exposure Time: 0.0013 s (1/800); ISO equiv: 200; Software: Nikon Transfer 1.5 W. Image was straightened by two degrees and re-sized; photo taken at 6:47 p.m.