Upper Tangent Arc, Halo, Sundogs and Glitter Path Observed from Vancouver
May 13, 2014
Photographer: Doug Farmer; Doug's Facebook site
Summary Authors: Doug Farmer; Jim Foster
Clearly visible on this splendid photo are the upper tangent arc (top center), a portion of the 22 degree circular halo and sundogs or parhelia. It was taken a little before sunset on July 4, 2009, from Vancouver, British Columbia. Ice crystals in cirrostratus clouds, covering much of the sky this summer afternoon, are responsible for these arcs and halos. The tangent arc forms when sunlight interacts with column-shaped hexagonal ice crystals. These columnar crystals drift with their long axes nearly horizontal. Sunlight enters through one of the prism's side faces and exits through an alternate face. Note also the glitter path on English Bay (lower center). See yesterday's Earth Science Picture of the Day.
Photo Details: Nikon D300 camera; 16 mm focal length: f/11 aperture; HDR 7 image sequence exposure times - 1/8000 sec. to 1/200 sec. (+/- 3 stops); ISO 200; Photomatix Pro Rev 4.2.7 software.