22-Degree Halo Over Dayton, Ohio
April 07, 2011
The photo above showing an eye-catching 22-degree halo and upper tangent arc was captured just before sunset over Dayton, Ohio on January 21, 2011. Halos form from pencil-shaped ice crystals, typically found in cirrus clouds, having no preferred orientation. Sunlight passing through the side faces of these crystals is refracted 22 degrees from the initial angle of incidence before exiting through an alternate side face. If the long axes of some of these same crystals are aligned so that they lie nearly in a horizontal orientation, upper tangent arcs (tangent to the top of the halo) may take shape. Always protect your eyes when looking toward the Sun.
Photo details: Camera Maker: Canon; Camera Model: Canon EOS DIGITAL REBEL XSi; Focal Length: 17mm; Aperture: f/5.6; Exposure Time: 0.0003 s (1/3200); ISO equiv: 400; Exposure Bias: none; Exposure: Manual; Exposure Mode: Manual; White Balance: Auto; Flash Fired: No (enforced); Orientation: Normal.