Upper Tangent Arc Sequence
January 29, 2013
Photographer: Zsofia Biro; Zsofia's Web site
Summary Authors: Zsofia Biro; Jim Foster
The photo sequence above showing the formation of an upper tangent arc was snapped from Budapest, Hungary, near sunset on December 13, 2012. Look for this sometimes quite bright and fan-shaped arc directly above the Sun. You’ll find these arcs tangent to the 22 degree halo, or where the halo would be if it were visible. It's formed by pencil-shaped hexagonal ice crystals found in cirrus clouds. These crystals are aligned more or less horizontally (like a pencil lying flat on a table) but have random orientations. Sunlight enters through one of the crystal's side faces and exits through an alternate side face. In the sequence at top, note how the jet contrail brightens and dims in concert with the upper tangent arc.
Photo details: Camera Maker: Canon; Camera Model: Canon EOS 1000D; Focal Length: 55.0mm; Aperture: f/5.6; Exposure Time: 0.0010 s (1/1000); ISO equiv: 100; Software: Adobe Photoshop CS3 Windows.