Upper Tangent Arc Sequence

January 29, 2013


Photographer: Zsofia Biro
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: 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.