Upper Tangent Arc from Orsta, Norway

April 23, 2013

Upper Tangent Arc from Orsta, Norway2

Photographer: Geir T. Oye; Geir's Web site
Summary Author: Geir T. Oye; Jim Foster

This jaunty upper tangent arc (UTA) was visible in the afternoon of December 24, 2012, from Orsta, Norway. It’s tangent to the 22 degree halo -- not visible here. To see UTA’s such as this, the Sun has to be low in the sky. However, once the Sun attains a position greater than about 29 degrees above the horizon, this upper arc and the corresponding lower one (directly below the Sun) will wrap around the 22 degree halo – circumscribed arc. UTAs form when the hexagonal ice crystals that compose cirrus clouds are aligned with their long axes horizontal, but they’re oriented in different directions. Sunlight entering one of the pencil-shaped crystals' side faces is bent before exiting through an alternate side face.

Photo details: Canon EOS 650D camera; sigma 17-70 mm lens; ISO 100; 1/320 sec. exposure. Photo was taken at 2:16 p.m.