Sundogs and Tangent Arc
August 11, 2003
The above photo was taken near Alamogordo, New Mexico and shows two sundogs (parhelia) on either side of the Sun, and a tangent arc above the Sun. These arcs are formed by sunlight refracting through hexagonal ice crystals in cirrus clouds. Discrete points of light occur because the crystals are oriented -- the sundogs are formed by flat crystals floating with their flat sides parallel to the ground, and the tangent arc is formed by columnar crystals floating with their long axis parallel to the ground. The faint circular halo connecting the bright spots results from refraction by more randomly oriented crystals. Due to the refractive index of ice, the bright points and the halo itself are 22 degrees away from the Sun. Note the jet contrail, which nearly bisects the halo.
This picture was taken with an HP C618 digital camera.