Upper Tangent and Supralateral Halo

April 24, 2006


Provided by: Franck Schwitter
Summary authors & editors: Franck Schwitter; Jim Foster; Les Cowley

The photo above showing two jaunty sky arcs was taken this past winter from near Crans Montana, in Switzerland. At lower center is an upper tangent arc, which is tangent to the 22 degree halo (not visible here), and at top is an especially bright and colorful portion of the 46 degree halo. Columnar ice crystals are responsible for forming tangent arcs (essentially a circumscribed arc at a lower solar angle). These crystals are aligned so that their axes are all in a horizontal direction. However, even though they're all horizontal to the surface, they're not all aligned in the same direction. Light enters one of the 6 side faces and exits through an alternate side face. The shape of tangent arcs change as the Sun moves up from and down toward the horizon. In this case, the Sun was about 10-15 degrees above the horizon.

The arc at top (curving downwards) is a supralateral arc -- also produced by columnar crytals. Note that a fragment of an upward curving arc (near the top most portion of the supralateral arc) can be detected. This is a portion of the circumzenithal arc. See also the Earth Science Picture of the Day for March 17, 2006.

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