Lower Tangent Arc
March 17, 2006
Provided by: Franck Schwitter
Summary authors & editors: Franck Schwitter; Jim Foster; Les Cowley
This stirring photo featuring a magnificent lower tangent arc was captured near Crans Montana, in Switzerland (taken earlier this winter). Though no green colors are visible, it looks like the work of a leprechaun. The arch like patch of light is tangent to the 22 degree halo (not pictured) and is found directly beneath the Sun. It bends downward or away from the Sun, and is positioned approximately 22 degrees below the Sun itself - about 2 fists held at arms length. In order to be seen as vividly as shown here, the Sun must be rather low in the sky, but at least 22 degrees above the horion. Columnar ice crystals, in this case floating just above the surface, are responsible for tangent arcs, both the upper an lower tangent arcs. 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 snow nucleation additives now being used in many snow machines produce near perfect hexagonal ice prisms. As a result, in ski areas, downwind of the snow making machines, we're able to see many more exceptional halo phenomena than in years past.
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