May 06, 2007
While skiing at Whistler, British Columbia (to be precise, on the Cloud Nine run near the top of Blackcomb mountain), we came across this strange phenomenon. Ice crystals in the air caught the Sun in such a way that there appeared to be a ball of light hovering just above the ground. There was a three dimensional element to the effect, which appeared to occupy a well-defined space.
This arch shaped light patch is a lower tangent arc. It's tangent to the 22 degree halo (not pictured) and is found approximately 22 degrees below the Sun itself. Columnar ice crystals, in this case floating just above the surface, are responsible for 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. It seems that the snow nucleation additives used in many snow machines produce nearly perfect hexagonal ice prisms. As a result, downwind of the snow making machines, exceptional halo phenomena can on occasion be viewed. See also the Earth Science Picture of the Day for March 17, 2006.