Arcs and mock-suns tower over and even shine among the skiers. Diamond dust, nearby ice crystals, made this. Geometry begets geometry. The precise curves and circles are creations of sunrays refracted and reflected by even more precisely figured hexagonal prism ice crystals**.
Diamond dust makes the best halos and ski areas have some of the best diamond dust. The crystals are a by-product of snow machines yet we should not be disappointed at this lowly origin for the displays are natural. Only a seed speck inside each ice crystal is courtesy of the ski slope. Snow machines spray out small particles, sometimes an enzyme, to act as nuclei for snow growth. Some nuclei escape and instead seed the natural growth of halo forming crystals up to several kilometres downwind. The crystals are oft of superb optical quality.
**We are certain of this for all but one of these halos, the faint V-shaped arc immediately over the sun, a Moilanen arc, whose origin is not yet explained. It was simulated in HaloSim at right using a rather artificial 37° prism.
The halos - rare - not so rare - frequent:
Upper tangent arc