Halo Display Above Granby, Colorado
December 02, 2009
The very best halo displays are produced by diamond dust, small ice crystals drifting in the air near the ground. These crystals are simple hexagonal plates and columns and are to be distinguished from snowflakes, which are much more complicated and larger. Combinations of a few crystal shapes, orientations and ray paths account for almost all the halos we ever see. Just four such combinations, plus a mystery one, made this magnificent display near Granby, Colorado. Even the glints from the individual diamond dust crystals are visible.
From top reading downwards, the bright and colorful strip is a circumzenithal arc(CZA). Plate shaped crystal drifting with their large faces almost horizontal are responsible for these lovely arcs. The upwards curving arc touching the CZA is rarer. It's a supralateral arc made by column crystals with their long axes horizontal. The next arc is quite faint. It’s a rare Parry arc from rather improbably, oriented column crystals that do not rotate as they fall. Below that is another much more common arc, an upper tangent made by horizontal columns crystals. Sun rays passing through randomly oriented crystals generated the familiar circular 22-degree halo.
And now the mystery halo. The small ‘V’ shaped halo between the sun and the 22-degree halo is a rare Moilanen arc, named after its Finnish discoverer. All we know is that it’s formed by refraction between ice crystal faces inclined an unusual 34 degrees to each other. Researchers in Finland have sampled the crystals of many diamond dust displays showing the arc but so far none with the required properties have been found. Its origin remains a mystery.
Coordinates for Granby Colorado: 40.086396N 105.936487W