Moilanen arc in Davos. Photo: Bertram Radelow
This report describes halo displays including Moilanen arcs (MAs) which occurred in Davos from 2005 to 2017, caused by snow gun induced diamond dust. Four of them are discussed in more detail. The MAs are presented in relation to the degree of ripening of the diamond dust crystals, and with respect to the other halo types in the displays.
Davos is a world-renowned winter sport resort. Because of the decreasing amount of natural snow over the past decades, numerous snow guns have been installed. In the winter season 2017/18, more than 150 of them are running.
Snow guns produce artificial snow made from clumps of small ice spheres without any discernible hexagonal crystal structures. However, a part of the water spray is converted into microscopically small ice spheres which drift off. These will serve as condensation seeds for larger crystals, often in column shape, but also plates do occur.
Remarkably, MAs are preferably generated during the first stage of crystal growth. Full-ripe crystals produce only dull MAs or none at all.
Author: Bertram Radelow, Davos, Switzerland
Concave and convexe Parry arc. Photos: Ruben Jakob
At the Fichtelgebirge, a low mountain range in northeastern Bavaria, there were similar halo phenomena as in the ore mountains circa 100km away (see article). In the morning, the high fog staunched and dissolved while passing the mountains. From afar, a very interesting cloud edge was seen at Mount Schneeberg (1051m above sea level). Shortly ahead Schneeberg, the temperature fell below -8 °C and rose to -5 °C in the fog. In the valley between the two mountains diamond dust was non-existent. I kept searching and trying to get out of the fog. Just in the village Grassemann (about 700m above sea level) there was a right parhelion and shortly afterwards a Supralateral arc visible above the fog. After 20 minutes, the fog suddenly came up to me and conjured a variety of halos in the sky. When I saw the concave and the convex Parry arc as well as the Moilanen arc distinctly, I was speechless. After that, the halo activity waned slowly. At 10 o’clock there were no more halos seen. The high fog broadened increasingly and the sun couldn’t shine through.
Grassemann 09.17 CET
Grassemann 09.18 CET
Grassemann 09.18 CET
Grassemann 09.21 CET
Grassemann 09.25 CET
Grassemann 09.26 CET
Grassemann 09.31 CET
Grassemann 09.43 CET
Ochsenkopf 11.13 CET
Ochsenkopf 11.16 CET
Ochsenkopf 11.24 CET
Ochsenkopf 11.27 CET
Ochsenkopf 11.38 CET
Ochsenkopf 11.40 CET
Ochsenkopf 11.41 CET
All in all, the following halo types were visible:
– Upper tangent arc
– Upper and lower sun pillar
– Circumzenithal arc
– 46°-halo and Supralateral arc
– Fragment of parhelic circle
– Upper concave and convex Parry arcs
– Upper Tape arcs
– Heliac arc
– Subhelic arc
– Moilanen arc
Afterwards I drove to the top of Mount Ochsenkopf (1024m above sea level). There was a second halo show starting at 11.10 CET. However, expect of the right infralateral arc with the Tape arc, there were no further halo types.
Author: Ruben Jacob, Burgkunstadt, Germany