Moilanen arc observations from Davos

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