The mystery deepens. In two previous posts we wondered why some displays are great in their column orientation halos even though the crystals have well caved ends. Here we show a case that appeared on November 22, 2015 in Rovaniemi, where crystals seem not much different, yet rare halos requiring basal faces are completely absent. Even the 46° supralateral arc gives just a whiff. Poor crystal orientations can’t explain the absence of rare halos as the tanget arc is quite sharp. Had we known only about this display, we would be quite happy to explain with cavities, but knowing about the other displays, it is quite puzzling.

Marko Mikkilä, Jarmo Moilanen, Marko Riikonen