Halo arcs from airborne, pyramidal ice crystals falling with their c axes in vertical orientation

Many halo arcs are caused by pyramidal crystals that have {1 0 -1 1} crystal faces. We treat halo arcs arising from pyramidal crystals that fall in the air with their c axes vertically oriented. To our knowledge only 6 of the 12 possible halo phenomena that belong to this category have been dealt with in the literature. Surprisingly the yet undiscussed halos are predicted to be of comparable intensity with those already treated. They are produced by reflections from pyramidal crystal basal faces. A theoretical summary and computer simulations are presented of the mentioned 12 halo phenomena and of the individual arcs into which they break in the sky. We give an overview to the current level of documentation of these phenomena by listing the first published photographs of each phenomenon known to the authors.