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 {10-11} crys tal faces. We treat halo arcs arising from pyramidal crystals that fal l in the air with their c axes vertically oriented. To our knowledge o nly 6 of the 12 possible halo phenomena that belong to this category h ave been dealt with in the literature. Surprisingly the yet undiscusse d halos are predicted to be of comparable intensity with those already treated. They are produced by reflections from pyramidal crystal basa l faces. A theoretical summary and computer simulations are presented of the mentioned 12 halo phenomena and of the individual arcs into whi ch they break in the sky. We give an overview to the current level of documentation of these phenomena by Listing the first published photog raphs of each phenomenon known to the authors. (C) 1998 Optical Societ y of America.