Abstract
The halo literature contains many reports of circular halos having radii other than the usual values of 22° and 46°.1 Such odd radius halos are supposed to form by refraction and reflection of sunlight in pyramidal ice crystals, that is, in crystals having pyramidal faces instead of, or in addition to, the usual prism and basal faces. In this talk I will review some photographic evidence for the existence of odd radius halos and pyramidal crystals, and I will use computer simulations to reexamine the classical explanation of odd radius halos. I find no doubt regarding the existence of odd radius halos and pyramidal crystals. And the basic model of refraction and reflection in pyramidal crystals seems entirely justified, but classical predictions are significantly qualified and extended by the results of the simulations.