Scheiner's Halo: Cubic Ice or Polycrystalline Hexagonal Ice?

Abstract

A new explanation is proposed for the rare Scheiner's halo, observed in the sky at an angle of 28° from the sun or moon. The existing explanation invokes the presence in the atmosphere of the cubic form of ice, ice Ic. However, extensive laboratory work has not demonstrated that ice Ic can form under conditions found in the atmosphere. We point out an alternative, that polycrystals of ice Ih (the ordinary hexagonal polymorph), in which specific orientation relations exist between adjacent crystals, are another possible cause of Scheiner's halo. Polycrystals with the appropriate orientation relation are not uncommon in the atmosphere, but concentrations sufficient to produce optical effects are expected to be rare. There appears to be no decisive evidence to rule out either of these explanations.