Halo polarization profiles and the interfacial angles of ice crystals

Polarization and radiance of various types of refraction halo in ice-crystal swarms that extend to ground level were measured as a function of scattering angle. Simultaneously, samples of the crystals that produce these halos were collected and replicated. The halo polarization peaks are wider than the Fraunhofer theory of diffraction predicts for the observed size distribution of the replicated crystals. The explanation we put forward is that the angles between crystal prism faces are not always exact integer multiples of 60°, and the basal faces are not always exactly parallel, as is usually assumed. The collected crystals confirm this. The widths of the halo polarization peaks can be explained if the distributions of the interfacial angles around their means reach their half-maximum values at a deviation of 0.49° ± 0.05°. This corresponds to a deviation of 0.35° ± 0.03° of the face normals from their crystallographic positions. The presence of variation in interfacial angles in low-level halos seems to ari e from the fact that the crystals are growing. Some hitherto unexplained features in halo displays can be understood by considering variations in the interfacial angles.