Crystals of hexagonal ice with (2 0 -2 3) Miller index faces explain exotic arcs in the Lascar halo display

Abstract

This article focuses on the 1997 Lascar halo display, during which very unusual arcs and halos were documented. Photographs have been analyzed with the aid of a specific image processing method developed by us. Using crystals of hexagonal ice with exotic (2 0 2 3) Miller index faces, it is possible to simulate all the features of the display with constant crystal populations and oriented crystals in plate orientation. The simulations perform better than the cubic ice explanation. The existence of (2 0 2 3) crystal faces is supported by a 1998 South Pole halo display.

Formation of atmospheric halos and applicability of geometric optics for calculating single-scattering properties of hexagonal ice crystals: Impacts of aspect ratio and ice crystal size

“…Several studies have simulated circumscribed halos and other optical phenomena by calculating ray paths that depend on the relative positions between the Sun and ice crystals (or orientations of ice crystals) and on the sunlight path through the faces of crystals [7][8][9][10][11][12][13][14][15][16][17]. For example, the 221 and 461 halos are caused by the minimum deviation of sunlight due to the refraction through a 601 prism formed by alternate side faces, and a 901 prism formed by the side and basal faces of hexagonal crystals, respectively.…”

“…Several programs (e.g., HaloSim [18] and HaloPoint [19]) using known minimum deviations of sunlight for the given shapes of ice crystals and the relative positions between the Sun and ice crystals are available for such simulations. Previous studies also estimated the geometries of ice crystals based on the angles at which halos were observed, in particular, to explain rarely seen circumscribed halos, such as 91, 181, 201, Scheiner's halo at $281 and 351 [20,21,13,16,17].…”

Dynamic signatures of the transition from stacking disordered to hexagonal ice: Dielectric and nuclear magnetic resonance studies

“…Starting with the observation of Scheiner's halo in 1629, claims were made about the occasional existence of cubic ice crystals in clouds, 1-3 e.g., in noctilucent mesospheric or polar stratospheric clouds. These claims are being debated since also polycrystals of hexagonal ice 4,5 or trigonal crystals may be of relevance in this context. 6 The higher vapor pressure and the lower thermal conductivity 7 of ice I c as compared to ice I h might be of importance in cloud freezing and persistent in-cloud supersaturations in cold cirrus.…”