Halo observations provide evidence of airborne cubic ice in the Earth’s atmosphere

Abstract

An ice crystal halo display that contains several previously unknown halo phenomena was observed in Northern Chile. Analysis of computer simulations of the halos demonstrates that most of the new halo arcs in the display can be explained by the presence of airborne and preferentially oriented crystals of cubic ice. These observations therefore provide evidence of the existence of the cubic phase of ice in the Earth's atmosphere.

Structure of ice crystallized from supercooled water

“…Although evidence from diffraction measurements is absent, crystals grown from water vapor at approximately 200 K can have a cubic habit (35) and 25% of crystals in the Antarctic stratosphere were solid cubes during one campaign (36). In addition, a halo at 28°from the sun has been photographed and is consistent with the rare presence of octahedral crystals of ice I c in the atmosphere (37). Perhaps the dearth of hard evidence for the existence of well-ordered ice I c indicates that it is rare and only forms under very specific conditions, which diffraction experiments have not yet accessed.…”

Production of Ice in Tropospheric Clouds: A Review

“…Evidence for cubic ice in the atmosphere includes photographs of a 28° halo (Riikonen et al 2000) and sampled crystals that had an unmistakable cubic morphology (Goodman et al 1989). Like supercooled water, the cubic form of ice is not a stable phase (Eisenberg and Kauzmann 1969), which may have consequences for ice in the atmosphere.…”

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.…”