Halo arcs from airborne, pyramidal ice crystals falling with their c axes in vertical orientation

Abstract

Many halo arcs are caused by pyramidal crystals that have {1 0 -1 1} crystal faces. We treat halo arcs arising from pyramidal crystals that fall in the air with their c axes vertically oriented. To our knowledge only 6 of the 12 possible halo phenomena that belong to this category have been dealt with in the literature. Surprisingly the yet undiscussed halos are predicted to be of comparable intensity with those already treated. They are produced by reflections from pyramidal crystal basal faces. A theoretical summary and computer simulations are presented of the mentioned 12 halo phenomena and of the individual arcs into which they break in the sky. We give an overview to the current level of documentation of these phenomena by listing the first published photographs of each phenomenon known to the authors.

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

“…This seems to be the first observation of the recently predicted 23°lower parhelion. 2 The inner edge of the broad haloing below the Sun is too faint to be measured reliably.…”

“…These crystals cause circular halos with radii of 9°, 18°, 20°, 23°, 24°, and 35°, as well as their associated arcs. 1,2 There is also some evidence of the presence of nontypical pyramidal crystals with high pyramid apex angles between 170°and 176°. These crystals are believed to give rise to elliptical halos: rare transient phenomena observed close to the position of the Sun in the sky.…”

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.…”

Symmetry in halo displays and symmetry in halo-making crystals

“…This paper explores potentials and limitations of crystal determinations from halo observations. One motivation is the expanding number of photographically documented observations of halo arcs resulting from pyramidal ice crystals 1,2 or perhaps from even more-exotic crystals. 3 A second motivation is the possibility that the Huygens probe 4 will detect halos resulting from exotic crystals during its descent in January 2005 through the atmosphere of the Saturnian satellite Titan.…”