Abstract
Abstract We are all familiar with the hexagonal shape of snow and ice crystals, and it is well established that their sixfold symmetry is derived from the arrangement of water molecules in a hexagonal crystal structure. However, atmospheric ice crystals with only threefold rotational symmetry are often observed, which is inconsistent with the hexagonal crystal structure of ordinary ice. These crystals are found in a wide range of different cloud types ranging from upper-tropospheric cirrus to contrails and diamond dust and they form at temperatures ranging from about −84° to −5°C. Recent experimental studies of ice crystal structures have shown that ice under a wide range of atmospheric conditions does not always conform to the standard hexagonal crystal structure. Instead, sequences of the hexagonal structure can be interlaced with cubic sequences to create stacking-disordered ice. This degrades the symmetry of the crystal structure so that, instead of having a hexagonal structure, they have a trigonal structure with a corresponding threefold symmetry. Hence, this implies that atmospheric ice crystals with threefold symmetry are made of stacking-disordered ice. We conclude that the presence of trigonal crystals in the atmosphere is consistent with rare Parry arc halos and also show that they have distinct radiative properties compared with hexagonal ice.
Highlights
For many years it has generally been assumed that ice crystals
recent crystallographic work on the structure of ice has shown that ice crystals contain stacking disorder
laboratory conditions is consistent with the presence of stacking-disordered ice
Summary
Isd. we ask the following: what is the shape (or habit) of an ice crystal that contains stacking disorder growing under isotropic conditions? The sidebar on “Crystallography and crystal shape” introduces the concept of crystallographic. | 1521 an equilateral-triangular shape (Fig. 7i). Similar to ice Ih crystals, ice Isd crystals can appear as plates or columns depending on the relative growth rates of the basal and prismatic faces. 1–4 are consistent with trigonal symmetry in the crystal structure and we use the term trigonal ice crystals when referring to these crystals
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