Trigonal ice crystals in Earth’s atmosphere

We are all familiar with the hexagonal shape of snow and ice crystals, and it is well established that their six-fold symmetry is derived from the arrangement of water molecules in a hexagonal crystal structure. However, atmospheric ice crystals with only three-fold 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 a 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 corresponding three-fold symmetry. Hence, this implies that atmospheric ice crystals with three-fold 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 to hexagonal ice.