Trigonal ice crystals in Earth's atmosphere

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.