From Wikipedia, the free encyclopedia

Ice I_h is the hexagonal crystal form of ordinary ice, or frozen water. Virtually all ice in the biosphere is ice I_h, with the exception only of a small amount of ice I_c which is occasionally present in the upper atmosphere. Ice I_h exhibits many peculiar properties which are relevant to the existence of life and regulation of global climate.^[1]

Ice I_h is stable down to −200 °C (73.1 K; −328.0 °F) and can exist at pressures up to 0.2 GPa. The crystal structure is characterized by hexagonal symmetry and near tetrahedral bonding angles.