The solid, crystalline form of water substance; it is found in the atmosphere as snow crystals, hail, ice pellets, etc. , and on the earth's surface in forms such as hoarfrost, rime, glaze, sea ice, glacier ice, ground ice, frazil, anchor ice, etc. This form of water is, strictly speaking, called ice I, the only one of the several known modifications of solid water substance that is stable at commonly occurring temperatures and pressures. (Some of the other forms have very unusual properties, ice VII, for example, being stable only at pressures above 22 400 kg cm−2, but then existing at temperatures up to about 100°C. ) Ice has an open structure because the water molecules bond to their neighbors covalently only in four directions; it therefore floats on higher density water, where broken molecular bonds permit closer packing. All commonly occurring forms of ice are crystalline, although large single crystals are relatively rare except in glaciers. The ice crystal lattice possesses hexagonal symmetry that manifests itself in the gross forms of such single crystals as are sometimes found in snow. At an air pressure of one atmosphere, ice melts at 0°C by definition of the Celsius temperature scale. (Strictly at equilibrium among water, ice, and vapor occurs at +0. 01°C, the triple point. ) On the other hand, ice does not invariably form in liquid water cooled below this temperature; it has a tendency to supercool, more so in the absence of ice nuclei. See Bernal–Fowler rules, ice crystal.