(1) Solid precipitation in the form of ice crystals, chiefly in complex branched hexagonal form and often agglomerated into snowflakes; or an accumulation of the same on the Earth's surface. (2) Solid precipitation that has accumulated on the summer surface on a glacier and that transforms to firn at the end of the mass-balance year. See zone. In this sense, which prevails almost universally in the study of mass balance, snow may contain ice in the form of lenses or pipes which are the result of refreezing of meltwater. (3) An accumulation of solid precipitation on a glacier that has not yet attained a density through compaction sufficient to restrict the circulation of air and water significantly. In this structural sense, the dividing line between snow and firn is diffuse but is conventionally taken to be near to a density of 400 kg m-3.