The conversion of snow to firn and then to glacier ice. Newly fallen snow has a variable density depending on the meteorological conditions of its formation and deposition. The density of dry snow increases rapidly at first, by the conversion of snowflakes to grains. Then, usually under the pressure of an increasing overburden of newer snow, density increases more slowly by settling of the grains to about 550 kg m-3, representing the maximum practically attainable packing. Snow becomes firn (in the structural sense) over a range of density beginning at about 400 kg m-3.Beyond the maximum packing density, even slower mechanisms of densification sintering and plastic deformation of the grains, and recrystallization become dominant. When the firn reaches a density of about 830 kg m-3, the pore spaces between crystals are closed off, air can no longer flow (as opposed to diffusing through the crystal lattices), and the substance is deemed to be glacier ice. When there has been no melting, densification rarely proceeds beyond 400 kg m-3 over the course of a typical mid-latitude winter. Depending on the accumulation (that is, loading) rate, glacier ice may be produced in times from a few years to a few centuries. Melting followed by refreezing can yield bulk densities near that of pure ice in times shorter than a day.