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 inc
reases 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.
Density is the ratio of the mass of an object to its volume. Snow has a density averaging about 0.1, firn has a density greater than 0.55, and glacier ice has a density of about 0.89. The density of u
nmineralized fresh water is 1. Glaciologists measure snowpack density frequently so that they may anticipate future water supplies, and to assess avalanche hazards. The density of a fresh snowpack is about 0.1; firn has a density of about 0.55 and glacier ice, of about 0.89. Each annual snow layer has a characteristic grain size and density.
The ratio of the mass of any substance to the volume that it occupies. Density is expressed in kg m3. The density of the matter constituting the glacier can range from as low as 10 kg m3, at the surfa
ce in unusual weather, to the density of pure ice at depths at which all air has been squeezed out of bubbles. It is very common to assume that the bulk density of the glacier is 900 kg m3. This reduced density is a rough-and-ready allowance for the presence of snow and firn, large voids (crevasses, moulins and subglacial cavities) and sediment. Where a large proportion of the glacier thickness consists of snow and firn, a bulk density even lower than 900 kg m3 is appropriate. Where there is relatively little snow or firn, and the temperature is very low, a higher density, approaching or even exceeding the conventional 917 kg m3, may be appropriate. In studies of mass balance, however, densities are never known with the accuracy of laboratory measurements of pure ice, which are made by measuring the lattice parameters of single crystals. Typical field instruments are hand-held corers and spring balances, and inaccuracies of the order of 48% are usual. Better accuracy is possible in principle with advanced devices such as neutron-scattering probes, but these are not in routine use. In some circumstances, such as when a load of low-density snow produces compensating densification at depth, the density of the mass gained or lost by the glacier may be assumed equal to the bulk density. See Sorge's law.
a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure of many-body systems, in particular atoms, molecules, and t
he condensed phases.
The ratio, expressed as a percentage, of the volume which a given quantity of snow would occupy if it were reduced to water, to the volume of the snow. When a snow sampler is used, it is the ratio exp
ressed as percentage of the scale reading on the sampler to the length of the snow core or sample.