In hydrologic terms, old snow on top of glaciers, granular and compact and not yet converted into ice. It is a transitional stage between snow and ice. Also called Neve. Rounded, well-bonded snow that
is older than one year; firn has a density greater than 550 kilograms per cubic-meter (35 pounds per cubic-foot); called n (1) Snow that has survived at least one ablation season but has not been transformed to glacier ice. This sense prevails in the study of mass balance. Snow becomes firn, by definition, at the instant when the mass-balance year ends. See zone. (2) Structurally, the metamorphic stage intermediate between snow and ice, in which the pore space is at least partially interconnected, allowing air and water to circulate; typical densities are 400830 kg m3. In this sense, the firn is generally up to a few tens of metres thick on a temperate glacier that is close to a steady state, and up to or more than 100 m thick in the dry snow zone on the ice sheets. Old snow that has recrystallized into a dense material. Unlike ordinary snow, the particles are to some extent joined; but, unlike ice, the air spaces in it still connect with each other. Sea ice terminology that describes old snow which has re-crystallized into a dense material. Unlike ordinary snow, particles are (to some extent) joined together; but, unlike ice, the air spaces in it still connect with each other. Well-bonded and compacted snow that has survived the summer season, but hasnot been transformed to glacier ice. Typical densities are 400-830 kg m3 (perennialsnow, n An intermediate stage in the transformation of snow to glacier ice. Snow becomes firn when it has been compressed so that no pore space remains between flakes or crystals, a process that takes less than a year. Old snow which has become granular and dense under the action of various processes of melting and refreezing, including sublimation and crystallization.
The zone of the glacier where the summer surface is underlain by firn instead of glacier ice. Changes in extent of the firn area, and thickness of the firn, complicate mass-balance calculations by the
geodetic method since Sorge's law no longer applies. The firn area is not the same as the accumulation zone.
The highest level to which the fresh snow on a glacier's surface retreats during the melting season. The line separating the accumulation area from the ablation area.
In hydrologic terms, the highest level to which the fresh snow on a glacier's surface retreats during the melting season. The line separating the accumulation area from the ablation area.
A line across the glacier, from edge to edge, that marks the transition between exposed glacier ice (below) and the snow-covered surface of a glacier (right). During the summer melt season, this line
migrates up-glacier. At the end of the melt season the firn line separates the accumulation zone from the ablation zone.
The set of points on the surface of a glacier delineating the firn area and, at the end of the mass-balance year, separating firn (usually above) from glacier ice (usually below).In steady state and e
quilibrium, and in the absence of superimposed ice, the firn line coincides with the equilibrium line. However, the equilibrium line will generally be above the firn line in a year of negative mass balance; in a year of positive mass balance it will in general be below the firn line of the previous year.
The boundary of the area of snow on a glacier surviving one year's ablation, thus becoming firn. In the absence of superimposed ice, this limit is equivalent to the equilibrium line. See climate snow
line.
A line that marks the limit on a mountain above which snow persists from one winter to the next is called the annual snowline, and this line on a glacier is called the firnline. Above the firnline, sn
ow that falls each year packs down and changes into glacier ice as air is slowly forced out of it. This part of the glacier is its accumulation area where more snow falls each year than is lost by melting. Below the firnline is the ablation area, where melting predominates. The minimum elevation of firn lying on a glacier surface; each year's firn line marks a glacier's annual equilibrium line; also called firn limit.
A thin, highly reflective sheet of clear ice formed at the snow surface; formed in spring when subfreezing air temperatures combine with subsurface melting (due to penetration of solar energy into the
snow).