| Term: | Firn |
| Definition: |
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. NOAAHydrology
Firn is old snow that has been recrystalized into a more dense substance. Firn has a density greater than 0.55. Snowflakes are compressed under the weight of the overlying snowpack. Individual crystal near the melting point have slick liquid edges allowing them to glide along other crystal planes and to readjust the space between them. Where the crystals touch they bond together, squeezing the air between them to the surface or into bubbles. During summer we might see the crystal metamorphosis occur more rapidly because of water percolation between the crystals. By summer's end the result is firn -- a compacted snow with the appearance of wet sugar, but with a hardness that makes it resistant to all but the most dedicated snow shovelers! Several years are usually required for the snow to settle and to season into the substance we call glacier ice. 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. NOAA-NWS 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 NSIDCCryosphere (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. IHPGlacierMassBalance 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. WMOSeaIce 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. ECCCanada 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 IACSSnow 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. WMOHydrology (from the German) Dense, old snow in which the crystals are partly joined together, but in which the air pockets still communicate with each other. It has a sugary texture. Swisseduc Old snow that has become granular and compacted (dense) as the result of various surface metamorphoses, mainly melting and refreezing but also including sublimation. The resulting particles are generally spherical and rather uniform. Firnification, the process of firn formation, is the first step in the transformation of snow into land ice (usually glacier ice). Some authorities restrict the use of firn to snow that has lasted through one summer, thereby distinguishing it from spring snow. Originally, the French term, "n v ," was equivalent to the German term, "firn," but there is a growing tendency, especially among British glaciologists, to use "n v s" for an area of firn, that is, generally for the accumulation area above or at the head of a glacier. AMSglossary Neve on a glacier that survives the year's ablation season. With time much of the firn is transformed into glacial ice. PhysicalGeography GCW |