| Term: | Firn line |
| Definition: |
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. NOAAHydrology
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. 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, snow 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. NSIDCCryosphere 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. NOAA-NWS 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 equilibrium, 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. IHPGlacierMassBalance Boundary on the surface of a glacier which separates the zone of accumulation from the zone of ablation. WMOHydrology 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. AMSglossary The line or zone on a glacier that separates bare ice from snow at the end of the ablation season (cf. Equilibrium line and Snow line). SPRI GCW |