Slow growth of grains and bonds within the snowpack resulting in a decrease of the specific surface area of snow. Causes particles to round off. Works at low temperature gradients, i.e., when excess w
ater vapour density is below the critical value for kinetic growth to occur. An extreme case of equilibrium growth is isothermal or equi-temperature growth in dry snow. This is the type of metamorphism that in nature occurs only in the centre of polar ice shields and may allow grains to develop facets. The latter is still a matter of research.
The line or zone on a glacier's surface where a year's ablation balances a year's accumulation (cf. Firn line). It is determined at the end of the ablation season, and commonly occurs at the boundary
between superimposed ice (q.v.) and glacier ice.
The set of points on the surface of the glacier where the climatic mass balance is zero at a given moment. The equilibrium line separates the accumulation zone from the ablation zone. It coincides wit
h the snowline only if all mass exchange occurs at the surface of the glacier and there is no superimposed ice. Unless qualified by a different adjective, references to the equilibrium line refer to the Annual equilibrium line. See also equilibrium-line altitude, firn line, snowline, transient equilibrium line, zone.
The spatially averaged boundary at a given moment, usually chosen as the seasonal mass budget minimum at the end of summer, between the region on a glacier where there is a net annual loss of ice mass
(ablation area) and that where there is a net annual gain (accumulation area). The altitude of this boundary is referred to as equilibrium line altitude (ELA).
The boundary on a glacier between the ablation area and accumulation area. No net mass is gained or lost at this location. In the absence of superimposed ice, this line is equal to the snow line at th
e end of the mass balance year.
The line or zone on a glacier where a year's ablation balances a year's accumulation. The equilibrium line is determined at the end of the ablation season (cf. Firn line and Snow line).
The spatially averaged altitude of the equilibrium line. The ELA may be determined by direct visual observation, but is generally determined, in the context of mass-balance measurements, by fitting a
curve to data representing surface mass balance as a function of altitude (see mass-balance profile). This is often an idealization, because the equilibrium line tends to span a range of altitudes. Many approximations of the ELA have been suggested; the glaciation level and the mid-range altitude are examples. The ELA is understood to be the Annual ELA unless it is qualified as the transient ELA.