What line separates a glacier's accumulation area from its ablation area?

The line separating a glacier's accumulation area from its ablation area is the equilibrium line.

At this boundary, the glacier's net surface mass balance is approximately zero: accumulated snow and ice equal the mass lost through melting, sublimation, evaporation, wind removal, or other ablation over the balance year. The upper region is the accumulation zone, while the lower region is the ablation zone.

Scientists often refer to the equilibrium line's elevation as the equilibrium-line altitude, or ELA. Its position changes from year to year with snowfall and summer temperatures, so it is not a permanent stripe painted across the ice. A higher ELA generally indicates that more of the glacier lies in a loss-dominated environment.

The equilibrium line is sometimes confused with the firn line. A firn line describes where old snow or firn persists, while the equilibrium line is defined by mass balance. In some conditions they may appear close, but they are not identical concepts.