'A glacier is simply the surplus ice that collects above the permanent snowline where the losses to summer melting are less than the gains from winter accumulation. A glacier flows downhill and crosses the permanent snowline from the area of snow gain to the zone of ice loss. The altitude of this permanent snowline is the equilibrium line: it marks the altitude at which snow gain (accumulation) is exactly balanced by melt (ablation) and is represented by the end-of-summer snowline.'