(1) All processes that reduce the mass of the glacier. (2) The mass lost by the operation of any of the processes of sense 1, expressed as a negative number. The main processes of ablation are melting and calving (or, when the glacier nourishes an ice shelf, ice discharge across the grounding line). On some glaciers sublimation, loss of windborne snow and avalanching are significant processes of ablation. 'Ablation', unqualified, is sometimes used as if it were a synonym of surface ablation, although internal ablation, basal ablation, and frontal ablation, especially calving, can all be significant in some contexts.