The difference between the amount of ice a glacier gains in winter and the amount lost in summer. A glacier which is gaining mass has a positive mass balance: more ice is added in the winter than is lost in the summer. A negative mass balance indicates that the glacier is losing mass.

Two related terms are accumulation* and ablation. In a glacier there are generally two distinct zones, the accumulation zone (where snow cover remains at the end of the summer) and (at lower altitude) the ablation zone, where all the winter snowfall melts over the summer.

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