Answer and explanation:
The glacial budget describes how ice accumulates and melts on a glacier, which ultimately determines whether or not a glacier advances or retreats. The balance of the accumulating ice is weighed against the melting ice, and whichever is greater determines whether the glacier will advance or retreat. In the zone of accumulation, the rate of annual snowfall is greater than the rate of melting. In other words, not all of the snow that falls each winter melts during the following summer, and the ice surface is always covered with snow. On the other hand, in the zone of melting or ablation more ice melts then accumulates as snow during the year. The equilibrium line marks the boundary between the two zones of accumulation and ablation. Below the equilibrium line, in the zone of melting, bare ice is exposed because the last winter’s snow had all melted; above that line, the ice is still mostly covered with snow from last winter. The position of the equilibrium line changes from year to year as a function of the balance between snow accumulation in the winter and snowmelt during the summer. More winter snow and less summer melting obviously favors the advance of the equilibrium line and of the glacier’s leading-edge (or terminus). Though, of these two variables, it is the summer melt that matters most to a glacier’s budget. Cool summers promote glacial advance and warm summers promote glacial retreat.
