with input-output accounting

B_n = inputs - outputs where B_n is the net mass balance across the whole glacier for a year

OR

B_n = accumulation - ablation

where B_n is the change in mass of the glacier, accumulation refers to all sources of snow and ice (inputs) into the glacier and ablation refers to all losses of ice (outputs)

all processes by which material is added to a glacier

snowfall

windblown snow

avalances off valley walls

rain that freezes in the snow or on the ice

accumulation takes place at or near the surface of the glacier

all processes by which material is lost from a glaicer

melting followed by runoff

evaporation (sublimation)

removal by wind

calving of icebergs

not to be confused with some surface melting and refreezing either within the snowpack or on the ice, this isn't ablation b/c water is not lost from the glacier

at the surface

in the case of calving at the glacier terminus

this is the term for how a glacier advances down its valley or an ice sheet will spread out accross a landscape when accumulation is greater than ablation on the glacier or ice sheet over a period of years

B_n has a postive value

this is the term for how a glacier will retreat up its valley or ice sheet will shrink in size when ablation is greater than accumulation over the glacier or ice sheet over a period of years

B_n has a negative value

imbalances between inputs (accumulation) and outputs (ablation) of ice)

glaciers also respond to changes in mass balance by thickening or thinning

found at each elevation on the glacier

the sum of the positive winter balance and the negative summer balance