Glaciers are masses of ice and debris which are continually changing and may be seen as an open system.
Inputs are precipitation and avalanches (things that increase the mass).
Outputs are evaporation and melt-water (things that decrease the mass).
Near the source where inputs are bigger than outputs it's called the zone of accumulation this is because there is more precipitation (the orographic effect). Temperature is lower here because of strong winds at a high altitude and and the reflective properties of snow. As temperatures are low, sublimation (solid into vapor) and other losses are low and melt-water is likely to refreeze
The zone of ablation is at a lower altitude towars the snout of the glacier. Outputs exceed inputs here. There is less snowfall and temperatures are higher, resulting in melting, sublimation, evaporation and calving.

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Folienset am The global distribution of cold environments, erstellt von Katy Thompson am 19/03/2016.