Loss of snow and ice from a glacier by sublimation, melting, and evaporation; and from the calving of icebergs, and avalanches. In temperate and subpolar regions melting is the major form of ablation; in the Antarctic, it is calving. The rate of loss varies with air temperature, relative humidity, wind speed, insolation, aspect, and the nature of the surface. In snowfields, ablation includes snow removed by the wind, and is affected by aspect, depth of snow, and the nature of the underlying surface.
The ablation sub-system, where annual ablation exceeds annual accumulation, lies between the firn line and the glacier snout. Where ablation occurs at the edges of glaciers, debris accumulates to form ablation moraine/ablation till. An ablation valley is a subsidiary valley formed beneath the crest of a lateral moraine and the valley side. Iturrizaga (2001) Geogr. J. 54 notes that ‘true ablation valleys’ can result from differences in insolation, and may act in the development of lateroglacial moraine valleys.