Refreezing of water within a glacier, between the summer surface and the bed, which goes undetected by measurements of surface mass balance. See mass-balance units, zone. Accumulation beneath the summer surface is the refreezing of surface meltwater (or freezing of rain) that is in transit and otherwise would have left the glacier as runoff. In the case of meltwater, it may be regarded as redistributing mass within the glacier. This may require careful accounting in the calculation of mass balance. Internal accumulation proceeds by the freezing of water that percolates early in the ablation season into firn that is still cold, heating the firn in the process, or by the freezing of retained pore water during the accumulation season, also releasing latent heat and thus slowing the downward advance of the winter cold wave. The term is reserved for refreezing beneath the summer surface, that is, within the firn or the ice. Meltwater that refreezes within the snow does not constitute internal accumulation since it is accounted for by end-of-season density measurements as part of conventional mass-balance measurements. Internal accumulation may be small in magnitude, and negligible on temperate glaciers, but if not accounted for it constitutes a bias towards overestimation of mass loss. In remote-sensing studies, it is not always possible to detect the summer surface. In addition models of the surface mass balance do not always distinguish between internal accumulation and refreezing within the snow. To avoid confusion, it is advisable to use 'internal accumulation' only in the sense given above and to use the more inclusive 'refreezing' only for 'internal accumulation plus refreezing within the snow'. Refreezing within the snow should be described as such explicitly.