Ice accumulated on the current summer surface, during the current mass-balance year, by the refreezing there of rain or meltwater. Superimposed ice is not the same thing as internal accumulation, which represents refreezing below the summer surface. Superimposed ice becomes glacier ice at the end of the mass-balance year. Superimposed ice requires special attention in conventional mass-balance programmes. In a pair of stake measurements, at the start and end of the ablation season, accumulation of superimposed ice causes a decrease of the distance from the top of the stake to the ice surface (regardless of any overlying snow). This decrease is real and not, for example, due to faulty book-keeping.