The part of the glacier where water from surface melting or rainfall percolates into the subsurface; see percolation, zone. In the upper percolation zone, above the wet-snow line, water percolates only into the snow. In the lower percolation zone, also called the wet-snow zone, water percolates into the firn below the summer surface. The lower percolation zone contains the slush zone. If, having percolated, the water refreezes, it warms its surroundings by releasing latent heat. If it refreezes in the firn, the result is internal accumulation. If it refreezes as a layer immediately above the summer surface, it forms superimposed ice. If this superimposed ice becomes exposed by continued surface ablation, the resulting superimposed ice zone is conventionally regarded as distinct from the percolation zone.