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Definition:
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The set of points on the surface of a glacier delineating the firn area and, at the end of the mass-balance year, separating firn (usually above) from glacier ice (usually below).In steady state and equilibrium, and in the absence of superimposed ice, the firn line coincides with the equilibrium line. However, the equilibrium line will generally be above the firn line in a year of negative mass balance; in a year of positive mass balance it will in general be below the firn line of the previous year.
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