Firn is a granular ice mass (density: 450-830 kg/m3) composed of crystals or aggregates of crystals of grain size up to 12 mm.

It is formed by recrystallisation and/or multiple freezing and freezing of snowflakes especially on the upper part of a glacier where it has not been compressed into ice. It is a transitional stage between snow and glacial ice. The speed of the process of converting snow into glacial ice depends on many factors, like the amount of snowfall, temperature fluctuations and amount of melting water.

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