Noun
firn (countable and uncountable, plural firns)
A type of old snow which has gone through multiple thaw and refreeze cycles and thus is made of numerous small icy grains, though it is not nearly as saturated with water as snowcone slush is; can be hard or somewhat soft depending on recent and current weather conditions.
Synonym: névé
Further, the resolution of ice-cores is determined by the time of closure of the firn (solidification time of snow into ice). Source: Internet
The average air composition at closing depth (measured top-down in firn) is only 7 years older than the air of the atmosphere (measured at the South Pole). Source: Internet