Plural of firn; granular snow that has partially compacted but hasn't yet become glacial ice, typically one to several years old.
Plural of firn, borrowed from German Firn (year-old snow) in the 19th century when glaciology became a modern science studying Alpine glaciers.
If you hike on glaciers, you're probably walking on firn—that crunchy, whitish snow that's denser than fresh snow but hasn't crystallized into the blue ice you see deeper down.
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