Also: Firn Spiegel, firn mirror, glacier fire
An ice crust formed on the snow surface on sunny, cold days. The sun's heat penetrates the surface snow layers and causes melt around the grains beneath the surface but meltwater at the surface is refrozen forming a thin layer of ice. This requires just the right heat balance.
Firnspiegel is highly reflective and mirror-like, giving it the colloquial names of firn mirror and glacier fire..
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