How does firn differ from fresh snow and glacier ice?

Firn serves as the intermediate stage between fresh snow and glacier ice, representing snow that has survived at least one melting season.12

Key Differences

• Fresh Snow: Characterized by low density (typically 50–250 kg/m³) and intricate, unstable crystal shapes like needles or stars. It is highly permeable to air and water.2
• Firn: A partially compacted, granular material with a density generally ranging from 300 to 840 kg/m³. It forms through the pressure of overlying snow, which causes recrystallization, localized melting, and the crushing of individual snowflakes.12
• Glacier Ice: The final stage of transformation, occurring when the material reaches a density of approximately 830–840 kg/m³. At this point, the air spaces between grains are sealed off, making the ice impermeable to air and water. Unlike the granular structure of firn, glacier ice consists of an aggregate of interlocking single crystals.12

The transition time from snow to glacier ice varies significantly based on the environment, ranging from a few years in warm, wet climates to thousands of years in polar regions.2