Firn and the Snowline
A glacier is not just frozen water. Between the snow that falls each winter and the dense blue ice you see at the toe lies firn — half-snow, half-ice — and a transition zone called the equilibrium line. Understanding firn and the snowline is essential to understanding what a glacier is and how it responds to climate.
From snow to ice
Fresh snow has a density of around 50-200 kg/m³. Glacier ice has a density of about 900 kg/m³. The intermediate material, called firn, is snow that has survived at least one summer and recrystallized: it has been compacted, sintered together, and may also have been wetted and refrozen. Firn densities range from roughly 400 to 800 kg/m³.
What is firn?
Firn is granular, slightly icy snow. You can dig into it with a hand or ice axe. In a deep snowpit it looks layered, with each year's summer surface showing as a dust line or icy crust. Eventually the spaces between the grains close off — typically at 50 to 100 metres depth on a cold glacier, less on a temperate one — and the firn becomes solid glacier ice.
The equilibrium line
The equilibrium line of a glacier is the boundary between the zone where snow accumulates over a year (the accumulation zone) and the zone where the glacier loses ice over the year (the ablation zone). At the equilibrium line, net balance is zero.
The equilibrium line altitude (ELA) is the single most important number for understanding a glacier's state. Watching it move up or down year by year tells you everything you need to know about whether the glacier is gaining or losing.
Snowline vs equilibrium line
The snowline at the end of summer is, on most temperate glaciers, a very good approximation of the ELA — because below the snowline the surface is bare ice (ablation zone), and above it is the remaining snow of the year (accumulation zone). Glaciologists photograph this line every September to estimate the year's mass balance.
Why snowline matters
A small rise in temperature does not directly remove ice — but it raises the snowline by hundreds of metres. On a small Alpine glacier, when the snowline rises above the top of the glacier in a given summer, the entire accumulation area disappears for that year. Without accumulation, the glacier loses mass everywhere on its surface — and once this happens routinely, the glacier cannot survive.
How firn changes a glacier
A thick firn zone acts as a buffer. Meltwater on the surface in summer percolates into the firn and refreezes there, releasing heat and warming the firn. In bad years, large quantities of meltwater leave the glacier through englacial conduits instead of staying as firn. Repeated bad years progressively destroy the firn zone, and the glacier becomes much more vulnerable.
Cold, polythermal and temperate
The structure of firn depends on temperature:
- Cold: firn stays below 0 °C; little meltwater; firn extends to great depth.
- Polythermal: firn includes some refrozen meltwater layers; typical of high Arctic and high Asian glaciers.
- Temperate: firn is at 0 °C; meltwater is everywhere; firn zone is shallow.
Why it matters
The snowline / equilibrium line is the practical handle for measuring climate change on glaciers. ELAs have risen by hundreds of metres on many Alpine glaciers since 1900. Tracking the ELA year-on-year tells us what is happening with much less expensive fieldwork than a full mass balance survey.
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The interactive map overlays late-summer snowline photographs for several well-monitored glaciers — useful for seeing how the line has moved over the last decades.