Why does snow become denser after it falls on a glacier?

Snow becomes denser after falling on a glacier through a process of transformation driven by compaction, recrystallization, and environmental factors. Initially, the intricate, unstable shapes of snow crystals undergo rounding as molecules evaporate from sharp points and condense into hollows, allowing grains to fit more closely together. Wind may also physically pack these crystals more tightly.1

As more snow accumulates, the weight of the overlying layers exerts pressure, causing the snow to compact and individual grains to recrystallize to minimize stress. If liquid water is present, the process accelerates through melting and refreezing, which fills the spaces between grains. Over time, these grains develop "necks" of ice that connect them, a process known as sintering.12

Eventually, the material transitions into firn, an intermediate stage between snow and glacier ice. Once the density reaches approximately 830 to 840 kilograms per cubic meter, air spaces are sealed off, rendering the ice impermeable. Further burial and continued stress compress remaining air bubbles, eventually forming clear, dense glacier ice.21