什么是 Firn; pore close-off？

Firn is snow that has survived at least one melt season and is being compacted into ice: denser than snow, still permeable to air. Pore close-off is the point in its compaction, at a density of about $820\text{ to }830\,\mathrm{kg}/\mathrm{m}^{3}$, where the pores between the grains are sealed into bubbles and the air in them is cut off from the atmosphere.