Figure 1.

(From Martinerie et al., ref. 3). (Upper) Closed porosity vs. density in the firn at Summit, Greenland. Note the monotonic relationship between the two properties. When density = 0.83, open porosity = 0. The decrease in closed porosity at higher densities is due to compression of bubbles. (Lower) Closed porosity vs. depth for maximum-density layers and minimum-density layers. Note how maximum-density layers are completely closed by 75-m depth, and minimum-density layers by 83-m depth. In the intervening depth interval, parts of the firn remain permeable but gases cannot migrate vertically.