Depth hoar

From HydroWiki

DEPTH HOAR: a kind of hoarfrost; ice crystals that develop by sublimation within a layer of dry snow; characterized by rapid recrystallization, usually caused by large temperature gradients; similar in physical origin to crevasse hoar; crystals are faceted, rather than rounded.

Definition from the National Snow and Ice Data Center.

Photo by Doug Chabot, Gallatin National Forest Avalanche Center Dec 24, 2006 Depth Hoar was 5 cm thick and was sitting on the icy bed surface of the old Oct snow.

Sam Colbeck, studies snow physics at the U.S. Army Cold Regions Research and Engineering Laboratory in New Hampshire. He explains the process this way: "The ground warms the lower crystals, and they're giving up water vapor to the crystals above." The process, he says, is "a form of cannibalism." Neither the density nor the thickness of the snowpack changes, but a weak, unstable and invisible layer forms nonetheless Reference.

Observations of slab avalanche releases in alpine terrain have led to the hypothesis that rocky outcrops can influence the spatial distributions of temperature and heat flow in dry alpine snow covers, and thus control the local distribution of depth hoar. The effect is stronger in the early winter than in the late winter because the rock has then had time to cool. In all cases we found that depth hoar growth occurs preferentially over the rock. This suggests that snow pit investigations made over soil can be misleading if rocky outcrops are present (Arons et al., 1998).

Depth hoar is common where the snowpack is not deep and air temperatures are very low. Strong temperature gradients between the base of the snowpack, which are typically near 0°C, and the surface of the snowpack drive vapor movement from high- to low-temperature regions in the snowpack. This causes snow crystal metamorphism where vapor is deposited on grains causing them to grow larger and eventually form the large faceted crystals known as depth hoar. Depth hoar has little cohesion and forms weak layers in snowpacks that often become buried leading to potential hazardous conditions suitable for avalanches.

Sources and references

Arons, E.M., Colbeck, S.C. and Gray, J., 1998. Depth-hoar growth rates near a rocky outcrop. Journal of Glaciology, 44(148): 477-484.