Field Experiments on the Development of Depth Hoar

Depth hoar is the coarsest grained snow structure that can form in the absence of the liquid phase. It consists of large well-developed skeletal crystals, formed by sublimation. Weak intercrystalline bonding causes depth hoar layers to be fragile. It forms in seasonal and perennial snow strata as a result of upward vapor transport along vapor pressure gradients produced by temperature gradients in the snow. Depth hoar strata are essential parts of the annual discontinuity that permits one to interpret snow stratigraphy where melting does not occur on the Greenland and Antarctic Ice Sheets. In mountain regions these layers contribute to the critically unstable structure that produces avalanches. Extreme development of depth hoar occurs in the shallow (50 to 80 cm) seasonal snowpacks of interior Alaska. An experimental arrangement employed in the Fairbanks area for several years allows us to observe the formation of depth hoar in the natural snowpack with strong temperature gradient and compare it with the lack of formation in adjacent control snowpacks that are not subjected to the strong gradients. The measured upward vapor fluxes in the snow during four successive years averaged 0.025 g cm−2 day−1, an order of magnitude greater than vapor fluxes calculated from pure diffusion models. To date, all models for depth hoar formation have been diffusion models, which specifically exclude the process of air convection in snow. Our results suggest that significant convection does occur and is responsible for the order of magnitude difference between our measured rates and the previously calculated rates.