Thermal conductivity measurements of depth hoar

Abstract

The effective thermal conductivity of snow (keff), which includes latent heat transfer due to vapor diffusion, was measured during three winters in Fairbanks, Alaska. In 1986-1987, keff of several layers of snow was monitored in detail as the snow metamorphosed into depth hoar. Measurements were made using a needle probe with an estimated accuracy of +8%; keff was found to decrease and then increase as the snow passed from new snow through several distinct stages of depth hoar. For depth hoar, keff ranged from 0.026 to 0.105, with an average value of 0.063 W m -1 K -1 . This is one half to one fourth the value suggested by most studies for snow of similar density. For depth hoar of a given type, keff can be represented as a linear function of temperature between 0 ø and -20øC but requires a nonlinear function for the range from 0 ø to -196øC. At -196øC the thermal conductivity of depth hoar approached that of still air, suggesting that conduction through the ice skeleton of the snow was limited and that the increase in keff at temperatures near 0øC is the result of the strong temperature dependence of water vapor density. This conclusion is consistent with the nature of the ice bonds in depth hoar, which are thin and relatively few in number. INTRODUCTIONThermal conductivity is a critical parameter that must be known in order to make calculations of heat transfer through the snow cover. Such calculations are important in modeling the energy exchange in snow-covered areas, modeling metamorphism of snow grains, and in determining the dominant processes that move heat and mass through the snow cover.Unfortunately, published values of the thermal conductivity of depth hoar are rare. Yet subarctic snow, consisting primarily of depth hoar, can be found in a circumpolar band that covers thousands of square kilometers [Pruitt, 1970[Pruitt, , 1984Benson, 1982]. Because these subarctic regions are often underlain by permafrost, knowledge of the thermal conductivity of the snow cover is of great importance in determining the depth and rate of frost penetration into the ground. Also, subnivean animals rely on the high insulation value of depth hoar to protect them from low temperatures. Without the development of a thick depth hoar layer, the same snow thickness would provide insufficient insulation for their survival [Pruitt, 1984;Adams, 1981].Functional relationships between conductivity and snow density are widely used to estimate the conductivity of a particular snow cover [e.g., Mellor, 1977]. With the exception of the work of Izumi and Huzioka [1975] and Lange [1985], these relationships have been derived from measurement series that did not include depth hoar. Consequently, they are not applicable to the subarctic snow. Even without considering depth hoar, values derived from the relationships show substantial scatter at a given density (Figure 1). The scatter indicates that density is not the only snow property affecting thermal conductivity.Texture, which includes the size, shape, bonding, and spatial arrangement o...

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