Weak layers such as buried surface hoar or depth hoar frequently form the failure plane of slab avalanches. Therefore, the mechanical properties of such layers in relation to their snow structure have been investigated. Since it is difficult to transport samples containing a weak layer into cold rooms, the mechanical measurements have to be made in situ.

We investigate strain-rate dependency of shear strength by measuring concurrently strength, deformation and acceleration, using a digital force gauge attached to a 0.05 m2 shear frame to which an accelerometer and a displacement sensor are fixed. In doing so, a dynamic force comparable to a driving skier is applied. The measurements cover a strain-rate range 10-2 to 1 s-1. The samples fail in a brittle manner. The shear-strength values cover the range 0.2–2.8 kPa. The dataset is also used to approximate the coefficient G, the shear modulus, for different weak layers.

The snow structure has been analysed macroscopically in the field and for some layers representative snow samples have been extracted in order to prepare, in the cold laboratory, single-sided serial planes with cuts every 0.1 mm recorded by video. The analysis of these snow samples should have given the relation between some mechanical properties (strength, strain) and the structural properties. Due to basic problems in defining the connection between complex snow grains (e.g. surface hoar), we were unable to complete this part in due time. Only preliminary results on this aspect are presented here. Based on our long-term database, containing macroscopic structural and strength data of weak layers, a relationship between snow type and shear strength has been established.

For operational snow-cover simulations, an adequate modelling of the evolution of buried weak layers is of crucial importance. Therefore, the processes governing snow metamorphism within weak layers before and after burial must be known in detail. At the study site of the Swiss Federal Institute for Snow and Avalanche Research, 2540 ma.s.l., a 2 cm thick weak layer of column-grown cup-shaped crystals formed beneath a thin crust in mid-January 1996. Exposed to near-surface processes for about 4 weeks, the layer was buried on 8 February and persisted in the snowpack until mid-April. Numerous temperature profiles and characterizations of both the weak and the adjacent layers were performed in situ. Snow-grain samples, as well as larger snow blocks, were taken to the cold laboratory for further analysis of the texture. The shear strength of the buried weak layer was also investigated by means of shear-frame tests. The field observations and measurements are compared with model simulations of snow temperature and stratigraphy. The comparison shows potential and problems in the modelling of weak-layer evolution.

The mass per unit horizontal area, vertical height and density of new snow accumulated on various slopes of 0° to 75° were measured. Although the mass of new snow on these slopes was nearly the same, vertical height increased and density decreased with increase of slope angle. Differences in heights and densities of new snow due to slope angle were explained by considering both accumulation and densification processes on the slopes.

Since 1975, the Norwegian Geotechnical Institute has performed field investigations of snow-creep forces on masts at the research site in Grasdalen, Stryn mountains in Norway. Two poles, with diameters of 419 and 219 mm, respectively, were erected at the site together with a retaining structure. On both poles, strain gauges were mounted in pairs every 0.5 m to find the axial stresses and the moments in different sections. In the middle section of the retaining structure, the beams and supporters were instrumented to obtain measurements of the strains and stresses. Snow glide was controlled by glide shoes mounted at the rock surface above the structures.

During the winter, snow profiles were made systematically; these included measurements of snow depth, density and temperature, and observations of snow type and moisture content. By relating the measured stresses and moments in the structures to the snow depth, it was possible to find the snow-pressure distribution. A comparison of the snow pressures with the “Body force index” (product of snow depth h, density ρ and acceleration due to gravity g), show a close relationship for the wall element. For the pole elements, the snow temperature during the winter is an added factor of high importance and the highest pressures on these elements occur in winters with long periods of 0°C isothermal snowpack.

Surface hoar growing for several clear and humid days were observed. During daytime, air and snow-surface temperature increased and relative humidity decreased, hence evaporation (sublimation) occurred at the snow surface. The amount of evaporation calculated using a bulk-transfer method suggests that the surface-hoar crystals which grew during the previous night should have disappeared but they were observed to survive on the snow surface even during the daytime. During the following night, new surface-hoar crystals formed on top of the older ones and grew even larger. This result indicates that, although the surface-hoar crystals evaporated into the air during the daytime, snow grains beneath the surface were warmed by solar radiation and evaporated to the air. They may partially condense into the surface-hoar crystals and make up for the reduction in size. Depth-hoar crystals formed beneath the snow surface for several days and the surface layer, composed of both types of hoar crystal, showed a very weak shear strength.

Disitributions of snow hardness minutely measured with a bandy-type digital load-gauge (push–pull gauge) are demonstrated. This push-pull gauge is of compact design and is useful for precise measurement of tension and compression loads. It can measure the maximum strength of a snowpack when it is destroyed by the attachment pushed horizontally into the side of the pit. Because it takes only a few seconds for one measurement, snow-hardness distribution can be measured at very small space intervals more quickly and with less effort than by using any previous hardness meter, such as a rammsonde, Canadian gauge, Kinosita-type hardness meter, and so on.

Snow-pit observations were made at Saiho, Sapporo and Minakami, Japan, in the winter seasons of 1996 and 1997. The snow hardness was measured with the push-pull gauge at regular intervals of 5 cm vertically and 10 cm horizontally. Some weak layers between harder layers could be detected with the push-pull gauge whereas they could not be using the rammsonde. The hardness of snow was observed to be almost uniform horizontally before snowmelt. Once meltwater infiltrated into the snowpack, its distribution became heterogeneous. It was revealed that the hardness of the fine-grained compacted snow layer with grain-sizes less than 0.5 mm showed a high correlation with the fourth power of the snow density.

This paper deals with air temperature, net-radiation and relative-humidity measurements which we performed in Austria in different zones of mountain forests in recent years. Our aim using such measurements was to approximate the general conditions for the formation of surface-hoar layers in low-density forest stands near the timber line. These layers are a necessary condition for snow-slab release and avalanche formation.

Our investigations showed that the total radiation balance was negative but less negative in a forest with a close canopy (on clear nights about −30 to −40 W m−2) than within a small clearing (6 m by 6 m). Moreover, the total radiation balance within the forest clearing was almost the same as that in an open field. There we measured −60 to −80 W m−2 on clear nights. Approximations of surface temperature (ts) showed that ts was below the dewpoint (tE) in an open field but did not reach tE in the forest; moreover, ts was very close to tE within the forest clearing. Water-vapor flux was positive only in open terrain (i.e. condensation between 0.1 × 10−6 and 0.9 × 10−6 kg m−2 s −1 and evaporation occurred within the clearing and in the forest.

The mechanism of sun-crust formation was investigated through laboratory experiments in a cold wind tunnel. The experiments were carried out by controlling the energy balance and the sun crust formed was consistent with that observed in Nature, i.e. a thin ice layer composed of ice particles and cavities due to internal melting beneath the sun-crust layer. The surface-cooling rate was between −50 and −100 W m−2, and the absorption of shortwave radiation exceeded 200 W m−2. The energy balance during the formation of the sun crust is consistent with the data observed in Nature.

The sources of H2O for the sun-crust formation were investigated through changes in δ18O. Taking into account that the δ value of sun crust was larger than that of the snow beneath it and the calculations of the latent-heat emission with the sun-crust formation, it was concluded that the retention of meltwater by capillary force and refreezing in a thin layer was the dominant mechanism of this metamorphism.

The transformation of dry snow to firn is described by the transition between densification by deformationless restacking and densification by power-law creep. The observed decrease with temperature of the density at the snow-firn transition seems to result from the competition between grain-boundary sliding and power-law creep. These two densification processess occur concurrently in snow, although there are probably micro-regions in which sliding alone occurs. Validation of a geometrical densification model developed for ceramics has been obtained from densification data from several Antarctic and Greenland sites and from the characterization of the structure of polar firn.

The regular packing of spheres or polyhedrons of various shapes linked by rigid bonds is presented and discussed as a model of snow structure. Basic structural parameters of this model are: the coordination number and introduced dimensionless factors of friability and rigidity. The snow densification is described as successive changes of these parameters. Use of the model allows us to relate the density increase from ~130 to ~320, ~550, ~700, ~820 and 917 kg m−3, while the coordination number of the structure increases accordingly from 3 (friable hexagonal) to 4 (tetrahedral), 6 (cubic), 8, 10, 12 (dense hexagonal). These structural changes are in good agreement with the critical densities established in experimental studies of snow densification and the physical properties of snow. It is shown that the model presented allows us to estimate the mechanical properties of ice-porous media: Young’s modulus, Poisson’s ratio and strength.

A dynamic finite-element computer program was used to examine the evolution of microstructure and its effect on continuum-scale deformation for the constant-speed uniaxial-strain compaction of an aggregate of roughly spherical elastic-plastic particles. Simulation results are used to explain some micromechanical aspects of snow compaction. Different compaction rates were used to examine the limits of quasi-static response and the effects of inertial stresses. Four stages of microstructurally controlled compaction were observed for quasi-static loading: particle re-arrangement, elastic deformation and two stages of plastic deformation. Elastic deformation follows the first critical density caused by the stable random loose-particle packing of rough spheres. Stage III compaction occurs by plastic deformation once stresses acting on particles exceed yield. Stage IV compaction follows a second critical density caused by the stable packing of deformed particles and is also through plastic deformation of particles. During high-speed compaction, inertial stresses propagate particle deformation from the loading platen into the aggregate. As a consequence, particle re-arrangement is limited so that the pressure-density ratio is larger for high-speed compaction than for quasi-static compaction at the same density. Hence, critical densities and the compaction-rate dependence of the pressure-density ratio during compaction of an aggregate of particles composed of rate-independent material are determined by the evolution of microstructure at different compaction rates. Observed pressure-density profiles for polar snow exhibit the same features of critical density and changes in the pressure-density ratio as found in the simulation and consist of four compaction stages: particle re-arrangement and three stages of creep particle deformation each following a critical density. Shear stresses appear to enhance the compaction during the stage III creep deformation of snow.

The available experimental data on acoustic parameters and dynamic elastic moduli of dry coherent snow-ice formations in their whole density range are analysed and generalized in this paper. A set of critical densities at which there are sufficiently sharp changes in the laws of these property variations have been discovered and are discussed. The empirical equations describing the elastic-wave propagation velocities (Vp and Vs), acoustic resistivities and elastic moduli as functions of the porosity factor of the medium are obtained for five sub-ranges in the density range from light snow to massive ice.

The authors present the results of snow hemispherical–directional reflectance measurements on natural snow in the 0.9–1.45 μm spectral range. The measurements were made in a cold laboratory on snow collected in the field. Some of the samples have been subjected to controlled metamorphism in the laboratory before measurements were made. In the first part, the adding–doubling model, experimental assumptions and methodology are described. In the second part, experimental results are discussed and compared with theoretical values for different typical snow types and for different stages of snow evolution when subjected to temperature-gradient and wetness metamorphisms.

The porosity of wet snow is often about 50%; however, liquid water generally fills less than 10% of this pore volume. In order to relate the irreducible water content trapped in snow to its characteristics, we have conducted experiments in a cold laboratory. The results show that irreducible water content, expressed as per cent of mass, depends only on porosity. Experimental studies were restricted to homogeneous wet snow samples. Therefore, we can only achieve a valid result in natural snowpacks when applying to an homogeneous layer of wet snow. Nevertheless, the results may be incorporated into snow-cover energy-balance models to improve the retention and percolation predictions. The thickness of the water-saturated layer observed at the base of the sample in our experiments, was related to the ratio of the mean convex radius of curvature to dry density.

Time-domain reflectometry (TDR) is widely used in soil physics to determine water content. Existing equipment and methods ran be adapted to measurements of snow wetness. The main advantages compared to other methods are flexibility in constructing sensors, minimal influence on snow cover during measurements and sensors can be multiplexed. We developed sensors suitable for continuous and non-continuous measurements of snow wetness and density, measured the apparent permittivity in different snow densities and snow types, and compared the measurements to existing mixing formulas for mixtures of snow and air. In dry snow, density was measured from 110 to 470 kg m−3. The residual error is 14 kg m −3 and the 95% confidence interval of our model is 3 kg m−3. To measure snow density and wetness continuously suitable sensors have been constructed. Their small size and high surface area to weight ratio minimizes their movement in the snowpack, except when they are exposed to intense solar radiation. Results show that changes in dry-snow density of less than 5 kgm−3 can be detected. Infiltration of even small amounts of water clearly shows up in the permittivity. At the surface of the snowpack, problems occur due to the formation of air pockets around the sensors during long-term measurements.

Precise measurements of temperature and density distributions in snow under an applied temperature gradient showed that alternation of evaporation and condensation zones is formed and causes the wavy patterns in quasi-steady temperature and density distributions. In samples with a snow density of 200–500 kg2 m−3 the wavelength was 3–7 cm and the amplitude was roughly 2°C. The present result gives a clue to explaining the wide range of previously measured water-vapor diffusion coefficients in snow.

This paper describes a method for estimating the depth of new snow, using hourly data of total snow depth and precipitation. As the snow cover is compacted continuously due to its own weight, the depth of new snow deposited since the previous time-step to the present time is given by a difference between the height of the present snow surface and the present is impacted height of the previous snow surface. Thus, based on viscous compression theory and an empirical relation between compressive viscosity and the density of snow, an equation has been derived to compute the time variation of the thickness of a snow layer due to viscous compression. Using this equation, the present height of the previous snow surface, which cannot be measured by simple means, was computed and the depth of daily new snow was estimated as its difference from the present measured total snow depth. The approximated results were found to be in good agreement with data measured in Tohkamachi during the three winters from 1992–93 to 1994–95. The standard deviation was 1.71 cm and the maximum difference between estimated values and observed values was ± 8 cm.

Melt-layer frequency and magnitude in polar and sub-polar ice cores have been interpreted as measures of past summer temperature, and calibrations have been proposed relating frequency of occurrence of ice layers in ice cores to past summer temperatures. But, observations in the percolation facies in Greenland and an analysis of the combined processes of meltwater infiltration and refreezing of water in snow indicate that, in addition to unusually high rates of meltwater input, formation of ice layers will also be facilitated by unusually cold initial conditions or early onset of melt. Uniform warming of both summer and winter conditions has the opposite effect and suppresses ice-layer formation in favor of uniform wetting and refreezing of the snowpack. Numerical modeling of infiltration and refreezing at a stratigraphic fine-to-coarse transition allows quantification of the effects of significant parameters (initial temperature, grain-size and density contrast across the stratigraphic transition, water-input rate and minimum impermeable-layer thickness). Calculations are made to distinguish threshold values of parameters at which infiltration progresses faster than refreezing, resulting in a break-through of water across the stratigraphic transition, from values leading to the formation of an ice layer when refreezing progresses faster than infiltration.

Today, it is possible to estimate the density of snow in many different ways (gravimetry, attenuation of gamma rays and velocity of electromagnetic waves). In this paper, we suggest using the acoustic properties of snow. First, we recall the main studies on the acoustic properties of snow. Then, we show, from laboratory measurements, the correlation between density and the acoustic parameters of snow. We found a very good correlation between density and the modulus of the normalized characteristic impedance. We also show that use ofa three-parameter model simulating the propagation of an acoustic wave in porous media allows one to calculate the density of snow from its acoustic porosity.

Using a direct simple-shear apparatus, snow samples (115 mm in diameter, 16–18 mm in height) taken from a so-called homogeneous layer (small rounded particles, density: 290 kg m−3) were tested in a cold laboratory. Experiments were performed for strain rates between 7 × 10−6 s−1 and 5 × 10−3 s−1 at test temperatures of −5°C, −10°C and −15°C. The effects of strain rate and temperature on failure stress, failure strain, stiffness (initial tangent modulus) and toughness were studied. The transition between the ductile and brittle (sudden fracture) state of failure was found to be at about 1 × 10−3 s−1 for the snow types tested, independent of temperature. Stiffness proved to be the most temperature-dependent property of alpine snow. It strongly increases with decreasing temperature. Failure strain and toughness decrease with decreasing temperature. Failure stress was found to increase slightly with decreasing temperature. The effect is not very distinct but close to statistically significant and might be partly hidden by the scatter in the stress data due to variations inherent in sampling and testing.