Snow Particle Size Research Articles

Light Attentuation and Visibility in Blowing Snow

Visibility in blowing snow was investigated by measurements of visibility using optical targets, light attenuation of parallel beam and snow concentration and mass flux of airborne snow particles. Visibility is thought to be inversely proportional to extinction coefficient of light according to Koschmeider’s equation of visibility. However, visually observed values are not in accordance with those calculated from extinction coefficient of light when visibility is at a very low range. The extinction coefficient of light is proportional to the total cross section of airborne snow particles in a light path. The total cross section is also in proportion to concentration of airborne snow particles. There are many reported investigations of relation between visibility and snow concentration with widely differing results. The extinction coefficient of light varies with the shape and size of the particles, but cannot totally account for observed differences. Transforming the snow concentration into mass flux of snow particles, a better relation between visibility and mass flux was found. These facts are explained in terms of the size of snow particles which are large enough to be seen. Visibility through visible particles is determined not only by the homogeneous attenuation of light discovered by Koschmeider, but also by the influence of visible particles themselves and their afterimage.

Properties of blowing snow

The size and shape of windblown snow particles determine not only the mass transported by turbulent fluxes but also the rate of phase change from ice to water vapor that occurs in this multiphase flow. These properties and particle densities dictate particle fall velocity and therefore the vertical distribution of mass and surface area, which strongly influence the gradients and fluxes of sensible heat and water vapor within the transport layer. Initial movement at the snow surface depends more on availability and impact forces of loose particles than on aerodynamic drag. Cohesion between surface particles and particle restitution coefficient are important properties that determine threshold wind speeds for snow transport. Threshold speeds for blowing snow vary over such a large range in nature that formulations predicting transport rate as a function of wind speed should include threshold speed as a parameter. The expression derived by Iversen et al. (1975) is compared with low‐level snow transport in the atmospheric boundary layer. Self‐similarity of wind profiles in blowing snow is a property of the flow that has been exploited for scale modeling of snow deposition around obstacles, both outdoors and in wind tunnels. Good quantitative results are obtained by careful attention to similitude requirements.

Microwave Emission From Snow and Glacier Ice

AbstractThe microwave emission from a model snow field, consisting of randomly spaced ice spheres which scatter independently, is calculated. Mie scattering and radiative transfer theory are applied in a manner similar to that used in calculating microwave and optical properties of clouds. The extinction coefficient is computed as a function of both microwave wavelength and ice-particle radius. Volume scattering by the individual ice particles in the snow field significantly decreases the computed emission for particle radii greater than a few hundredths of the microwave wavelength. Since the mean annual temperature and the accumulation rate of dry polar firn mainly determine the grain sizes upon which the microwave emission depends, these two parameters account for the main features of the 1.55 cm emission observed from Greenland and Antarctica with the Nimbus-5 scanning radiometer. For snow particle sizes normally encountered, most of the calculated radiation emanates from a layer on the order of 10 m in thickness at a wavelength of 2.8 cm, and less at shorter wavelengths. A marked increase in emission from wet versus dry snow is predicted, a result which is consistent with observations. The model results indicate that the characteristic grain sizes in the radiating layers, dry-firn accumulation rales, areas of summer melting, and physical temperatures, can be determined from multispectral microwave observations.

Microwave Emission From Snow and Glacier Ice

AbstractThe microwave emission from a model snow field, consisting of randomly spaced ice spheres which scatter independently, is calculated. Mie scattering and radiative transfer theory are applied in a manner similar to that used in calculating microwave and optical properties of clouds. The extinction coefficient is computed as a function of both microwave wavelength and ice-particle radius. Volume scattering by the individual ice particles in the snow field significantly decreases the computed emission for particle radii greater than a few hundredths of the microwave wavelength. Since the mean annual temperature and the accumulation rate of dry polar firn mainly determine the grain sizes upon which the microwave emission depends, these two parameters account for the main features of the 1.55 cm emission observed from Greenland and Antarctica with the Nimbus-5 scanning radiometer. For snow particle sizes normally encountered, most of the calculated radiation emanates from a layer on the order of 10 m in thickness at a wavelength of 2.8 cm, and less at shorter wavelengths. A marked increase in emission from wet versus dry snow is predicted, a result which is consistent with observations. The model results indicate that the characteristic grain sizes in the radiating layers, dry-firn accumulation rales, areas of summer melting, and physical temperatures, can be determined from multispectral microwave observations.

Age Hardening of Snow at the South Pole

The age hardening of artificially and naturally compacted snow has been investigated at the South Pole. Results show that the age-hardening process is greatly retarded at low temperatures. Artificially compacted samples of density 0.55 g./cm.3 attained a compressive strength of less than 3.0 kg./cm.2 after one year’s ageing at −49° C. Exposure to solar radiation accelerated the age hardening. Irradiated samples attained a strength of 6.0 kg./cm.2 after 100 hr., increasing to a virtual maximum of 8.0 kg./cm.2 at the end of 600 hr. Compressive strengths increased with decrease in snow-particle size and with increasing angularity of the particles. Below 3 m. the strength of naturally compacted snow was found to increase rapidly with increase in density. Naturally compacted snow of density 0.55 g./cm.3 possessed considerably greater strength than any of the age-hardened samples of artificially compacted snow of the same density. Thin-section studies show that age hardening can be correlated with the formation and growth of intergranular bonds, and that bond growth falls off rapidly with decreasing temperature. In view of the low strengths found in both naturally compacted snows near the surface and in artificially compacted snow at the South Pole, “cut-and-cover” under-snow camp construction may not prove too practical at the South Pole.

Age Hardening of Snow at the South Pole

The age hardening of artificially and naturally compacted snow has been investigated at the South Pole. Results show that the age-hardening process is greatly retarded at low temperatures. Artificially compacted samples of density 0.55 g./cm.3 attained a compressive strength of less than 3.0 kg./cm.2 after one year’s ageing at −49° C. Exposure to solar radiation accelerated the age hardening. Irradiated samples attained a strength of 6.0 kg./cm.2 after 100 hr., increasing to a virtual maximum of 8.0 kg./cm.2 at the end of 600 hr. Compressive strengths increased with decrease in snow-particle size and with increasing angularity of the particles. Below 3 m. the strength of naturally compacted snow was found to increase rapidly with increase in density. Naturally compacted snow of density 0.55 g./cm.3 possessed considerably greater strength than any of the age-hardened samples of artificially compacted snow of the same density. Thin-section studies show that age hardening can be correlated with the formation and growth of intergranular bonds, and that bond growth falls off rapidly with decreasing temperature. In view of the low strengths found in both naturally compacted snows near the surface and in artificially compacted snow at the South Pole, “cut-and-cover” under-snow camp construction may not prove too practical at the South Pole.

Effective thermal conductivity of ventilated snow

Abstract : Thermal conductivities of unconsolidated snow particles with air flowing in a direction parallel but opposite to the energy flow have been investigated for the mass flow rate ranging approximately from 10 to 40 times 10 to the -4 g/sq cmsec based on the total cross-sectional area of the flask containing the snow sample. The results are interpreted as being the effective thermal conductivity of snow. In the experimental range for snow densities from 0.376 to 0.472 g/cu cm and corresponding snow particle sizes of 0.07 to 0.22 cm nominal diameter, the results can be represented well by the following leastsquares equation, k sub e = 0.0014 + 0.58 G where k sub e is the effective thermal conductivity of sknow in cal/cm-sec-C and G is the mass flow rate of dry air in g/cu cm-sec. When there is no flow, or G = 0, k sub e reduces to a constant value of 0.0014 cal/cm-sec-C, equivalent to the thermal conductivity of snow with motionless fluid. The value of 0.0014 is in good agreement with the data reported by Abel's (1893) and Kondrat'eva (1945). (Author)

Compressive Strength Properties of Snow

AbstractThe compressive strength of snow cylinders was investigated as a function of the age of the snow from which the cylinders were made, the snow particle size and the age of the cylinders. The results show that the compressive strength is reduced if the snow is older, if the particle size is smaller, or if the cylinders are younger. The variation with age of the cylinders can be represented by an equation similar to that for a first-order chemical reaction. The effect of adding small quantities of various gases to the atmosphere in which the cylinders were kept was also investigated; carbon dioxide and methane had no measurable effect, but ammonia lowered the strength of the cylinders. All the strength measurements were carried out at −10° C.

Compressive Strength Properties of Snow

Abstract The compressive strength of snow cylinders was investigated as a function of the age of the snow from which the cylinders were made, the snow particle size and the age of the cylinders. The results show that the compressive strength is reduced if the snow is older, if the particle size is smaller, or if the cylinders are younger. The variation with age of the cylinders can be represented by an equation similar to that for a first-order chemical reaction. The effect of adding small quantities of various gases to the atmosphere in which the cylinders were kept was also investigated; carbon dioxide and methane had no measurable effect, but ammonia lowered the strength of the cylinders. All the strength measurements were carried out at −10° C.