At the head of a tunnel driven to bedrock in Blue Glacier, Washington, the mechanism of sliding of the glacier over bedrock has been investigated. This mechanism involves (1) regelation-slip, which operates through the combined action of heat transport and mass transport (liquid and solid) in the immediate neighborhood of the glacier sole; (2) plastic flow, promoted by stress concentrations in the basal ice. We have observed and/or measured the following features of the basal slip process: 1. Slip rate in relation to internal deformation of the ice; 2. Time-variations of the slip rate; 3. Freezing of basal ice to bedrock upon release of overburden pressure; 4. Formation of a regelation layer in the basal ice, and detailed behavior of this layer in relation to bedrock obstacles and to incorporated debris particles; 5. Local separation of ice from bedrock and continuous formation of regelation spicules in the open cavities thus created; 6. Plastic deformation of basal ice as recorded in the warping of foliation planes and of the regelation layer. Simple experiments to test our interpretation of the regelation layer have been carried out, in which regelation flow of solid cubes of different materials frozen into blocks of ice was produced. The field measurements and laboratory results are used to test the theory by Weertman (1957, 1962) of the basal slip mechanism. It is found that the theoretical “controlling obstacle size” and “controlling obstacle spacing” that should correspond to our observations are about an order of magnitude too small. This quantitative failure represents an overemphasis in the theory on the importance of plastic flow as compared to regelation. A new theory has been constructed which gives results in better agreement with observation.
Natural snow covers are recrystallized to depth hoar through sublimation. The internal water‐vapor flux responsible for this recrystallization depends on temperature, temperature gradient, ambient atmospheric pressure, and snow structure. If a model for temperature distribution between ice framework and pore volume is assumed for low‐density snow, this flux can be calculated in terms of the diffusion process and its relation to the climatic variables quantitatively demonstrated. Observed depth‐hoar‐formation times agree with the calculated flux values and show that depth hoar forms as a consequence of internal mass translation resulting from the vapor flux itself, rather than from the smaller mass accumulation term.
A new classification of snow on the ground is based on the major physical processes involved in the metamorphism of a snow cover. The major divisions are based on (I) the mechanical damage to snow crystals during precipitation, (II) the transport of water vapor at constant temperature because of surface-energy differences, (III) the transport of water vapor along a thermal gradient, and (IV) firnification because of melting and refreezing, and pressure consolidation.
The penetration of radiation into snow is assumed to follow Fick's second law of diffusion with a term for simultaneous absorption. From this, the distribution of radiant energy in a deep homogeneous snow cover, a laminated snow cover, and a snow cover with an absorbing surface beneath is deduced. An equation for albedo is also derived. The penetration phenomenon is then analyzed in terms of the physical processes occurring and formulated as a random walk process. After a correction is made for a certain amount of nondiffuse radiation in the snow, more accurate equations for energy distribution and albedo are obtained. These results are applied to the theory of radiation measurement in snow. After experimental procedures were established on the Blue Glacier summer firn, field data were collected from winter snow at Alta, Utah, and used to check the theoretical equations for albedo and transmission to a black surface. The agreement of theory and experiment is very satisfactory.
ABSTRACT. Conventional avalanche forecasting is practiced as a mix of deterministic treatment for snow and weather parameters and inductive logic to reach actual forecast decisions. Inductive logic of the scientific method dominates, making frequent use of iteration and redundancy to minimize decision uncertainties. The mental processes involved are holistic rather than analytical. Elementary information theory can be used rationally to sort data categories for minimum entropy and optimize inductive reasoning. Recognizing these principles affords a chance to improve the practice and teaching of conventional forecasting techniques. RESUME. Les processus fondamentaux de la prevision conventionnelle des avalanches. La pn!vision conventionnelle des avalanches est pratiquee comme un melange d'un traitement deterministe des parametres relatifs a la neige et a la meteorologie et d'une logique inductive pour aboutir aux decisions reelles du previsionniste.La logique inductive de la methode scientifique domine, faisant un usage frequent de l'iteration et de la redondance pour diminuer les incertitudes de la decision. Les processus mentaux mis en oeuvre sont globaux plus qu'analytiques. On peut utiliser rationnellement une theorie elementaire de l'information pour choisir des categories de donnee d'enthropie minimum et optimiser le raisonnement inductif. La reconnaissance de ces principes apporte une chance d'ameliorer la pratique et l'enseignement des techniques conventionnelles de prevision.
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