Snow

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IN his Friday evening discourse at the Royal Institution on January 22, Mr. Gerald Seligman discussed “The Nature of Snow”. He commenced by showing how the molecular structure of ice accounts for the hexagonal structure of ice crystals. He reproduced Tyndall's experiment of forming ‘negative crystals' in ice by passing a beam of light through it. A cinematographic reproduction of the same experiment showed the formation of these ‘crystals' under high magnification. Ice evaporation is stimulated by a comparatively high temperature, and the reverse process? reconsolidation?by a low temperature. Mr. Seligman stressed the importance of sublimation in every question affecting snow and snowcraft. The life-histories of differenttypes of snowflakes under varying atmospheric conditions were then dealt with. Passing now to firnification, or the changes of the snowflake after lying on the ground, Mr. Seligman showed the complete cycle from newly-fallen snow to advanced firn snow by photomicrographs taken by him in the Alps and by scenic views illustrating the different kinds of snow surfaces encountered by the mountaineer or explorer. The process is essentially one of a consolidation of the fine, newly-fallen snow, occasionally containing as much as 89/90 of air, to one of pure ice containing little or no air at all. The final stage of firnification, the genesis of glacier ice, has never been fully recorded, and this is to form the subject of a research now in course of preparation by the lecturer who showed tools for the purpose lying on the table. Dealing briefly with new snow avalanches, Mr. Seligman used some of his microphotographs of firnification to show why snow slopes do not as a rule become dangerous until the temperature has risen after a snowfall? a practical point of value to the ski-runner. Turning next to the effects of wind upon snow, Mr. Seligman showed photographs of a number of wind-formations, including sastrugi, cornices, and snow ‘wind-packed’ or hardened by a wet wind. Wind-packed snow causes wind-slab avalanches, which consist of hard brittle drifts of snow lying loosely on steep slopes. These, if disturbed, break into blocks and form dangerous avalanches. A knowledge of the weather conditions which cause this type of avalanche has already proved of value to Everest climbers.