Secrets of the Snow
Visual Clues to Avalanche and Ski Conditions
Publication Year: 2001
Published by: University of Washington Press
Title Page, Copyright
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In 1969 I completed a book called Field Guide to Snow Crystals, published by the University of Washington Press. This book subsequently went out of print in the 1980s, but was revived as a reprint edition in 1992 by the International Glaciological Society and is still available today. The Field Guide told only half of the story, that of snow in the microscopic world. The present volume tells the other...
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This book is about snow in the mountains. Whenever snow lies on steep slopes, it may become unstable and slide away as an avalanche. Hence there will be frequent references to snow stability and clues to interpreting it. But this book is not about avalanches per se; texts dealing specifically with avalanches are listed in the bibliography. This book is intended for the interested layperson, for ski tourers, for avalanche workers, and for alpine snow country dwellers everywhere. ...
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To begin, compare figures 1, 2, 3, and 4. Each is a mid-winter scene of snow-covered alpine terrain, but startling differences exist among the overall appearances of these landscapes. The degree to which the snow covers the landscape varies widely, owing to wide differences in snow-cover depth and distribution. These basic differences stem from equally basic differences in climate. This is the starting point for evaluating ski conditions and snow stability: what kind of...
Wind on Snow
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Like the sea, the snow surface is constantly shaped by the wind. Unlike the sea, snow carries the history of this shaping within its internal and external textures. Reading this history is a vital part of reading the snow surface, for wind deposition of snow usually governs both the skiing quality and the mechanical character of unstable slab layers prone to avalanching. For sophisticated forecasting of snow conditions, instrumentation like recording...
Local Wind Features
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Figures 6, 7, and 8 illustrate the way wind influences can be interpreted by examination of local features. In figure 6, a layer of fresh snow exhibits a smooth, featureless surface except where broken by ski tracks in the foreground. The drifts are shaped by the building, an obstacle to the free flow of the wind. The sharp, clean edges of the drifts and roof deposits indicate drifting under moderate winds, as does the featureless character of the snow surface, which would...
Small-Scale Wind Features
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Figure 12 shifts to a fine detail of snow structure (scale given by ski poles). The many thin layers, each only a few millimeters thick, are formed as small variations in hardness engendered by fluctuations of wind velocity during deposition. Subsequently, part of the deposited snow has been eroded by a stronger wind that etched the exposed layer edges to reveal the hardness variations. This feature...
Other Snow-Surface Features
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The discussion of snow surface texture now moves to other features unrelated to the degree of wind action. These are features characteristic of winter snow and subfreezing temperatures. There is another whole domain of surface textures produced by various forms of summer ablation, but these largely relate to firn fields on glaciers and play little part in winter ski conditions or avalanche formation. Surface melt features and wet snow conditions will be...
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Returning to a wider view of surface features, one of the commonest of all is new snow sluffing on steep slopes, typically above 35 degrees. In figure 34 a benched hillside has produced dry, new snow sluffs on the steep faces of the benches. Most exhibit the typical point origin of loose snow avalanches and start below obvious trees or rock outcrops, which provide falling snow clods as triggers. Extensive sluffing like that seen here indicates a general stabilizing trend in...
Snow in Trees
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The way that snow lies (or does not lie) in trees contains many clues to the character of recent snowfall and weather. These clues add to the store of visual snow information available to the alert observer. When predominantly stellar crystals fall in light wind, the deposited snow exhibits canopying as the crystals interlock during accumulation. Cushions of snow begin to build up on exposed objects, notably including tree branches, until the enveloping snow...
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Melting snow offers another whole spectrum of surface features serving as clues to snow behavior and stability. These clues are especially important in the sometimes rapid transition from a subfreezing snow cover or new snowfall to a melting state with free water present. The sequence of sunball formation provides easily recognizable clues to fast-changing ski conditions and the evolution of wet snow avalanches. Sunballs are rolling lumps of snow that start as clods...
Other Snow Features
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Many other related topics besides visual snow features, such as snow stratigraphy, mechanical properties, thermodynamics, and acoustic behavior, contribute to interpreting snow conditions. Although a full treatment of these topics is far beyond the scope of this book, a few lesser-known sidelights are introduced below. The discussions to this point have made several references to the kinesthetic sense, the “feel” of the snow as it reacts beneath skis. ...
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Publication Year: 2001