Secrets of the Snow: Visual Clues to Avalanche and Ski Conditions

Other Snow Features

Other Snow Features 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. In this respect there is a curious feature to the stick-slip process illustrated in figure 68. I once attached a microphone to one of my skis and connected it to a tape recorder. Skiing in a wide variety of snow conditions produced a collection of characteristic snow sounds that varied with the type and metamorphosed state of snow crystals. One of these sounds was a clear record of ski chatter while making a turn in snow like that of figure 68. Sound analysis plots of this particular record are shown in figure 69, where the chatter interval is labelled with a bracket. The lower plot shows waveform, or amplitude of the total sound as a function of time. The upper plot is a spectrum analysis, where frequency is shown as a function of the same time interval in seconds, with intensity of the sound at different frequencies depicted by darkness of the plot. The physically perceived ski chatter clearly was recorded in the waveform, but the loss of higher spectrum frequencies during chatter was an unexpected result. The point of this discussion is what came in the next test. The ski-microphone combination was sensitive enough to pick up weak chatter even when there was no perceived sensation through skis, boots, and feet. This raises the question of how much of the “feel” 86 of snow is a subconscious perception of similar vibrations. The mechanical characteristics of the skis obviously must play a part. There are some opportunities here for interesting research. Not all snow features are exhibited at the surface. In figure 70 a rotary snow plow has exposed from ground to surface a spring crosssection of the snow cover along a roadside. A signpost in the middle has locally inhibited snow settlement during winter accumulation. The highly visible layering in the snow brings out the fact that settlement is inhibited not just above the post but in diminishing amount over a distance ten times or more the diameter of the post. The post is not just supporting the snow immediately above it, but a much wider, and thus heavier, body of snow. This is why snow settlement can exert very large forces on buried structures. Consider the example of a cabin in the deep snows of a maritime climate. As winter snow accumulates, it can pile up deeper and deeper on the roof, and the cabin needs to have the structural strength to support this snow load. If the surrounding snow cover on the ground piles Other Snow Features 87 69 Period of stick-slip chatter 70 North Cascades, Washington up deep enough to connect with the roof snow, then the situation changes radically. Now the cabin has to support not only the roof load but also the downward pull of settling snow for a large surrounding area. Most structures are not designed for these kinds of forces and sometimes will collapse. The layers visible in the upper half of the snow in figure 70 stand out because melt has caused dirt particles preferentially to collect on variations in grain size or on ice layers. There are deliberate ways to bring out this stratigraphy for inspection. The variations in grain size and type can be developed like a photographic film by spraying the snow-pit wall with a water-soluble dye (fig. 71). Quickly passing a torch flame over the dyed surface enhances the effect. An unplanned version of this appears in figure 72, where heat and smoke from a bonfire built next to a snow face have brought out the layered structure of the snow. The variations in hardness can be delineated by stroking the pit wall with a soft brush, as in figure 73. Reference was made earlier to the phenomenon of age-hardening in snow, where disturbed snow gains strength with time owing to new bonds growing among the grains. Figure 74 illustrates an unusual example of this. A shallow snow cover consisting entirely of fragile, almost cohesionless depth...