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

Small-Scale Wind Features

Small-Scale Wind Features 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 serves as a reminder of the layered structure of snow that can lead to the formation of failure planes. Part of a layer of new snow may 12 Wasatch Mountains, Utah slide away as a soft slab with no obvious distinction from the rest of the new snow remaining behind, but in each case some minor fluctuation in snow deposition, perhaps a brief wind gust or interruption of snowfall, has left behind the seed of a failure plane and later slab avalanche release. Figure 13 moves to a larger scale with evidence of thicker layering . Again, a snowfall has been deposited with wind and weather variations during the storm. Again, a subsequent wind has eroded this deposit. The steps in the eroded surface exhibit the original 22 Small-Scale Wind Features 13 Wasatch Mountains, Utah deposition layers, which are especially clear in the upper part of the photo where the eroded surface intersects the layers at a steeper angle. This picture speaks of a crust breakable by skis and repeats the message about snow stability: new snow has fallen, then later been transported by the wind, hence lee slopes should be suspect. Similar layering is seen in figure 14, where the photo covers an area about one meter wide. In this case the overhanging weight of a small cornice has caused the snow to deform so that the layers follow the bending cornice curve. Small-Scale Wind Features 23 14 Wasatch Mountains, Utah Layering also occurs on a larger scale. In figure 15, the thick depositions of successive snowstorms are visible in this large cornice. The added weight of each storm bends downward the previous layers until the lowermost one rests on the lee snow surface below. Continuity of these layers with snow or rock anchors to the windward gives substantial strength to the cornice and resists its fall. Once a cornice does fall and leaves a fracture face, as in the foreground of figure 16 (where another cornice is in the act of falling in mid-distance), a different situation prevails. When a new cornice builds outward from the face left behind by the first cornice’s fall, there is no continuity between old and new snow layers, leaving the new cornice poorly attached to the snow behind it. Second generation cornices are much more unstable than first generation ones. Beware! 24 Small-Scale Wind Features 15 Wasatch Mountains, Utah Small-Scale Wind Features 25 16 Bridger Range, Montana The etching of deposition layering (figs. 12, 13 and 14) is possible only in relatively hard snow that can resist complete removal by strong wind. Softer, looser snow seldom exhibits such layer etching because it is simply blown away, but it does develop other surface textures more closely related to behavior of the wind rather than the layering of the snow. Figure 17 shows a transition to softer snow (skiing quality improves!) where moderate wind effects have delineated some of the layering, but where surface rippling from wind eddies also begins to appear. This rippling is more pronounced in figure 18 (still softer snow) and becomes prominent in figure 19, where the surface texture is now dominated by the complex interaction of wind and snow, with crystals being sorted by size and minor density variations along the snow surface. The main evidence about snow conditions to be gleaned from this kind of surface is that the 17 Wasatch Mountains, Utah fresh snow is relatively soft (it may harden with time) and the wind has been in the low to moderate range below 10 to 12 m/s (meters per second). Skiing may be good and soft slab formation possible if other factors are favorable for avalanching. Before continuing to examine new snow surfaces, it is appropriate at this point to consider briefly the type of rippled pattern in figure 20. At first glance...