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

Melting Snow

Melting Snow 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 falling from trees, cornices, rocks, or the passage of skis. They are properly called sunballs because their downhill progress and growth depends on subsurface melt in snow that comes about from solar radiation. This phenomenon is less common in snow wet by rain, except when deep wetting may produce large balls or wheels. Figure 57 shows the earliest stage of sunball formation. A fall of new snow is exposed to the sun late in winter, when the near-surface layers immediately absorb transmitted solar radiation and melt begins. As the first traces of liquid water appear, the snow becomes slightly sticky. Good deep powder skiing may still prevail at this point, for the bulk of the new snow is predominantly dry, but dislodged lumps of snow can roll and even grow, as they have in this photo. This is not yet the stage where wet snow avalanching begins, but it can signal the early onset of natural releases in unstable soft slabs. In figure 58, melt has progressed to the next stage. Appreciable liquid water now appears in the top few centimeters of snow. Even very small snow lumps roll downhill and accrete wet snow. Avalanching at this point is usually limited to very thin surface layers 71 57 Wasatch Mountains, Utah Melting Snow 73 58 Wasatch Mountains, Utah on high-angle slopes. Skiing quality definitely takes a turn for the worse. Before going on to subsequent melt stages, it will be useful to consider a less frequently observed version of the stage in figure 58. This is illustrated in figure 59, where close inspection of the shadows cast by the sunball tracks reveals that these stand as ridges above the surrounding snow surface instead of being formed as grooves depressed below the surface. In this case an earlier sunball episode much like that of figure 58 has terminated at that stage. Subsequent weather has allowed the snow to settle and stabilize without additional melt. Like the ski tracks in figure 9 (p. 18), the snow initially compressed as grooves by the rolling sunballs has hardened and failed to follow the settlement of the surrounding snow, ending up as ridges. The sunball tracks in figure 59 may be as much as several days old and no longer speak of current snow conditions. The settlement suggests that any wet snow avalanche hazard may have diminished by the time this photo was taken. As solar heating continues on south exposures during a clear day, more and more random lumps of snow fall from rocks and trees to find an increasingly wet snow surface on which to roll. The transition from conditions illustrated in figure 58 to those in figure 60 is often rapid. The sunballs increase in number and reach larger sizes as they descend over wet snow. Serious wet snow avalanche danger is still quite limited, but if this transition has been rapid it signals a clear warning that snow stability may also rapidly deteriorate in the next hour or two. Such deterioration is already approaching in figure 61, where significant free water has penetrated 30 cm or more, and the sunballs are starting to turn into larger snow wheels. A similar situation appears in figure 62, where the deepening penetration of melt-water in the snow allows the highly deformable wet snow to form incipient wheels and deep grooves. When the snow reaches this stage, wet snow avalanches are not far behind. Figure 63 is a good exam74 Melting Snow Melting Snow 75 59 Wasatch Mountains, Utah ple of the avalanches, along with a fine specimen of a snow wheel a full meter in diameter. To sum up the nature of these clues to developing wet avalanche conditions, small sunballs are seldom concurrent with big avalanches unless an unstable slab condition exists for other reasons, but they point the way to changes to come. When large sunballs evolve...