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

Local Wind Features

Local Wind Features 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 have been etched with micro-relief by stronger wind. The small cornices on the roof ridge indicate that the prevailing storm winds were toward the camera, and the size and location of the extensive snowdrifts point to accumulation from several storms. Although the snow surface and appearance of ski tracks indicate a fresh snowfall, perhaps just ended as the cloudy sky begins to break, the tree shows no accumulation at all, suggesting it must have been swept free by the wind. The ski tracks reflect at least 15–20 cm of soft snow, possibly that of the inferred fresh fall. The overall deduction is that of an appreciable new snowfall accompanied by moderate winds, a situation likely to produce good powder skiing and potentially some soft slab avalanche formation. The degree of instability would have to be determined from other factors such as density of snow, character of the previous snow surface, or, more directly, test skiing to see if fracturing will propagate. The roof drifts on that building might be a safe place to begin such testing. Figure 7 presents a more complex wind history. The size of the cornices in the center indicate that prevailing storm winds blew across the ridge from right to left, transporting snow from the wind13 ward timbered slope to the cornices and the lee slope below them. Recent winds have altered this pattern. First, a wind blew in the opposite direction from the prevailing one and transported an appreciable amount of snow. It could have accompanied snowfall or picked up previously deposited snow from the surface. This drifted snow formed eddies around the cornices, whose normally lee faces acted in this case as windward obstacles, leading here to 14 Local Wind Features 6 San Juan Mountains, Colorado the formation of miniature “wind scoops.” This transported snow has had a chance to age-harden—at least a few hours is a reasonable guess—and then a strong wind has blown upslope to erode this age-hardened surface and etch the final texture visible in the photo. These features suggest that prevailing lee slopes where slab avalanches might normally be found have been wind-scoured. On the other hand, the countervailing wind may have deposited slabs in Local Wind Features 15 7 Wasatch Mountains, Utah unexpected places. In any case, ski conditions are going to be changeable from slope to slope. Figure 8 speaks very clearly to wind action on snow, lots of wind. The scoured and scalloped surface is formed by high wind velocity, probably hurricane force at times. The original snow must have been deposited and reworked by a sequence of windstorms, ensuring agehardening and hard, high-density snow. If new snow has recently fallen, it too has been blasted away by the wind, while the hardened old snow beneath remained. With no evidence of melt around the exposed grass patches, the inferred site and season can be high altitude , early to mid winter, and probably both. Scarcity of snow on the distant slopes points to very early winter. Elsewhere the snow cover will be very shallow, with depth hoar formation likely, and highly localized deep drifts may exist on lee slopes, with surface erosion by wind scour and sublimation. Very scattered, hard slab avalanches may be possible, and ski conditions certainly will be poor if any skiable surface can be found at all. Figure 9 presents another complex situation. The snow obviously must be solid and hard, for the skier hardly makes a dent in the surface . The peculiar surrounding drifts are old ski tracks made earlier in a layer of new snow deposited on top of the hard surface. Later a strong wind completely removed this new snow except where it had age-hardened following compression under passing skis. If the hard surface also exists on nearby lee avalanche slopes, it may provide a good sliding surface for a slab consisting of...