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

Other Snow-Surface Features

Other Snow-Surface Features 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 discussed later. Figure 25 depicts a close-up view of a snow surface spanning an area about 15 cm wide. The feathery crystals are surface hoar deposited by condensation of atmospheric water vapor directly as ice at subfreezing temperatures, without the presence of a liquid 25 San Juan Mountains, Colorado phase (dew). They form a fragile, loose structure at the surface that severely inhibits bonding of a subsequent snowfall to the older snow underneath. A thick layer of surface hoar can in itself release as small, loose-snow sluffs, but these offer little danger. On a solid base, it can also provide very fast and pleasant skiing. Once buried beneath accumulating snow, however, surface hoar becomes one of the commonest causes of slab avalanche release by providing a very weak layer that encourages failure at the slab base. Such a buried layer is visible in figure 26, where a thin slab of snow cut vertically 36 Other Snow-Surface Features 26 Wasatch Mountains, Utah from a snow cover is illuminated from behind by sunlight. Figure 27 gives a close-up view. The thin, bright layer across the slab is buried surface hoar, with a thin, dark band of ice crust, prominent in figure 27, just beneath it. This is an especially favorable combination for slab avalanche release. Evaluating such a hazard is tricky because surface hoar forms erratically across the landscape according to local surface cooling and variations in supply of atmospheric moisture. Once formed, it can be easily and erratically swept away by wind. Other Snow-Surface Features 37 27 Wasatch Mountains, Utah Figure 28 shows the fuzzy appearance of a snow surface coated with surface hoar. (No, the camera is not out of focus.) An earlier deposition of rime on the grass stalks has occurred from wind-blown, supercooled cloud droplets. A more complete discussion of surface hoar and rime, with microphotographs, is given in Field Guide to Snow Crystals, the companion volume to Secrets of the Snow. In favorable light conditions, surface hoar can reflect many bright pinpoints of sunlight from crystal surfaces, which provides a sparkling appearance. In figure 29, however, the many bright points 38 Other Snow-Surface Features 28 Rocky Mountains, Colorado of specular reflection are from a different source, the surfaces of large, rime-free stellar crystals deposited by snowfall. Lack of rime on such crystals produces a new snow type that can form shallow, very unstable slabs under the right wind and temperature conditions . Such snow does not usually build up into deep and dangerous slabs unless the wind is strong enough to drift and fragment the crystals. The tendency to form unstable soft slabs of dangerous size Other Snow-Surface Features 39 29 Mount Yotei, Hokkaido increases in new snow as the degree of riming on the crystals increases. The observer must be cautious, though, for the presence or absence of rime on crystals at the surface may not reflect what lies underneath. The rough snow surface in figure 30 is produced by the complex interaction of sunlight, melt, and evaporation, which leads to enhancement of surface relief. The net result is to generate an increasingly rough surface characterized by small pinnacles of snow, called “nieves penitentes” when they reach appreciable size. Full development of these pinnacles can reach a relief of 2 meters or more, but this latter situation is limited to high altitude glaciers and snowfields, primarily in the tropics. Incipient formation of small 40 Other Snow-Surface Features 30 Wasatch Mountains, Utah nieves penitentes, like those shown here, is fairly common on southfacing slopes of temperate-zone snowfields when the sun altitude is high and the air is dry. In practical terms, this means it is mostly encountered at higher altitudes, lower latitudes, and toward spring. There are two clues here...