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Show Types of Avalanche* The specific conditions 1 land under which avalanches occur are varied. Some ava-hes take place following snowfalls, others during windstorms, some under ring conditions; the type of break- away, the shape of the terminal dump and type of the component snow, all these vary. Different also is the action of various avalanches against obstacles; some avalanches roll over high obstructions, others, on the other hand, are blocked and pile up at small ones. Avalanche volume and the relief of the slide slope are only partly responsible for avalanche peculiarities. We must first of all look at snow condition for the answer to the question of avalanche causes. The difference in the external qualities of snow serves as a good index in determing the causes and conditions of the avalanche process, namely, for typing avalanches, One is sure to observe transitional types also. In particular, in most dry avalanches a loose windblown snow is always present in great amount, yet the characteristics of the particular avalanche type can be more or less precisely stated as a rule. It should be noted that avalanches of a given size are specific to a given form of relief, at the same time they depend typologically on weather conditions at the time of snow deposition. During the same winter avalanches of a certain type are observed not to occur, while those of another type are very frequent. Below we shall describe the avalanche types observed in Chibin. A. Avalanches Caused by Precipitation or Drifting of Fresh Snow. 1) " Top ( snow)":** Many avalanches occur on steep slopes following a heavy snowfall in still weather. Insignificant amounts of freshly fallen, unbound snow will pick up the downslope snowcover, and the forefront of the avalanche gets progressively wider. As the slide grows in volume, the snow no longer scatters downward, but begins to flow as a stream. The avalanche is always accompanied by a cloud of snow dust. The rupture is at an angle, or in steps with a large-scale slide. Any type of snow, even quite loose snow, can provide the undersur-face for the slide. The dump has the form of a roller, the fore part of which lies over undisturbed snow, or in larger avalanches, the form of a flattened, elongated cone. The dumps of this type of avalanche are very loose and homogeneous, covered with small, fragile snow balls. Snow density in these deposits is 0.12- 0.35. Such avalanches break away on steep, precipitous slopes of 45- 60°, mostly from under rocks or snow cornices. Their volume is not great, several tens of m3, only a very few attaining volumes of 2000- 3000 m3. Due to their small volume they do not travel far, stopping on relatively steep parts of a slope. They usually occur during the first 24 hours following a snowfall. An increase of 10 cm or more in snow thickness is critical. As mentioned above, the greater the increase in snow thickness, the less the slope grade needed to start avalanching. Such avalanches are frequent on mountain tops; and where conditions are favorable, the steeper slopes may be covered with them. They are not observed every year. There were nine snowfalls during the period we observed this type of avalanche. In two slides fresh, unbound snow avalanched; in the * The substance of this chapter was presented by the writer to the Presidium of the Academy of Science of the USSR at a conference on snow problems. ** The names here given to the types of avalanche snow are merely the working terms in use at the weather station. - 45- |