| OCR Text |
Show a more gentle slope is more likely to produce slides depends on the nature of the snow. Crevices often appear in the rupture step, parallel to the face, for the entire height and several centimeters in width. A step- type break- away may occur in loose windblown snow and in exceptional cases, in loose fresh snow ( avalanche of freshly fallen snow on April 2, 1935, on the southwest slope of Mount Ukspor). as well as in firm, strong snow. In cirques and on ( gentle) slopes the snow lies in an even cover only during still weather. With wind, some spots are blown bare and others show piled snow; after a windstorm a mountain side is covered with random patches of new snow which contrast sharply with the base of older, grayish snow. Since the edges of such snow patches often protrude when sliding occurs, the upper edge of the break- away for an avalanche of this type of snow can scarcely be made out; the break- away does not form a step here, but rather a line below which the snow has disappeared and above which an edging or rim 2- 3 cm high is left. Seen from above, a line rupture presents a more irregular front than a step rupture and is less dependent on relief features. Line rupture is sometimes observed in the spaces between step ruptures with large avalanches. If the avalanche is set off by the collapse of a snow cornice or by the descent of a snowball or stone, the break- away is limited to the spot of impact. The track of the descending snow gets wider and wider with its progress here, describing, at least in its upper portion, a triangle the apex of which is at the point of origin. This type of angular rupture is characteristic of avalanches of loose or partially melted snow. In steep- sided cirques that are not precipitous at the top, avalanches always originate under cornices, near summits, at elevations of 900- 1000 m above sea level. In more gradually sloped cirques the break- away takes place much lower down. Avalanches are produced in loges at elevations of 500- 600 m above sea level, at the lower third of mountain sides, probably because avalanches of loose windblown snow mostly originate here as a result of snow accumulation on the lower, more gentle slopes at about this level. On wide, even slopes the position of the upper edge of the break- away depends on the relief and may occur at any layer floor. A 35,000 m^ avalanche of January 25, 1938, on the west slope of Aykuayventchorr, headed toward Chibin, originated at an elevation of 400- 450 m above sea level, only 50- 100 m above the valley. Of 230 avalanches breaking away at known heights, 110 originated at elevations of 600 m or less above sea level, i. e., in the lower half of the mountain side. For any one site, slides of different type will break away at different heights. Avalanche Movement Avalanche movement, at least at the beginning, varies as a function of the snow condition. At the break- away a homogeneous layer of unbound snow will loosen, fall apart throughout its entire thickness and start down slope. The amount of moving snow increases rapidly as it gathers snow of the same type in - 33- |
