OCR Text |
Show small. " Outpour" is one of the features accompanying an air wave. When a wave reaches a building, it may tear out windows and doors, although the avalanche volume may not be great. When even a weak air wave reaches a forest, it covers the upslope side of the tree trunks with a dense, solid coating of snow dust, the cross- section of which is triangular with the apex uphill. Lower down the avalanche slope where the snow cloud has settled, the surface is covered with a loose, mossy coat. Cases are known where a powerful air wave has snapped off power- line poles 100 m down slope from the avalanche deposit and carried them another 50 m. Even small avalanches can set up air waves. An observed avalanche of loose snow, only a few thousand m3 in volume, stopping 20 m short of a frame building, was accompanied by an air wave that demolished the building completely by crushing it against a brick structure close by. The air wave of an avalanche does not move close to the snowcover surface, but somewhat higher, often snapping off treetops and leaving no tracks on the snow, even close to the terminal deposit. Air wave movement is in a straight line. In an avalanche of December 22, 1936, the snow stream divided in two and the air stream passed between in the primary stream direction. To explain an air wave, it is necessary to observe that the rear part of a slide moves with greater speed than the fore part because of reduced friction in the already smoothed trough. This difference in speed is greatest at the steepest grade on the slope. As a result, the snow at the rear of the avalanche rolls forward and accumulates in the fore part, and this growing, forward roller drives an air wave ahead of the slide. When the avalanche takes off the snowcover, the forward roller grows especially fast. But even without this roller, the moving snow stream displaces a layer of air. In the tail of an avalanche, in the space cleared by the slide, air is sucked in from above and from the sides and drawn in the general direction of the stream. When the avalanche dies and the snow roller is dispersed, the enormous flow of air accompanying the snow stream continues on. Still further impetus is provided by the tendency of air compressed by motion to stabilize its pressure and volume. Especially powerful air waves are generated by avalanches from cirques where snow is sucked into the avalanche stream from all sides. According to M. Pitken-en, who accidentally set off an avalanche while making his way over a dangerous mountainside, he was carried from the top of a cirque to the foot of the slope, a distance of 1 km, on a snow slab. A tremendous wind ( air wave) arose the moment following break- away, accompanied by a cloud of snow dust that obscured everything from view. The Causes of Avalanches Snow lying on a slope tends to shift downward under gravity. Linkage with the underlying surface, bonding ( cohesion) with the snow above and to the sides in the same layer, and the propping action of the downslope snow, all resist this downward pull. Most significant is the link with the underlying surface. Where an underlying protrusion, or other surface feature making for adhesion, is altered or displaced, the bond with the surrounding snow is impaired. Loose snow has this bond, but it is quite weak. The bond ( cohesion) with the surrounding snow is of paramount importance only in strong, firm snow, especially where the - 41- |