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Show Figure 9. Slab fracture. Note that it broke at the sharpest part of the curve on a convex slope. Fracture plane is perpendicular to the slope. The convex slope is avalanche prone because the snow layers settling upon it are placed under tension. Avalanches almost always fracture at the sharpest point on the curve, get up to full speed instantly, and pulverize rapidly. ( Figure 9). The steepest part of a convex slope is generally near the bottom, leading to a sudden transition and poor anchorage at the toe of a snow layer. On a concave surface, snow is under compression. The steepest part of the slope is generally near the top, the transition is more gradual and snow anchorage at the toe is better than for a convex slope. A slab avalanche on a concave slope gets under way more slowly while overcoming its anchorage and has a tendency to travel for some distance as an almost continuous sheet. While the treacherous " hangfire" avalanche can occur anywhere, it is most likely on a concave slope. Hangfire and delayed action are two different things. The hangfire is simply an avalanche which holds back a - 23 - |