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Show which the ski pole fell the last foot by its own weight. Hair- trigger slab conditions thus existed next to the ground; the entire thickness of snow was resting almost upon a cushion of air. In spite of the fact that the initial testing had given no warning of hazard conditions, the party leader became increasingly worried about slab avalanches as the party progressed. He later said that he sensed a kind of " hollow feeling" to the snow. Finally, he led the party off the northeast slope and onto the southeast side of the spur. About fifteen seconds later the slide occurred. The slab avalanche that the party observed involved the entire north side of the spur. As far as could be determined, the whole slope came away as a single slab, almost at the same instant. As it moved, it was split up into blocks by large trees and buried outcrops. Smaller trees were overridden, bent, broken and torn loose. The fracture line ran approximately down the crest of the spur. On the lower, rounded slope, it cut across the crest, involving part of the southeast side of the spur. The snow ran for one- half mile before decreased gradient brought it to a stop. The slab was approximately five and a half feet thick, and swept the slope clean of snow down to grass and bedrock. COMMENTS The ski party may have triggered the slide, but a rapid change in temperature or heavy wind gusts could also have been responsible. This particular example illustrates the ineffectiveness of shallow probing in determining the existence of avalanche conditions. Fortunately the party leader, an experienced mountaineer, sensed the possible danger and led the party out of the slide path before the avalanche occurred. We wish more cases like this could be reported in these pages. Unfortunately, the " non- accidents" seldom find their way into the files; usually only the disasters are recorded. ( Note: See report 62- 8 for similar accident.) No. 60- 5 LAPLATTA PEAK. COLORADO 19 March I960 WEATHER FACTORS Temperatures during January, February, and the first part of March had been below normal, setting records in several places. On 13 March the cold arctic air was replaced by a warm wind, and temperatures were above normal on the weekend of the accident. The long cold spell had produced depth hoar on most slopes, and avalanche activity was very much in evidence throughout the mountains on 19 and 20 March. The accident area faced west and was warmed by the afternoon sun, but, because of the elevation ( 11,500 feet) it cooled to a very low temperature at night. 59 |