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Show path. A human being caught in one would be killed or severely injured, property damage would be heavy. The medium- large class of avalanches recognizes the vulnerability of humans as compared to property. Injury is likely and death possible to a person caught in an avalanche of this size, property damage would probably be moderate. The medium avalanche is only mildly dangerous to humans and property damage would be light if it occurred at all. The small avalanche is harmless. This method of classification has its disadvantages. In borderline cases, the opinion of two observers might differ. With this exception, the classification by hazard is rapid, flexible and accurate. TYPE Dry snow avalanches ( Figure 12) are composed of loose snow, possibly drifted but not windpacked. They start at a point, and travel at high speed on a gradually widening path, increasing in size as they descend. If of major proportions they can be accompanied by destructive air blast, being composed of light materials, they move in the air as much as on the ground. Danger to human beings is from suffocation in this cloud of snow dust as well as from physical violence. A dry snow avalanche is almost always shallow at the start and depends for volume on the snow it can pick up during its run. Thus a dry snow avalanche of dangerous size can only occur on a long slidepath, or from a large accumulation zone which funnels into a constricted outrun. Occasionally heavy snowfall at low temperatures produces the phenomenon of the " wild snow" avalanche, formless masses pouring down the mountainside. They are really avalanches of air and snow mixed. Damp and wet snow avalanches ( Figure 13) often resemble those of dry snow with the same arrowhead point of origin, gradually becoming wider. Being heavier and stickier, they develop more friction and therefore travel at a slower pace. An alert skier can generally outrun them and the principal hazard is to fixed installations. The comparatively low speed causes them to stop rather suddenly when they lose momentum and to pile up in towering masses of snowballs and snow " boulders". This is in contrast to the dry slide which tends to spread out like the splash of a wave. Damp and wet slides solidify the moment they are released from the pressure of motion, adding to the difficulty of rescue or clearing operations. Wet slides have a distinctive habit of channeling. The moving snow constructs its own banks and flows between them like a river of slush, often in unexpected directions. The damp sunslides of midwinter are generally shallow. But the wet avalanches of spring, caused by deep thawing either from rain or prolonged temperatures above freezing, often involve enormous masses of snow and debris and incalcuable destructive power. The coastal alpine zone specializes in a type of midwinter damp or wet snow avalanche with dangerous potentialities. This is caused by very wet snow or rain falling on an unconsolidated surface. Strictly speaking, this is a combination avalanche and should be so classified. - 30 - |