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Show neutral. Other factors are more important in determining the severity of the hazard. The prolonged cold of the high alpine zone has a different effect. It preserves any instabilities which exist by retarding settlement and leads to the hazard typical of this zone, the delayed action slab formed in snow wind- transported from the surface. The inverted storm, beginning cold and ending with thawing temperatures, is an extreme example of departure from normal. It is an avalanche breeder well known to the coastal alpine zone. The mechanics are obvious. The wet snow, having little stability, begins to slide and carries along the unconsolidated dry snow beneath. The excessive weight of a wet layer may also overcome the cohesion of a dry layer which might be stable enough if left to itself. Sudden temperature changes in either direction produce a certain amount of tension in a snowfield, sufficient on occasion to set the stage for an avalanche release. Temperature plays an obvious part in governing snow types. Fluctuations in the upper air strata, beyond the reach of observation, are probably as important as at ground level. For the avalanche observer, therefore, the fact that the thermometer stands at this or that level is less important than the effect as revealed by snow types and settlement rates. Temperature also influences delayed action avalanche hazard. The effect of prolonged cold following a storm has already been mentioned. Strong sun action after a storm seldom fails to produce a cycle of sunslides. These are an excellent guide to stabilization of the surface. They indicate that sun action is or has been effective and that tensions have been relieved. The penetration of sunslides in loose new snow is generally shallow. If slab on the surface is involved, the situation is more threatening. Penetrations in depth are likely. A disastrous avalanche in Idaho in 1952 was apparently a temperature release of slab caused by an intrusion of warm wind. The shallow sunslides of midwinter merge into the deeper wet avalanches of late winter and spring. The latter require temperatures well above freezing for 24 hours or longer. Rain and warm wind are more effective thawing agents than sunlight since much of the sun's radiated heat is reflected into the air by the white snow surface. FACTORS STILL TO BE EVALUATED Some investigators believe that relative humidity plays an important part in the formation of slab. We have no reason to think otherwise but data for evaluation are difficult to obtain. During a storm, relative humidity gages go to a high level and stay there, leaving little room for interpretation. In the case of slabs developing in snow wind- transported from the surface, relative humidity readings would have to come from the deposition zone to be of any value. In addition to the mechanical difficulties, relative humidity readings at sub- freezing temperatures are subject to wide margins of error. - 52 - |