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Show 5. Average Density ( water content per inch of snow) is normally another expression of snow type. But it has some special characteristics of interest for the avalanche observer and is therefore discussed separately. The dry snows typical of the alpine zones have .07 to .09 inch of water per inch of new snow. The very dry snow, typical of the high alpine zone but found occasionally in other zones, is very much lighter. Average densities as low as .04 are common. Damp and wet snows can have average densities running as high as .14. These are normal average density characteristics and have no unusual significance. However, we often find radical departures from the average densities expected, especially on the high side. This can only be the result of wind action on the falling snow and is significant. We have learned that abnormal densities usually indicate conditions favorable to slab. The relationship is not invariable, but an average density of .11 or .12 in a dry or transition type of snow is a danger flag of the brightest hue. 6. Snbwfall Intensity ( inches of snowfall per hour). Like the amount of new snow, the rate at which it falls affects the possibility of avalanche action. We have learned from observation that at low intensities, the stabilizing factors such as settlement usually keep the hazard at moderate levels. The dividing line is in the neighborhood of 1" per hour. Like 12" new snow depth, this is merely an indicator. Snowfall intensity must be interpreted by the forecaster in connection with the other factors, particularly wind action. But we very seldom find hazardous direct action avalanche conditions building up if the S. I. is from ,6 down, and very frequently encounter, them if the S. I. is from .8 up. On the other hand, the very highest snow intensities ( we have recorded 3 inches per hour) usually occur without wind action and without hazard. 7. Precipitation Intensity ( moisture content per hour of snowfall). This factor has been in use only a short time. Its background is described in Appendix A. It promises to be the simplest and handiest indicator of direct avalanche hazard yet discovered. It also yields valuable clues to the likelihood of delayed action avalanches. Precipitation intensity combines elements of S. I., snow type, and average density. Observation to date leads to the following conclusion: If P. I. is continuously at or above .10 inch per hour, with strong wind action and in the absence of sluff cycles, avalanche hazard is critical whenever the P. I. Factor reaches 1. The P. I. Factor is P. I. times duration in hours. - 49 - |