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Show On slopes of more than about 30° inclination, typical powder avalanches can form from as yet unloosened snow after exceeding a velocity of 15 to 20 m/ sec. To begin with the nucleus of the avalanche is surrounded by a turbulent layer of air with suspended snow, which increases with increasing velocity and running time, so that finally the entire mass is transformed into a more or less uniform aerosol, which, according to observations and computations, behaves very similar to a compressible fluid of corresponding density. The velocity conforms to Equation ( 4) with ( 9) h' = ( Y / Y) ( h + h ) = ( Y0/ Y > h ( 1 + m) where h = h = 0, m = h / h h « is the height of the whirled up layer of natural snow cover lying in front of and under the avalanche. With a solid frozen snow surface hA = 0, while in exceptional cases it can be true that h « = h, if the natural snow cover consists of dry powder snow throughout. Tne evaluation of the measurements of Canton Forester Dr. M. Oechslin [ 4] ( Figure 24) by Equations ( 4) and ( 9) shows that, as a rule, the disintegration of the natural snow layer overrun may be neglected and thus in Equation ( 9) hA = m = 0. The calculated average curve shown in Figure 24 corresponds to an average density for the natural snow cover of 75 kg/ m3# xne deviations of the individual values from the calculated average curve are caused by the deviations of the effective densities from their average value. Evaluation of the individual measurements leads to the conclusion that powder avalanches, as a rule, are formed when the density of the natural snow cover is less than 100 kg/ m3, i. e., mostly in fresh new snow. In nature, phenomena of motion proceed in such a way that the potential energy tends toward a minimum, in the course of which, the kinetic energy reaches the maximum possible under the given circumstances. The highest possible boundary value of the kinetic energy is reached when Equation ( 4), including consideration of Equation ( 9), yields a maximum, i. e., for Y = 2 YL-This optimum of disintegration and aereation can, however, only be reached, if the powder avalanche occurs in terrain of comparatively small gradient ( Figure B), because at the avalanche front the statically distributed force, neglecting the vertical acceleration, must counter balance the damming force of the air which is pushed ahead, that is, from consideration of Equation ( 4) and neglecting \ i ( Y - Y ) h Y / Y - Y v / 2g = L o o L ( YL/ 2g ) £ ho ( Y J Y ) ( 1 - Y L / Y ) sin • from which ( 10) Y = < YL/ 2g ) 5 sin\| r Moreover the damming pressure of the air p = Yj_ v / 2g remains equal 28 |