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Stratification of Snow Deposits The stability of a given layer of snow depends not only upon its texture and internal cohesion, but also on the cohesion between the snow layer and the underlying surface. For when a vertical section is cut from the topmost surface of the snow to the ground, it is seen that each successive snowfall forms a separate and distinct layer of snow. In general, the lower layers have larger, better consolidated crystals. The difference between the layers is due to the difference in weather during and between successive snowfalls, and to the greater age of lower layers. Bond between layers.—A new layer will be well bonded to the previous layer if it falls when the previous layer is wet, or if the falling snow itself is somewhat damp. This is particularly true if the temperature falls after the snowfall has commenced, thereby freezing the new snow to the old snow. If, on the other hand, the old snow is-fairly well packed and the temperature is well below freezing, the bond is likely to be very poor until the new layer has also settled. The bond is particularly poor if the new snow falls on crusted snow. The crust may be due to wind, but is even worse if it is due to thawing; and then refreezing to form a smooth icy surface. In other words a layer of loose, unconsolidated snow lying on a smooth crust is apt to avalanche. |
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