Snowpack Snow layers develop one by one, storm by storm, or through melt-freeze cycles. At any given time, the snowpack in a particular area is composed of several layers with different characteristics of thickness, strength, hardness, and density. Snow Fresh, dry “powder” snow has a low density of 50 to 80 kg/ m2 and does not stick together in a ball. Well-settled “old” snow can reach densities between 300 and 400 kg/m2, and features strong bonds between the snow grains. Wet snow , which is typical of late-season snow, includes liquid water between the snow grains and can be of high density but low strength. A snowpack is commonly warmer than the surrounding air and colder than the ground, which causes the snow crystals within the snowpack to transform. In very cold weather, water vapour rising from the warmer ground and lower snow layers is deposited through deposition onto the snow and ice grains, and forms depth hoar crystals at the base of the snowpack. In clear, cold, and calm weather conditions, water vapour from the atmosphere can be deposited on the snow surface, also forming hoar crystals. Hoar is characterized by relatively large crystals and low strength. Snowpack Stability
The presence of weak layers within the snowpack decreases its stability, as does the accumulation of deep layers on steep slopes. The orientation of the slope with respect to the dominant wind direction or the sun can also determine how much snow an area receives. On downwind (lee) slopes, the wind decelerates and dumps snow, and can contribute to the formation of precariously suspended cornices. Large masses of snow can compact and when the wind blows, it can bring it together and further down, eventually it can collapse. Snow Avalanches Most avalanches occur naturally during or soon after snow storms, with the 24 hours following a heavy snowstorm being the most critical. Avalanches commonly initiate on steep slopes when snowfall builds to 0.5 to 1.5 m thick. But snow thicknesses can reach 2 to 5 m before failing in big avalanches that can be devastating. Snow Avalanches – 1910 Rogers Pass, BC Rogers Pass is a major transportation corridor through the Rockies, and includes the routes of the Canadian Pacific Railway and Trans-Canada Highway. On the evening of 4 March 1910 a group of night workers were busy removing the seven metres of hard-packed snow an avalanche from Cheops Mountain had dumped on the tracks a few hours before. A second avalanche, from the ominously named Avalanche Mountain located on the opposite side of the pass, caught them in action. Of the party of 63 men, only one survived. Size of Snow Avalanches Avalanches vary from the size-1 sluff to very large, size-5 events that can destroy a village or flatten a forest.
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