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Snowmans Avalanche FAQ - [Contents]
Back to Avalanche Safety Questions Snow density is often expressed as a percentage of the density of water. The density of water is 1 g/cm3. This is 1000 kg/m3 [1 g/cm3 times (1 kg / 1000 g) times (100 cm / 1 m)3]. So if snow density is measured at 110 kg/m3 that can be expressed as 110/1000 or 11%. The higher the density of the snow is as a percentage the closer it is to water. So these two things are closely related. To express the water equivalent you multiply the snow depth (in cm) times the density in percentage, then muliply by 10 since the water equivalent should be in mm. So 11 cm of your 11% density snow is equivalent to 12.1 mm of water. [11 cm time .11 (11%/100) times 10 mm/cm.] New snowfall is most usefully expressed in terms of it's equivalent water (rainfall) in mm since this is a measure of how much load was added to the top of the snowpack. Specifying 11 cm is not so useful since it could be 7% density (and add a small load) or 20% density (and add a larger load). The units above follow the standards used in snow science and meteorology - density is generally in kg/m3 or percent, snow depth is usually measured in cm, and water equivalent is given in mm. This is one of those areas of avalanche lore which is prone to sensationalism. There is not a lot of firm quantitative data on the phenomena. Some slides, particularly large ones, appear to be preceded by an air blast or pressure wave. Destructive airblasts are not common. There are certain paths where the phenomena occurs regularly though. When it does occur the airblast may extend about 100 meters beyond a major path. These pressure waves are not true shock waves since the speeds involved in avalanches are subsonic. The speed of sound inside an air/snow mixture may permit shock waves within the flow, but that would be a different phenomena than what you are asking about. I am not familiar with any studies on this, or any efforts to determine if it actually occurs. Most of the mass transfer and momentum (i.e. destructive power) of an avalanche is in the layer closest to the ground where most of the snow is. The dust cloud, although sometimes spectacular, has a much lower potential for destruction. The density of air is about ten percent that of snow/dust/air mixture so the destructive potential for an air blast is low. Accidents are very rarely associated with any kind of air blast. I am not familiar with any such incidents, but have heard references to rare events where the air blast directly caused injury. The majority of victims survive the actual avalanche, although I'm not sure how many of them are injured. Those that do not survive the ride are most likely killed by trauma (collisions with trees, rocks, etc). Once buried the chances for survival depend on quick, efficient rescue and often on prompt and correct first aid also. There are various reporting and classification schemes for avalanches. Type of Avalanche:
HS Hard Slab Trigger:
N Natural Size (*In relation to Path*):
Running Surface:
G Ran to Ground in Starting Zone Example: HS-AS-4-O :A hard slab running on old snow in the starting zone, large in relation to path, ski released. One of the problems with this system is that the size is in relation to the path, so "Class 3" or "Medium" can vary with the slide path. A reporting system used in Canada has the following size classifications:
Other systems have been proposed by Japanese and Swiss researchers, but have never come into use. There is an international system which is based on morphological classification. It does not have categories that could lead to a term such as 'Class 3'. (Int Comm on Snow and Ice. 1981. 'Avalanche Atlas. Illustrated International Avalanche Classification.' Paris:UNESCO 265pp.) There is a trend towards standardizing such things, including transceiver frequencies and hazard categories. |
