"Snowmans" Avalanche FAQs Jim Frankenfield snowman@aip.org This is a collection of postings I have made to various newsgroups. They are not intended to replace or compete with the FAQ which is currently under development (as of 2/14/95) and which should be much more extensive. Contents: ======== I Beacons II Classification of Avalanches III Danger Scale (Europe/Canada) IV Shock Waves I Beacons ========= Avalanche Transceivers (Beacons) by Jim Frankenfield In recent discussions on several internet bulletin boards I have found that a certain amount of confusion exists concerning beacon frequencies and search methods. The frequency standard in this country is changing. History The avalanche transceiver was introduced in this country about 25 years ago. The original units operated at a frequency of 2275Hz. Further development of the technology in Europe led to widespread use of units operating at 457kHz (457000Hz) there. In 1986 the European frequency was adopted as the international standard. 457kHz has been adopted as the US standard effective January 1, 1996. Due to the adoption of the European frequency as the international standard, dual-frequency units have dominated the US market for several years. Beginning this year, single-frequency 457kHz units will be actively marketed in the US. Range There seems to be a lot of confusion concerning the effective range of the various units. The higher European frequency (457kHz) is a better operating frequency, and beacons operating at only this frequency will transmit further (per unit of power) than beacons operating at only the lower (2275Hz) frequency. This is due to the electromagnetic properties of snow and their variation with frequency. Dual-frequency units have a lower range partly due to the fact that part of their power goes into transmitting at the lower inefficient frequency and partly because the antenna coil can not be designed to be ideal for both frequencies at once. Search Methods, Features, and Practice In addition to the 'traditional' search method there is another method known as the 'field line' or 'induction' method. This involves following the magnetic field lines towards the source. References have been made, erroneously, to 'beacons which work this way'. This is a method of searching, not a type of transceiver. The method can be used with any type of beacon, and has been in use by some people for quite some time. When done properly, it is claimed to be a faster method. New units are touting all kinds of features, some of which may help one conduct a field-line search more efficiently. While some of the new features are probably helpful, they do not reduce the need for practice. Very few people invest enough time in practice. Learning to use a new method involves an even larger investment of time. Practice often. If you are unfortunate enough to have to search for a person use the method you know best from your practice sessions. Implications With the advent of single frequency 457kHz units on the US market there is now a possibility that two units will not be able to 'communicate' with each other. People owning the old 2275Hz-only units should purchase new units soon. It would be advisable to do a beacon test at the trailhead or parking area. All participants should put their units in receive mode, and then one person at a time should transmit. Make a note of any beacons which cannot hear each other. This also checks that each unit is functioning properly. Put new batteries in now, and carry a spare set in your pack in case you find you need them at the trailhead check. II Cassification of Avalanches ============================== There are various reporting and classification schemes for avalanches. Type of Avalanche: HS Hard Slab SS Soft Slab WS Wet Slab L Loose WL Wet Loose Trigger: N Natural AS Artificial, Ski AE Artificial, Hand Charge AA Artificial, Artillery AL Artificial, Avalauncher AO Artificial, Other Size (*In relation to Path*): 1 Sluff 2 Small 3 Medium 4 Large 5 Major/Maximum Running Surface: G Ran to Ground in Starting Zone O Ran on old snow surface 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: Size Description Typ Mass Typ Length Typ Impact Pres 1 Relatively Harmless <10 t 10m 1 kPa 2 Could bury, injure, or kill a person 100 t 100m 10 kPa 3 Could bury a car, destroy a small bldg, or break trees 1000t 1000m 100 kPa 4 Could destroy a rail car ... 10000t 2000m 500kPa 5 Largest known 100000t 3000m 1000kPa 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. III Danger Scale (Europe/Canada) =============================== November 9 avalanche bulletin CANADIAN AVALANCHE CENTRE AVALANCHE INFORMATION 9 NOVEMBER 1994 9 NOVEMBER 1994 A number of European countries have agreed on a unified scale that has five levels of avalanche danger. This scale will be used in Canada by national & provincial parks, ski areas and the Canadian Avalanche Centre in all information for the public. The avalanche danger may be described in one word by radio, TV or print medias similar words are used to descibe fire and swimming danger in the summer. No decision should be made on this one word description but it must be used as a prompt for getting more detailed information if planning a trip into avalanche terrain. The following scale of words and definitions are a formal guideline to avalanche conditions. More detail will be given in Bulletins about weather, snowpack, avalanche activity and current information. The Bulletin from the Canadian Avalanche Centre is not a forecast but primarily a summary of prevailing conditions to help make the best informed decision before heading out into the backcountry. The new danger scale follows: AVALANCHE DANGER SCALE Degree of Danger Avalanche Activity __________________________________________________________________________ 1 LOW On a few very steep extreme slopes, triggering is green possible only with high additional loads such as a group of skiers, snow machine or avalanche blasting. Only a few natural avalanches possible. 2 MODERATE On some isolated steep slopes triggering is possible with yellow high additional loads such as skiers, snow machine or avalanche blasting. Large natural avalanches are not likely. 3 CONSIDERABLE On selected areas of steep terrain, triggering is ochre possible sometimes even with low additional loads such as a skier or walker. Medium and occasionally large natural avalanches may occur. 4 HIGH On many steep slopes, triggering is probable even with orange low additional loads such as a skier or walker. Frequent medium or large natural avalanches are likely. 5 EXTREME Even on moderately steep slopes, triggering is likely red with or without additional loads. Numerous large natural avalanches are certain. IV Shock Waves ============== >Is it true that just before an avalanche there is a shock wave? We were >having this conversation yesterday, and someone remarked that in most >cases this shock wave is the cause of all avalanche related accidents. 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.