Avalanche

• Direction - snowpack evolution is influenced by solar heating and wind. In medium latitudes on the northern hemisphere, more accidents happen on shady slopes with northern and north-eastern aspects. Slopes sheltered from the wind tend to gather more snow. Cornices also accumulate on the downwind side of ridges, and can contribute to avalanche danger.

• Profile - convex slopes are statistically more dangerous than concave. Reasons lie partly in human behaviour, and the tensile strength of snow layers versus the compression strength.

• Surface - base avalanches are more common on slopes covered with grass surfaces than slopes with dwarf pines. Boulders or buried vegetation may create weak areas within the snow pack.
  

  Snow

  
  Structure of the snowpack determines avalanche danger. Unfortunately relations between easily observable properties of snow layers (strength, grain size, grain type, temperature) and avalanche danger are complex and not yet fully understood. Additionally snow cover varies in space and so does stability of snow.
• New snow - has not had time to bond with the layers below, especially if it is light and powdery.
• Snow depth - snow that is above the layer of boulders and plants on the slope has none of these natural objects to help anchor it to the slope, and is therefore more dangerous. Naturally this is just the type of snow needed for snowsports such as skiing.
• Snow crystal shape - small ball-shaped snow crystals act as ball bearings and are potentially dangerous, while crystals that interlock are more stable.
• Snow compaction - compacted snow is less likely to move than light powdery layers.
  

  Weather

  
  Weather determines the evolution of snowpack. The most important factors are heating by solar radiation, radiational cooling, temperature gradients in snow, and snowfall amounts and type.
  Most avalanches happen during or soon after a storm.

• Temperature - if the temperature is high enough for gentle freeze-thaw cycles to take place, the melting and re-freezing of water in the snow stabilizes the snow crystals. Temperatures rising significantly over the freezing point may cause the whole slope to avalanche, especially in spring. Persistent low temperatures cause the snow to not gain stability from the freeze-thaw action, and may contribute to an internal "depth hoar" frost layer, where there is a high temperature gradient within the snow.

• Wind - anything more than a gentle wind can contribute to rapid build up of snow on sheltered slopes (downwind), while the wind pressure can also stabilize other slopes. "Wind slab" is a particularly fragile structure -- heavily loaded, poorly bonded. Even on a clear day, wind can quickly shift snow-load to the snow pack.

• Heavy snowfall - cause instability, both through the additional weight, and because the snow has insufficient time to bond.

• Rain - in the short-term causes instability through additional load and possible lubrication of lower layers.
  

  Avalanche avoidance

  
  Due to the complexity of the subject, winter travelling in the backcountry (off-piste) is never 100% safe. Good avalanche safety is a continous process, including route selection and examination of the snowpack, weather conditions, and human factors. Several well-known good habits can also minimise the risk. If local authorities issue avalanche risk reports, they should be considered and all warnings heeded. Never follow in the tracks of others without your own evaluations; snow conditions are almost certain to have changed since they were made. Observe the terrain and note obvious avalanche paths where vegetation is missing or damaged, where there are few surface anchors, and below cornices or ice formations. Avoid travel below others who might trigger an avalanche.
  

  The group

• Do not travel alone. There will be no-one to witness your burial and start the rescue.

• Traversing - is to be avoided to minimise cutting across the slope.

• Risk exposure - minimise the number of people on the slope. Maintain separation. Ideally one person should pass over the slope into an avalanche protected area before the next one leaves protective cover. Route selection should also consider what dangers lie above and below the route, and the consequences of an unexpected avalanche (i.e., unlikely to occur, but deadly if it does). Stop or camp only in safe locations. Wear warm gear to delay hypothermia if buried. Plan escape routes.

• Group size - the party should be large enough to perform a rescue, but additional people will increase the disturbance to the slope. Members should be aware of their duties to search.

• Leadership - If you find yourself in a potentially dangerous avalanche situation, you should seriously question the choice of route, why your safety is being put in jeopardy, and what alternatives might be safer than pressing on.
  

  Human survival and avalanche rescue

  
  Even small avalanches are a serious danger to life, even with properly trained and equipped companions who avoid the avalanche. Between 55 and 65 percent of victims buried in the open are killed, and only 80 percent of the victims remaining on the surface survive. (McClung, p.177). Thus, there is at least a one in five chance that you will be killed if caught in a significant avalanche.

In some cases avalanche victims are not located until spring thaw melts the snow, or even years later when objects emerge from a glacier.

Search and rescue equipment

Avalanche cords

Beacons

Recent digital models also attempt to give visual indications of direction and distance to victims and require less practice to be useful. Some inexpensive beacons also operate only as transmitters, relying on others not to become buried with the only receivers. There are also passive transponder devices that can be inserted into equipment, but they require specialized search equipment that might only be found near an organized sports area.

Mobile phones can seriously disrupt the ability of an ARVA to locate buried victims; phones should be switched off at the same time as the ARVA is switched on in the morning.

Probes

Probing can be a very time-consuming process if a thorough search is undertaken for a victim without a beacon. In the U.S., 86 percent of the 140 victims found (since 1950) by probing were already dead. Outside Online, Feb 2000 Survival/rescue more than 2 m deep is relatively rare (about 4 percent). Probes should be used immediately after a visual search for surface clues, in coordination with the beacon search.

Shovels

A large scoop and sturdy handle are important. Shovels are also useful for digging snow pits as part of evaluating the snowpack for hidden hazards, such as weak layers supporting large loads.

Other devices

More back-country adventurers are also carrying EPIRBs (Emergency position-indicating rescue beacons) containing GPS. This device can quickly notify search and rescue of an emergency and the general location (within 100 yards), but only if the person with the EPIRB has survived the avalanche and can activate the device manually. With modern mobile phone developments, an emergency GPS transmittter may also become more widely available (again, for use by a rescuer, because a victim may be unconscious or completely immobilised beneath dense snow).

Prior to the arrival of beacons, some brave skiers carried a 50-ft ball of nylon cord, tied to the skier, with direction and distance markings attached at intervals. When traversing a dangerous area, the ball would be dropped or thrown and the unravelled string would trail behind. If caught in an avalanche, the hope was that a segment of the string would be quickly found and indicate the distance and direction to the victim.

Although it will be very inefficient, some rescue equipment can also be hastily improvised: ski poles can become short probes, skis or snowboards can be used as shovels.

A First aid kit and equipment will also be useful for assisting survivors who may have cuts, broken bones, or other injuries, in addition to hypothermia.

Witnesses as rescuers

Witnesses to an avalanche that engulfs people are frequently limited to those in the party involved in the avalanche. Those not caught should try to note the locations where the avalanched person or people were seen. This is such an important priority it should be discussed before initially entering an avalanche area. Once the avalanche has stopped, and there is no danger of secondary slides, these points should be marked with objects for reference. Survivors should then be counted to see who may be lost. If the area is safe to enter, a visual search of the likely burial areas should begin (along a downslope trajectory from the marked points last seen). Some victims are buried partially or shallowly and can be located quickly by making a visual scan of the avalanche debris and pulling out any clothing or equipment found. It may be attached to someone buried.

Alert others if a radio is available, especially if help is nearby, but do NOT waste valuable resources by sending a searcher for help at this point. Switch transceivers to receive mode and check them. Select likely burial areas and search them, listening for beeps (or voices), expanding to other areas of the avalanche, always looking and listening for other clues (movement, equipment, body parts). Probe randomly in probable burial areas. Mark any points where signal was received or equipment found. Only after the first 15 minutes of searching should consideration be given to sending someone for help. Continue scanning and probing near marked clues and other likely burial areas. After 30-60 minutes, consider sending a searcher to get more help, as it is more likely than not that any remaining victims have not survived.

Line probes are arranged in most likely burial areas and marked as searched. Continue searching and probing the area until you can live with the fact that your companion may be dead because you didn't try hard enough to find him or her. Avoid contaminating the scent of the avalanche area with urine, food, spit, blood, etc, in case search dogs arrive.

The areas where buried victims are most likely to be found are: below the marked point last seen, along the line of flow of the avalanche, around trees and rocks or other obstacles, near the bottom runout of the debris, along edges of the avalanche track, and in low spots where the snow may collect (gullies, crevasses, creeks, ditches along roads, etc). Although less likely, other areas should not be ignored if initial searches are not fruitful.

Once a buried victim is found and his or her head is freed, perform first aid (airway, breathing, circulation/pulse, arterial bleeding, spinal injuries, fractures, shock, hypothermia, internal injuries, etc), according to local law and custom.

Victims

European avalanche risk table

In Europe, the avalanche risk is widely rated on the following scale, which was adopted in April 1993 to replace the earlier non-standard national schemes. Descriptions were last updated in May 2003 to enhance uniformity pdf.

[1] Stability:

[2] additional load:

Gradient:

Avalanche size:

United States Avalanche Danger Scale

In the United States, the following avalanche danger scale is used.

Probability and trigger	Degree and distribution of danger	Recommended action in back country
Low (green)	Natural avalanches very unlikely. Human triggered avalanches unlikely. Generally stable snow. Isolated areas of instability.	Travel is generally safe. Normal caution advised.
Moderate (yellow)	Natural avalanches unlikely. Human triggered avalanches possible. Unstable slabs possible on steep terrain.	Use caution in steeper terrain.
Considerable (orange)	Natural avalanches possible. Human triggered avalanches probable. Unstable slabs probable on steep terrain.	Be increasingly cautious in steeper terrain.
High (red)	Natural and human triggered avalanches likely. Unstable slabs likely on a variety of aspects and slope angles.	Travel in avalanche terrain is not recommended. Safest travel on windward ridges of lower angle slopes without steeper terrain above.
Extreme (red/black border)	Widespread natural or human triggered avalanches certain. Extremely unstable slabs certain on most aspects and slope angles. Large destructive avalanches possible.	Travel in avalanche terrain should be avoided and travel confined to low angle terrain well away from avalanche path run-outs.

See also

External links

References