An avalanche is a mass of moving snow which has been triggered by a natural or man-made occurence. Avalanches occur when the pull of gravity, plus the effect of the trigger, creates stress which exceeds the strength of the bond between layers in the snowpack.
The four ingredients for an avalanche
The majority of slab avalanches start on areas between 30 and 45 degrees. While this is the most critical zone, any slope over 15 degrees must be regarded as possible avalanche terrain and attention given to the terrain features in relation to the snowpack.
Most avalanches occur during or soon after prolonged or heavy snowfall. As a general rule give it 24 hours.
A Weak Layer
The snowpack is made up of layers that may differ in strength, crystal size and their adhesion to each other. Weak bonding and weak layers are generally the cause of slab avalanches. Overtime the snowpack can change. This can make the snowpack weaker or stronger!
If someone is buried in an avalanche it is most likely the victim was the trigger. Other natural triggers may be; heating by the sun, cornice collapse and wind loaded snow etc.
- Compacted avalanche debris will weigh in excess of 500kg per cubic metre – half a ton.
- 60% of avalanche victims who died whilst buried were showing something on the surface.
- You have a 90% chance of survival if you are found in the first 15 minutes.
- If you immediately alert a rescue, the fastest they could be there on average is after 30 minutes. You have halved the survival chance. You must search and try to find them yourself.
- Most avalanches are triggered by it’s own victims.
- Of those who survive the avalanche and are buried, 50% will be dead after 30 minutes. 90% will be dead after 2 hours.
- If you are buried by more than 1 meter of snow, it is unlikely that you will be dug out until after 2 hours.
- Nearly 20% of people avalanched will die before the avalanche stops.
Actions to take
- You only have 15 minutes. Use your eyes to search the site.
- Mark the spot where the victim(s) were taken from by the avalanche.
- Mark the spot where they were last seen during the avalanche.
- Using these two spots as a guide, continue down that line into the debris, it may narrow down the search area.
- Search with transceiver.
- When located on the surface with the transceiver, use the probe to feel the victim.
- Leave the probe in when the victim is felt.
- Dig down to the probe.
- Clear the face and chest.