This information is simply for research purposes and does not replace the knowledge you will gain by taking an actual Avalanche Safety Course. It is highly recommended that everybody take an avalanche safety course before heading into the backcountry. You can find courses near you on the AIARE Website or by Googling “avalanche level 1 class”.
The official definition of an avalanche according to Dictionary.com is a large mass of snow, ice, etc. detached from a mountain slope and sliding or falling suddenly downward. While this seems pretty simple, not every avalanche is the same. There are many forms of avalanches that can occur. It’s important to know what causes these and what they are to stay safe in the backcountry.
Loose Dry Avalanches
You have probably heard of the term “sluff” when talking about skiing in powder. Sluff is simply a small loose dry avalanche. These avalanches are triggered at a single point either from a skier, a falling cornice, or even a falling rock and fan out as the snow moves down the mountain. While this doesn’t sound life threatening, these can move a lot of snow and can bury you in a terrain trap. Along with that, this sluff can trigger larger, more dangerous avalanches.
In the picture of Allison to the right, you can see a fan of dry snow below where she entered that pitch. This is a small amount of sluff that was caused by her making turns down a steeper pitch.
Loose Wet Avalanches
As the name suggests, this kind of avalanche is caused by wet/melting snow. These avalanches can be caused by consistent warm temperatures (no overnight freezing) and/or rain. It will be obvious if this is an issue because the temperature has either been over freezing for over 24 hours or it has rained at high elevations. When the conditions have been like this but you still decide to go ski, the best in field observation that points to this kind of avalanche is roller balls. You can think about this as the snowball effect. If you send a small ball of snow down the slope, does snow continually stick to it causing the ball to grow? When you see this, you know the snow is wet and there is a potential for a loose wet avalanche. These kinds of slides are more dangerous than loose dry because wet snow is a lot heavier than dry snow. So, if you are in a loose wet slide, it is very easy to have your skis wiped out from under you.
When triggered, these also continue down the mountain and make a fan shape. If you see traces of one of these while out in the field, how do you tell if it’s a loose wet or loose dry avalanche? Well, if you see large balls of snow all clumped and sticking together in the run out of the slide, you can assume it’s a loose wet because that wet snow caused it to become cohesive and allow the falling snow to stick to itself. If the snow was dry, you will most likely not see these cohesive balls of snow in the run out.
In the example above, you can see Allison coming down and the snow that she is sending into the air looks dry but if you look at the tracks that are closer, you can see chunks of snow have balled up. This is a sign of wet snow that can lead to a loose wet avalanche.
When snow falls on top of older snow from a prior storm, it can cause a weak, unstable layer in the snow pack. If this weak layer collapses or releases, a slab avalanche will occur where a large body of snow will break and slide down the mountain.
Loose wet avalanches are likely to trigger a wet slab avalanche if there is a weak layer in the snow pack. Even if there isn’t a loose wet avalanche as a trigger, these avalanches can be caused by other factors. When you have a recent snow layer on top of a weak layer, there is already some weakness in the snow pack. But as soon as you introduce rain or warming temperatures that causes the new snow to become wet and heavy, that weak layer has even more potential to break and release a wet slab.
These slides are very dangerous. The reason being is as I said above, wet snow is heavy and will take you down quickly. Also, you are moving with a whole slab and not just loose snow. Slabs generally break around you and are extremely hard to get out of.
Storm Slab Avalanche
These avalanches are caused when the density of new or falling snow is higher than the density of the older snow surface causing a weak layer. This can also happen simply because the new snow hasn’t settled in with the snow pack.
There are many signs you can look for to identify if storm slabs are a risk when you are out on the skin track. If you look at steep parts of the mountains you are around, do you see natural slides? If so, storm slab risk is very high. If it has snowed recently and you are out, you can see if the new snow has settled by looking at the bases of trees. What you are looking for is called coning. What happens is the snow settles around trees and creates snow cones that reach up the tree trunks.
Bottom line, if it snowed the night before, you are most likely at risk of a storm slab avalanche.
Wind Slab Avalanche
Wind Slabs are created by wind loading. Wind Loading is where the wind takes snow from the windward side the a ridge and deposits it on the leeward side of the ridge. This creates deep sections of snow that can also be very active when it comes to avalanches.
Wind has a very significant impact on the snow. There are many signs to look for:
- Snow being blown over the ridge
- Wind Effected Snow
- Cornices on the ridge
- Snow drifts
- Variable snow surface
All of these can be persistent slabs or deep slabs. Persistent slabs are slabs that break loose because of a persistent weak layer caused by surface hoar, facetted snow, or depth hoar. These propagate and cause a large surface area of snow to go at once. Signs of these are whumping sounds and cracking when skinning.
Deep slabs are slabs that break due to a weak layer very far down in the snow pack. These slabs often take large impacts to trigger due to how deep the weak layer is. These impacts include cornice falls, large rock falls, or explosives.
For more information on Avalanches, check out the Sierra Avalanches Center Website.
Take this knowledge into the backcountry with you, be observant, be aware, and stay safe.
If you are looking to sign up for a professional avalanche class, check out these resources where you can find one near you:
- AIARE Course Providers: https://aiare.info/providers_list.php
- Upcoming Public Avalanche Training Courses: https://aiare.info/course_list.php
- Choosing an Avalanche Course: https://avtraining.org/choosing-an-avalanche-course/
To refresh yourself on more avalanche safety, check out four more avalanche safety articles here!