Although individual flakes weigh almost nothing, large accumulations of snow can be immensely heavy. When snow builds up on a sloped surface, gravity continually tries to pull that snow downhill. The steeper the slope, the greater the pull of gravity will be. Because snow sticks together (cohesion), it will often resist the pull of gravity. However, if the weight eventually becomes too heavy (overloading) or if something disturbs the snow structure (such as a skier), snow can break loose and tumble downhill. This may result in a chain reaction, causing more snow to break free and tumble down the slope, potentially burying anything and anyone caught in its path.
Types of Avalanches
According to MetEd, there are two primary types of avalanches. A “loose snow avalanche” typically starts at a single point and gathers momentum as loosened snow tumbles downhill. This may also be called a “sluff avalanche” and often displays an inverted “V” pattern. The second type is a “slab avalanche,” which occurs when a large section of compressed snow breaks loose and slides downhill. Snow slabs are often formed by compressed, windblown snow. Slabs can form in a single layer (from one large snowfall) or multiple layers (from several different snowfalls over time). A slab avalanche can usually be identified by a noticeable fracture line that appears where the slab detached from the surrounding snowpack.
Most avalanches occur on 30° to 45° slopes, although slopes greater than 45° can also be prone to slides in the right conditions. Snowfall, wind and temperature changes are all key factors in forming avalanche conditions. In general, slab avalanches tend to be more dangerous than loose snow avalanches, but all avalanches are dangerous. If a large snow slab breaks free, especially on a steep grade with a smooth sliding surface, the resulting avalanche can be very powerful.
Weak, less dense layers of snow in between denser, more cohesive layers add to the likelihood that a slab may break free of the surrounding snowpack. This is called “unstable snowpack.” Weak layers can form with sudden and/or prolonged temperature changes. Aside from a steep slope grade, a smooth surface beneath an avalanche trigger point can result in less resistance and greater avalanche momentum. A smooth, icy surface or “crust” may form when rain falls onto snow and freezes overnight, or when the snow surface melts during the day and refreezes at night. When combined, the following three conditions create an ideal environment for an avalanche to occur:
- A large, heavy slab of snow or recent heavy snowfall
- Unstable snowpack
- A smooth sliding service
Avalanche experts can often make educated guesses about how weather will contribute to potential avalanche conditions. By tracking the temperature before, during and after a snowfall and keeping a log of previous snowfall conditions, avalanche experts can usually determine if an area will have good snow bonding or poor bonding. Poor snow bonding (and subsequently unstable snowpack) is a leading contributor to avalanche conditions. Always check an avalanche report before visiting an area. If a report is unavailable, it’s possible to test snowpack by digging a series of snowpits in the area you’ll be visiting. However, it’s strongly recommended that you take an avalanche safety course before attempting to test snow conditions in the backcountry. Check out our blog post on How to Dig a Snowpit for more information.
On sloped, snow-covered terrain, be on the lookout for potential avalanche warning signs before choosing your route. First, determine if the slope angle of an area you are about to traverse is greater than 30°. If so, it’s important to look for signs that could indicate potential slide conditions. These include signs of previous avalanches, signs of unstable snowpack and/or large blocks of windblown snow. If an area has recently experienced heavy precipitation within the past 24 hours and/or significant shifts in temperature, it’s important to be extra cautious. Identifying (and avoiding) potential trigger points and terrain traps is also important. Check out our blog post on How to Read Avalanche Terrain when Route-Finding for more details.
When snowpack is unstable, avalanches occur naturally in alpine conditions. These natural avalanches may be triggered by overloading from heavy snowfall, strong winds and other factors. However, unexpected avalanches can also be triggered by a skier, snowboarder, snowshoer or snow-mobiler. It doesn’t always take much to trigger a slide. According to the National Avalanche Center, “in 90 percent of avalanche accidents, the victim or someone in the victim’s party triggers the avalanche.”
In-Bounds vs. Out-of-Bounds Terrain
In order to mitigate risk to skiers and snowboarders, many established ski resorts actively monitor in-bounds terrain for avalanche conditions. Some resorts have special zoning maps that ski patrol uses to periodically check for the formation of avalanche conditions, allowing them to temporarily close off specific areas as needed. Cornices with large, overhanging snow slabs are one example of an avalanche hot spot that must be frequently inspected. Resorts may also take measures to intentionally trigger avalanches (always making sure the area is completely clear of people and equipment first), using small explosive charges. This technique is called “artificial triggering,” and allows a dangerous area to be cleared of unstable snowpack by forcing an avalanche to occur in a controlled environment.
Although avalanches are much less common on properly maintained, in-bounds ski resort terrain, they can still occur, so it’s important for skiers and snowboarders to keep an eye out for warning signs and pay close attention to resort boundary lines. Ducking under ropes to hunt for powder can be a very dangerous proposition. Before venturing out of bounds, people must be aware of the risks and should be equipped with avalanche safety equipment (and also know how to effectively use this equipment).
The most important part of avalanche safety is knowledge. Understanding how to identify and avoid potential avalanche conditions is a specialized skill. Of course, even experienced backcountry skiers can be caught off guard, which is why carrying transceivers and other avalanche rescue equipment is essential for all out-of-bounds/backcountry skiing and snowboarding.
- Important Safety Warning: Anyone planning to use avalanche rescue equipment should take an avalanche safety course from a reputable outdoor school or organization. Even having all the right knowledge and gear does not guarantee rescue in an avalanche burial scenario.
An avalanche transceiver (often called an avalanche beacon) is a device capable of emitting and also pinpointing a pulsing signal. All transceivers should have at least two modes: transmit and receive. When heading into the backcountry, skiers should all turn their beacons to transmit mode. This means that everyone’s beacon will be constantly emitting a signal. In the event that a skier or skiers become buried in an avalanche, the other members of the group will switch their beacons to receive mode (also called search mode) and use their transceivers to pinpoint the buried victims and hopefully rescue them.
Using avalanche beacons effectively takes skill and practice. Because buried victims often have a limited air supply, pinpointing them quickly is critical for survival. Before heading into the backcountry, learn how to correctly use all the features of a new avalanche beacon and practice pinpointing both single and multiple targets. (The mode used to pinpoint individual victims in a multi-victim burial may be different from the mode used to pinpoint a single victim, depending on the beacon’s features).
Once a transceiver is used to pinpoint a buried victim, a long metal probe is used to further pinpoint the exact location of that victim underneath the snow. This should give rescuers a better idea of exactly where to dig and how deeply the victim is buried. Avalanche probes are typically made of hollow aluminum rods divided into segments that can be quickly assembled into a single, long probe.
An avalanche shovel or “rescue shovel” is a small- to medium-sized shovel with a detachable handle. These shovels are available in a variety of designs and crafted of lightweight materials. Most are small enough to fit inside a backpack or be strapped securely to the exterior of a pack. Backpacks for skiing and snowboarding often have a dedicated compartment on the front designed to hold an avalanche shovel.
Some ski jackets and pants are equipped with built-in RECCO® passive avalanche recovery reflectors. These reflectors don’t actively emit any signal. However, ski patrol units at many larger resorts are equipped with RECCO® detectors, which are designed to pinpoint avalanche victims wearing RECCO® reflectors. Both ski patrol units and helicopter searchers can pinpoint burial victims using a RECCO® detector (if the victims are wearing reflectors and within range). The RECCO® system is not a substitute for avalanche transceivers or companion rescue. However, RECCO® reflectors can potentially help rescuers find burial victims who might otherwise be extremely difficult to locate.
Intended to help keep victims closer to the surface of an avalanche, backpacks with built-in, deployable air bags have become increasingly popular among backcountry skiers and snowboarders. Several well-known brands now offer avalanche airbags, including Black Diamond Equipment, Mammut and Backcountry Access. According to ABS, increasing the volume of a skier with inflated air bags decreases the likelihood of that skier being completely buried or buried as deeply as they would be without an airbag. However, in order to gain any possible benefit from an air bag, the victim must deploy the airbag before losing the ability to do so. Unless the bag is self-deploying, this requires a fast reaction time. Avalanche airbags are not a substitute for avalanche transceivers or companion rescue.
Getting caught in an avalanche can be a worst-nightmare scenario, even with the proper safety gear. Depending on the type and severity of the avalanche, there may not be much a person can do to avoid getting buried. If that person does get buried, chances are they’ll be stuck until friends or rescuers can dig them out. However, according to Avalanche.org, there are a few tactics that may help:
- If you’re skiing or snowboarding and get knocked down by an avalanche, attempt to ditch your skis or board before you get swept up completely.
- Attempt to stay on your back with your feet pointing downhill, if possible. Swim with arms and feet and try and stay on the surface.
- If trees are nearby, attempt to grab ahold of a tree trunk and hold on.
- If you find yourself pulled underneath the surface, push outward with your hands and clear as much snow from around your mouth and face as possible.
- If you do get buried, try hard to remain as calm as possible and control breathing. This will help conserve oxygen and may give rescuers more time.
- Always carry avalanche safety equipment in the backcountry and know how to use it, and never go out alone.
- Avalanches most commonly occur on 30° to 45° slopes, and in a variety of snow conditions. Even in-bounds terrain can be hazardous. Learn to recognize avalanche danger signs.
- Research snow conditions and avalanche reports before you head out.
- Never venture into the backcountry or out-of-bounds terrain alone.
- When skiing or riding off-piste, every member of a group should carry a beacon, probe and shovel. All members should know how to effectively use avalanche rescue equipment and techniques.
- Members of a group should stay spread out and always cross potentially hazardous slopes one at a time. This will decrease the likelihood of multiple burials during a slide, leaving others available to perform a rescue.
- Always check your beacon’s battery level before heading out. Store your beacon securely on your person with a strap or tether.
- Practice location and rescue techniques in a safe, controlled environment before venturing out of bounds or into the backcountry.
This guide is intended to provide an introduction to basic avalanche information and gear. This is not a substitute for an avalanche safety course or program. Backcountry skiing, snowboarding, snowmobiling, climbing and other related activities are inherently dangerous. Anyone purchasing or using equipment for this purpose is personally responsible for getting proper instruction on its correct and safe use. Please seek out a certified instructor to address any questions you may have about avalanche safety gear or techniques.