This page contains some helpful resources for educating yourself on avalanche danger. Our Slope or Nope Model operates under the assumption that you
will make informed decisions in the backcountry based on conditions you observe throughout the day. The concept of spatial variability, where snow stability changes dramatically
over time and space, makes general predicting difficult so you must use your own observations to make the right call on the spot. In the following we provide some basic
information and point you to good sources to education yourself further.
Basic elements needed for an avalanche
- Slab (load): Consolidated mass of snow
- Weak layer under slab that propagates instability over a distance
- Steep slope: Generally greater than 30 degree slope angle
- Trigger (deformation): e.g. weight of a human, cornice fall, etc.
Dangerous avalanche terrain characteristics (Tremper 2014)
- Is between 30-50 degrees (35-45 is most likely to slide)
- Has a wind load on it: leeward or cross-loaded.
- Has few anchors like trees.
- Has bad consequences if a slide occurs.
Safer avalanche terrain characteristics (Tremper 2014)
- Is gentle (0-25 degrees) or very steep (55+)
- Has densely spaced anchors
- Has less dangerous consequences if a slide occurs
- Is on the windward side of a slope.
Ways to limit risk during winter in the backcountry
- Travel mostly above or clear of steep slopes, not on or underneath them: Terrain travel choices are very important so that you limit your time on a area where
avalanches assist you. During route planning, it is useful to look at maps with avalanche shading and also familiarize yourself with the look of slopes above 30 degree
steepness. If in doubt, use a slope angle gauge to measure steepness. For example, If you want to get up a mountain, look for traveling on the top of a ridge rather than on
the slope that could slide.
- Recognize signs of instability: Look for wind pillows, cornices, and other snow features as indicators of wind direction and dangerous loading. Monitor snowfall
throughout the day. Watch for evidence of previous avalanches or shooting cracks in the snow. Listen and recognize whumpfing or collapsing of the snowpack. Perform stability
tests like slope cuts and Extended Column Tests. Warnings like these should influence your decisions.
- Travel in small groups: Have a partner with knowledge of avalanche rescue and how to make decisions about stability in the backcountry. Make sure your group isn't
too big though, because communication breaks down and accidents more likely to happen with larger groups.
- If you choose to travel on or under an avalanche slope, expose one at a time and be quick about it: Avoid staying together in avalanche terrain. Space out, and keep
eyes on your partners when they enter the danger zone.
- When in avalanche terrain, recognize consequences: If a slide occurs, where will it take you? Snow that takes you over cliffs or into gulleys or trees are much more
- Know the weather forecast and current conditions: Conditions change rapidly, so ensure you are in a situation where you are prepared for what is coming can react
- Have the appropriate gear and know how to use it: The standard gear are beacon, shovel, and probe for backcountry rescue. Small emergency kits such as temporary
shelters and insulators are important to have also.
- Bring a communication system and devices: Have a good plan for communication and practice regular communication with good radios that work in inclement weather. Avoid
reliance on cell phone, but have them for emergency correspondence.
- Know the recommendations for avalanche danger and prior observations where you plan to travel: Knowing the tendency is will give you a head start on good decision
making for the day, and might even prompt a change of plans before you go out. This is the concept of "Know before you go."
Major indicators of instability that should prompt staying away from steep slopes:
- Wind intensity and direction: Generally greater than 19 kph (12 mph)
- Temperature above freezing for 24 hours
- Signs of avalanche: Recent avalanche activity.
- High rate of snowfall: Over 30 cm new snow in 24 hours.
- Whumpfing, cracking, collapsing (AIARE curriculum and Tremper 2013)
Five Structural Lemons
- Weak layer in top 100 cm of snowpack
- Weak layer thickness 10 cm or less
- Hand hardness difference greater than 1 step
- Weak layer grain type-persistent (SH, DH, FC)
- Grain size difference greater than 1 mm (McCammon & Schweizer 2002)
Some general rules of thumb from McClung & Schaerer (2009)
- Sun effect
- Shaded slopes stabilize slowly. In the spring they stabilize more quickly as temperature increases
- Sunny slopes stabilize quicker but in spring they can rapidly become unstable even within a daily cycle.
- Bonding of snow
- Temp below -5°C (23°F) new snow does not bond as quickly. (McClung & Schaerer 2009)
- Bonds form more rapidly under higher weight
- Poor bonds result from mismatches in crystal size, layers over and under crusts, and sometimes changes in crystal riming from one layer to next.
- All persistent layers form during high temperature gradients at and above snowpack (surface hoar), near surface layers (facets and radiation recrystallization), or at the bottom of the snowpack (depth hoar).
- Weak bonds between layers may be more common cause of avalanching than low strength within layers.
- Persistent forms last for several days are are due to layers including buried surface hoar, near surface faceted snow, depth hoar, crusts with weak bonds to snowpack, and radiation recrystallization, or other combinations of these forms
- Nonpersistent avy conditions are from new snow such as stellar crystals due to temperature, wind, and humidity variations in snow deposition. These generally settle out after 24 hours.
A variety of tests exist to standardize the study of snow. Tests are only useful if they reveal instability. In other words, a test with no propagation score does not give a
green light to enter avalanche terrain. The following are a list of resources for learning more about stability tests and their results:
Tremper, B., 2014: Staying alive in avalanche terrain. Seattle (WA): The Mountaineers Books.
Tremper, B., 2013: Avalanche essentials: A step-by-step system for safety and survival. Seattle (WA): Mountaineers Books.
Ferguson, S. A., E. R. LaChapelle, and E. R. LaChapelle, 2003: The ABCs of avalanche safety. Seattle, WA: Mountaineers Books.
Know Before You Go: http://kbyg.org/
More technical resources
McClung, D., and Schaerer, P., 2006: The avalanche handbook. Seattle, WA: Mountaineers Books.
International Snow Science Workshop proceedings: http://arc.lib.montana.edu/snow-science/index.php?view=default
Snow, Weather, and Avalanche Guidelines: http://www.americanavalancheassociation.org/wp-content/uploads/2017/01/AAA_SWAG_Web.pdf