Avalanche danger scale explained, level by level
The five avalanche danger levels are Low, Moderate, Considerable, High and Extreme. Considerable means human-triggered avalanches are likely and is the rating associated with the largest number of accidents, because it is the highest level on which most people still travel. The rating is a summary: the avalanche problem, aspect and elevation are what translate into route choice.
The danger scale is the headline on every avalanche forecast and it is the least useful part of it. It compresses likelihood, expected size and distribution into one word, which is genuinely helpful for a quick read and completely insufficient for planning a route. What plans a route is the avalanche problem, the aspects and elevations it lives on, and the discussion underneath.
The five levels
| Level | Name | Likelihood | Size and distribution | Travel advice |
|---|---|---|---|---|
| 1 | Low | Natural and human-triggered avalanches unlikely. | Small avalanches in isolated areas or extreme terrain. | Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. |
| 2 | Moderate | Natural avalanches unlikely, human-triggered avalanches possible. | Small avalanches in specific areas, or large avalanches in isolated areas. | Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully and identify features of concern. |
| 3 | Considerable | Natural avalanches possible, human-triggered avalanches likely. | Small avalanches in many areas, large avalanches in specific areas, or very large avalanches in isolated areas. | Dangerous avalanche conditions. Careful snowpack evaluation, cautious route finding and conservative decision making are essential. |
| 4 | High | Natural avalanches likely, human-triggered avalanches very likely. | Large avalanches in many areas, or very large avalanches in specific areas. | Very dangerous avalanche conditions. Travel in avalanche terrain is not recommended. |
| 5 | Extreme | Natural and human-triggered avalanches certain. | Very large avalanches in many areas. | Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain. |
This is the North American public avalanche danger scale as published by avalanche forecast centres. It is a summary of conditions, and it is deliberately written in terms of likelihood and size together, because a small avalanche that is very likely and a very large avalanche that is unlikely can both produce a Considerable rating for entirely different reasons.
Considerable is the one that catches people
Published accident analyses consistently associate the largest number of avalanche fatalities with the Considerable rating rather than with High or Extreme. That is not because Considerable is the most dangerous set of conditions. It is because it is the highest rating on which large numbers of people still go out.
High and Extreme keep almost everybody at home, and Low and Moderate genuinely do describe less dangerous conditions. Considerable sits exactly where the exposure is: dangerous conditions, human-triggered avalanches likely, and a large number of parties in the terrain anyway. If you take one thing from this page, take that.
The nine avalanche problems
| Problem | What it is |
|---|---|
| Loose dry | Sluffing of unconsolidated dry snow, usually on steep terrain. Small on its own, dangerous where it can carry you into terrain you cannot survive. |
| Loose wet | The same thing in wet snow, typically after warming or rain. Heavy, and it entrains more mass as it runs. |
| Storm slab | A cohesive layer of recent storm snow failing within itself. Common, widespread, and often the reason a forecast rises after a storm. |
| Wind slab | Snow transported and deposited by wind, denser and stiffer than what fell. Found on leeward aspects and behind terrain features, and it can persist for days. |
| Persistent slab | A slab sitting on a buried weak layer that will not go away, such as facets or surface hoar. Harder to trigger, far more likely to be large and fatal, and the problem that most experienced people are caught by. |
| Deep persistent slab | A persistent weak layer deep in the snowpack, often near the ground. Rare to trigger and catastrophic when triggered. Uncertainty is high and the only reliable answer is terrain avoidance. |
| Wet slab | A slab failing due to water percolating to a weak layer. A spring and warm-storm problem, often with a clear daily timing pattern. |
| Cornice fall | Overhanging snow at a ridgeline breaking off, often triggering a slope beneath. Dangerous both to stand on and to stand under. |
| Glide avalanche | The entire snowpack sliding on the ground, usually on smooth rock or grass. Effectively unpredictable in timing, so the answer is to avoid glide cracks entirely. |
The problem type matters more than the rating, because different problems demand different responses. A wind slab problem is spatially specific and you can often manage it by avoiding leeward features. A persistent slab problem is not manageable in the same way: it can be hard to trigger, it produces large avalanches, and the standard advice from every forecast centre is conservative terrain choice rather than clever route finding.
The uncertainty attached to a problem is part of the forecast too, and it is why an experienced party will sometimes stay on low-angle terrain at Moderate with a deep persistent slab and ski steeper terrain at Considerable with a storm slab. That judgement is what an avalanche course teaches and what no reference page can give you.
Reading the aspect and elevation rose
The rose is the circular diagram showing danger by compass aspect and by elevation band, usually split into below treeline, near treeline and alpine. It is the part of a forecast that translates directly into route choice, because it tells you which slopes are the problem rather than only how bad things are overall.
Practically: if the rose shows the problem on north through east above treeline, a tour that stays on southerly aspects below treeline is a genuinely different exposure. That is the daily work of backcountry travel, and it is why the same rating can mean go and no-go depending on the plan. How to read an avalanche forecast works through a full example.
Slope angle, for cross-reference
| Angle | What happens there |
|---|---|
| Under 20 degrees | Too shallow for a slab to release on its own. Still exposed to what comes from above: a slope this gentle can sit directly beneath a start zone, and a runout is where debris piles deepest. |
| 20 to 25 degrees | Below the usual slab range, but wet snow and glide avalanches do release here in spring conditions and on very weak snowpacks. Treated as low angle rather than as safe. |
| 25 to 30 degrees | The lower edge of the range. Slab releases become possible, and the number matters less than the fact that measurement error, convexities and steeper rollovers put you in the next band without noticing. |
| 30 to 35 degrees | Inside the range where most avalanches happen, and the band that catches people out because it does not feel steep. Plenty of low-angle powder skiing sits right on this line. |
| 35 to 40 degrees | The peak of the distribution. More avalanches release in this band than in any other, and it is also the angle most skiers consider the best skiing on the mountain. Those two facts are the entire problem. |
| 40 to 45 degrees | Still firmly in the range. Consequences climb sharply because a slide here moves faster and a fall here does not stop. |
| Over 45 degrees | Slabs form less readily because snow sheds continuously as it falls, so loose-snow sluffing dominates. This is not a safety improvement: sluff management becomes its own hazard, and a fall is unsurvivable in the wrong terrain. |
Most slabs release between 30 and 45 degrees, peaking between 35 and 40. Low angle terrain reduces the chance of triggering a slab under your own skis and does nothing about what is above you: a gentle runout beneath a loaded start zone is where debris piles deepest. The slope angle reference covers this in full.
The equipment does not change with the rating
Digital transceiver
A three-antenna digital beacon, worn on the body under an outer layer, switched to transmit, and checked at the trailhead against every other beacon in the party. A beacon in a pack pocket is not a beacon.
Probe, 240 cm or longer
A beacon search finishes at roughly a two to three metre circle. The probe converts that circle into a point, and a strike tells you the exact depth to dig to. 240 cm is the practical floor and 280 to 320 cm is better in deep snowpacks.
Metal-bladed shovel
Avalanche debris sets up like concrete within minutes and plastic blades fail in it. The shovel is where almost all of the rescue time goes, and a burial of one metre means moving roughly a tonne of snow.
Every member of every party carries all three, every time, at every rating, along with the training to use them under time pressure. What changes with the rating is where you go, not what you carry. Read avalanche safety basics for the order that keeps people alive and companion rescue for what the first fifteen minutes actually look like.
Frequently asked questions
What do the five avalanche danger levels mean?
Low is generally safe conditions with isolated unstable features. Moderate means heightened danger on specific terrain features. Considerable means human-triggered avalanches are likely and dangerous conditions exist. High means travel in avalanche terrain is not recommended. Extreme means avoid all avalanche terrain. The scale is about likelihood and size together, not about one or the other.
Which danger level kills the most people?
Considerable, the middle of the scale, by a wide margin in most published accident analyses. It is not because Considerable is the most dangerous rating; it is because it is the highest rating on which large numbers of people still go out. High and Extreme keep people at home, and Low and Moderate genuinely are less dangerous.
Is the danger rating enough to plan a day?
No. The rating is a summary. The useful content of a forecast is the avalanche problem, the aspects and elevations where it lives, the likelihood and expected size, and the discussion. A Considerable rating driven by a persistent slab on north-facing terrain above treeline calls for completely different choices than a Considerable driven by wind slab on isolated leeward features.
What is an aspect and elevation rose?
The circular diagram on a forecast that shows danger by compass aspect and by elevation band, usually below treeline, near treeline and alpine. It is the part of the forecast that translates directly into route choice, because it tells you which slopes are the problem rather than only how bad things are overall.
Does a low danger rating mean I do not need my gear?
No. Avalanches happen at every rating, including Low, and a transceiver, probe and shovel are carried by every member of every party any time you are in or under avalanche terrain. The rating changes terrain choice, not equipment. It also does not change the training requirement.
Where do I find my regional forecast?
In the United States, through your local avalanche centre, which are indexed at avalanche.org. In Canada, Avalanche Canada. Read it on the day of travel, not the night before, because a forecast read yesterday describes yesterday snowpack.
How we choose: we compare published manufacturer specifications, published certification standards and verified owner reviews across retailers. We do not test equipment in person, and every figure quoted here is the manufacturer's published number unless we say otherwise. This is researched buying guidance and not professional instruction: binding release values are set by a certified technician on a calibrated jig, and avalanche terrain requires formal training and a current regional forecast that no article can replace.
