Access path to BEM. Photo Courtesy of SAIS Glencoe
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| Current high avalanche risk and the big avalanche today on BEM made me think a reminder of of the Danger Scale might be worth posting. |
The Avalanche Danger Scale uses five progressively increasing danger levels: Low, Moderate, Considerable, High and Extreme. It indicates the likelihood of avalanches, how they might be triggered and recommended actions in the back country. However, the wording is very brief and does not include a meaningful indication of risk. Below is an explanation of each danger level, including the transitions between levels, signs of instability at each level and the implications of slope angle, aspect and elevation.
Low
Travel is generally safe. The snow pack is well bonded and natural avalanches will not be seen except for small sluffs on extremely steep slopes. Human-triggered avalanches are unlikely except in isolated locations in extreme terrain. The danger will usually be from wind-driven snow in gullies and chutes or deposited across very steep open slopes near ridge lines. Ski or board one by one as smoothly as possible without falling if you suspect the formation of wind slab. Be aware of shaded, north to east aspects where the danger may be transitioning to Moderate. There are few fatalities at this danger level.
Moderate
This is the most difficult danger level for back country skiers and boarders to assess snow stability. Many of the usual indicators such as cracks, settling, whumpfing and signs of recent avalanche are absent, especially at the lower end of the moderate level. Key indicators are any recent snowfall, and wind deposition. Snow pack tests may help assess stability.
Conditions are generally favorable for travel providing routes are chosen carefully. The snow pack is only moderately bonded on some steep slopes. Areas of danger are usually restricted to certain types of terrain such as bowls and gullies. The altitude, aspect and type of terrain where danger can be expected are usually detailed in the Avalanche Forecast. Remote triggering is unlikely, so you only need to be concerned about the steepness of nearby terrain features.
Human-triggered avalanches are possible. Ski or board carefully, one by one, in suspect terrain and avoid high loading of the snow pack by spreading people out on the uphill track. Carefully evaluate the stability of very steep slopes (steeper than 35°) and aspects identified as potentially dangerous in the Avalanche Forecast.
Be especially careful if the higher elevation band in the forecast, or the danger on other aspects, is Considerable. There is a significant difference in instability between Moderate and Considerable. Don’t get sucked onto higher, steeper and more dangerous slopes. Although naturally triggered avalanches are not expected, ice climbers should watch out for the sun warming steep collection zones above their climbs.
If deep-slab instability due to a persistent weak layer is mentioned in the Avalanche Forecast, you need to pay careful attention to the terrain. Avalanches from such a layer are not only likely to be large and extensive, they are completely unpredictable. Unless you have specific local knowledge, keep off large open slopes at this danger level if the forecast warns of a persistent weak layer.
Considerable
Conditions have become much less favorable. The snow pack is only moderately or poorly bonded over a much larger area of the terrain. Human triggering is possible by a single skier on steep slopes and aspects mentioned in the Avalanche Forecast. Remote triggering of avalanches is possible, so the maximum steepness of the slope above you should be used when deciding if you want to continue.
Instability indicators mentioned in Moderate danger above will likely be present. Back country touring at this danger level requires good route finding skills, and experience in recognizing dangerous terrain and evaluating slope stability.
Keep to slopes of less than 35°, especially slopes at the altitude and aspect indicated in the Avalanche Forecast. Remember that remote triggering is possible. Typically the scree fans at the bottom of gullies starts out at around 30° and the slope steepens as it gets higher. Keep off such slopes at this hazard level. The remarks about persistent weak layers in the previous section on Moderate danger level also apply to this danger level.
High
Conditions have become dangerous, most often as a result of significant amounts of new snow, snowfall accompanied by wind or the snow pack becoming isothermal and threatening wet-snow avalanches. The snow pack is poorly bonded over large areas and human triggering is likely on steep slopes (steeper than 30°). Remote triggering is likely and large natural avalanches are to be expected.
Stay on slopes that are flatter than 30° for any part of the slope and be aware of the potential for avalanches from slopes above. If you do decide to walk ski or board on less steep slopes, be very aware of the surrounding terrain to avoid inadvertently crossing the bottom of steeper slopes or cutting down a steep convex rollover.
Usually this level of hazard is only present for a few days at a time. The smart back country traveler will stay in simple terrain until conditions improve. If you are caught out on a multi-day trip you may have to dig in and wait for travel conditions to improve and the avalanche danger to lessen.
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| Glencoe 19th March 2013 |
These circles in the avalanche forecast. My take is to think
of them as landmines blown by the wind, lurking in eddies from cross loading
when the wind blows across as well as down or over a slope, the color of them
is the sensitivity of the pressure plate to you the trigger. If there are
enough of them the explosion will propagate setting of others, or if the
surrounding slope is weak enough then it will slide with it. As you can see there are areas of High on slopes with a "considerable" risk. A lethal combo of detonator in red - and explosive on orange making for events that will take lives if you don't tread warily.
Extreme
Extreme danger levels are rare in Scotland as usually this level is associated with buildings and roads or alpine villages under threat, and usually the result of unusually large amounts of new snow. The snow pack is weakly bonded and unstable. Numerous large avalanches are likely. The weight of the new snow can trigger avalanches on layers buried deep in the snow pack. Natural avalanches can release on slopes of less than 30° Back country touring is not recommended and often impossible. Avoid all avalanche terrain and keep well away from avalanche path run outs.
