Thursday, 8 December 2011

Avalanche Danger Scale Re Post

As it's near the ski season I will re post some stuff  just to give a heads up for folk on interpreting the forecast.  This post was from when the SAIS forecast first changed to the wheel type view and was to help folk understand how it related to the European scale which is identical to the one below now.

Click for a larger size

The new scale is the result of collaboration between avalanche professionals in Canada, the USA and Europe. It is a North American Danger Scale with Icons standardized internationally (recognizable by Europeans visiting Noth America.)

Key changes:
  • The danger levels have been reversed — Extreme is now on top.
  • Extreme and High have been grouped as backcountry travel is not reccommended at either level.
  • Icons have been added to give an at-a-glance indication of the risk of travel.
  • Travel advice is given for each danger level.
  • The likelyhood of avalanches information has been retained from the previous scale.
  • The last column gives the potential size and extent of avalanches.
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 backcountry. 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.

Travel is generally safe. The snowpack 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.
This is the most difficult danger level for backcountry 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. Snowpack tests may help assess stability.

Conditions are generally favourable for travel providing routes are chosen carefully. The snowpack 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 snowpack 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.

Conditions have become much less favourable. The snowpack 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. Backcountry touring at this danger level requires good routefinding 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 talus 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.

Conditions have become dangerous, most often as a result of significant amounts of new snow, snowfall accompanied by wind or the snowpack becoming isothermal and threatening wet-snow avalanches. The snowpack 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 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 backcountry traveller 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.

Extreme danger levels are rare and usually the result of unusually large amounts of new snow. The snowpack 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 snowpack. Natural avalanches can release on slopes of less than 30°
Backcountry touring is not recommended and often impossible. Avoid all avalanche terrain and keep well away from avalanche path runouts.

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