Dear HAT forum,
At the end of July I will climb in a region of Kyrgyzstan, in "summer alpine" conditions: -4c to +10c was typical at the same time of year in 2009. This far, most of what I have learnt about avalanche avoidance has concerned skiers and winter or spring conditions. So I would be extremely grateful for any advice – or for pointers to good materials – on avalanche avoidance for summer alpinists.
Henry kindly spoke with me about this yesterday. He asked me to post my notes from our conversation in order to invite comments, corrections and contributions. They are very brief – aimed at my climbing partners – and can be found below. If you would like to cast a critical eye, please give particular attention to the depths of snowfall/wind-accumulation I have referred to.
For details the climbs we plan, see http://kyrgyzstan2011.kclamc.org
All the best,
Charlie
AVALANCHE AVOIDANCE FOR SUMMER ALPINISTS
Notes from a conversation with Henry Schniewind - 18th June 2011
Most materials about the development and avoidance of avalanches deal with winter and spring conditions and the behaviour of skiers. So, given the conditions encountered on the 2009 KCLAMC Kyrgyzstan expedition (-4°C to +10°C), I asked Henry for supplementary advice for the summer alpinist. He commented on my assumptions, shared information and advice – and lessons from a personal experience of avalanches on Denali. In summary:
DEVELOPMENT
All forms of winter or spring avalanche are possible in the summer: slab, powder and point release. Summer avalanche activity resembles spring activity.
However, given the heat of the day in summer, activity is often especially intense the first day after snowfall:
Slab avalanches release as weak layers are activated by percolation and as the bonds within the slab break down;
Powder avalanches occur as snowflake fingers break down (although powder avalanches may also occur due to weight of accumulation alone – after 25-30cm snowfall or wind-accumulation);
Point-release wet-snow slough is more significant than in winter;
Trigger by cornice-collapse is more likely than in winter (while trigger by serac-collapse is much the same at any time of year).
But this also means that stabilisation takes place more quickly (1 day or so) than in the winter (3 days or so in 'inter-mountain climates' such as the Alps) due to purging and snow transformation.
In summer, the effect of aspect is especially pronounced. The day after snowfall, east-facing slopes are the first to release as they catch the sun. After 10-15cm snowfall, these slopes go crazy in the morning.
How much snowfall is significant? On expansive slopes, as little as 5cm snowfall can gather into a serious avalanche. However, in general, 30cm snowfall or wind-accumulation gives grounds for concern.
In the summer, can unstable layers persist under deep snow, protected from the heat of the day?
75cm – 1m snow is enough to protect a weak layer, preventing stabilisation.
Due to its homogenous nature, neve is unlikely to harbour instabilities – unless it is a crevasse!
Henry was on Denali in good, unusually predictable weather, after three weeks without snowfall. The sense of security proved false when fresh snow led to avalanches onto his camp. He descended amid intense activity.
AVOIDANCE
In fresh snow, avoid camping below east-facing slopes, they are often the first to avalanche.
Unlike skiers, climbers can choose routes that avoid deep snow!
While stress in slabs is concentrated along ridges and it is easier to trigger an avalanche on a ridge than on an open slope, it's still best to be above the danger than below!
In unstable conditions, spread out, to minimise the load on each patch of snow and to reduce the number of victims in the event of an avalanche.
Belay climbers across suspect snow from a point of safety.
Beware during and just after snowfall – allow a day for snow to stabilise.
75cm quick pits can be useful, but only as one indicator: “Relying on one piece of data is like getting married on your first date. Sometimes it works, but generally it's not a good idea”. Definitely worth doing if you have doubts about a specific slope aspect. See: http://www.fsavalanche.org/Default.aspx?ContentId=60&LinkId=65&ParentLin...
Refer to all other indicators: whuumpfs! cracks; avalanches on slopes with similar aspect and altitude etc.
After writing the notes above, I emailed Henry with the questions (some of them big) listed below. Again, he suggested posting them for your consideration. What do you think?
Does the 25-30° rule apply to all forms of avalanche? Will powder and point-release avalanches take place at lower angles?
When your camp was avalanched on Denali, were you pitched below east-facing slopes?
Under what conditions do weak layers tend to form in the summer?
There's been no talk of transceivers and probes in my party. Do they only become indispensable for skiers, with their deep-snow habit?
Here are Henry's replies to the first two questions. Any further comments and advice would be most welcome:
ON SLOPE ANGLE: The slope angle for avalanches has more to do with climate and temperature than type of avalanche
In general, the colder it is, the lower the possible slope angle for an avalanche (except for very wet slush flow avalanches that have huge amounts of free water in them, and they occur mainly around the arctic circle). So that is why my colleagues in North American from ‘continental climates’ talk about avalanches that can release on slopes as low as 25° maybe even lower! In ‘intermountain climates’ like the N. French Alps, many people will say that it is impossible to have an avalanche below 28° (that was until this winter when we had a mega weak layer and cold temps that lead to avalanches releasing on 25° slopes.. some have said even slightly lower..). In ‘maritime climates’ it’s rare to get avalanches below 30° except when it is really cold, due to the sticky wet snow effect.
THE AVALANCHES ON DENALI: it was a shaded leeward slope and it didn’t really matter what the aspect was because at our high camp it was never above freezing. they were cold light loose snow ‘sluffs’ that built up as they came down slopes that started 300 m above us – But my point was that they gave us a good warning for what was to come (e.g. the new snow avalanches that go crazy after a snow storm and catch you unaware if it has been nice weather for some time). Had we stayed in that spot for a day or two more, we would have all been buried.