Snow Climates and Stability – Can they be easily compared?

Recently we had a question from one of our former students whether there was any published information about avalanche climates and which one was the most unstable, or how they might compare to one another from a stability standpoint.  The easy answer is that, to my knowledge, there are no studies that address this question directly.  Nor are there likely to be any in the future.  First, the authors would have to classify criteria for what was meant by the term stability, then they would have to quantify what size and type of avalanches would qualify for their study, then ….  This simple question becomes a messy answer really quickly.

Continental Climate

Continental – Colorado

Maritime – Washington

Tetons – Intermountain

So let’s back up a little bit and define the snow climates (something that has been done in peer-reviewed journals) and then try to answer the question without using the cop-out of “It Depends…”.

When we talk about different snow climates, we often think of several variables:

Temperature (mean air temp)

December Temperature Gradient (measured within the snowpack as a whole)

Snow Depth

Total Precipitation 

Snow Water Equivalent

Common Weak Layers

Maritime snowpacks tend to be warm, with deep snow, lots of precip, high density snow, and the most common weak layers tend to be new snow and partially settled particles (storm instabilities).  Most of the avalanches are direct-action avalanches; meaning during the storm or within a day following the storm.

On the flip side, continental climates tend to be COLD, with shallow snow, much less total precipitation, lower density snow, and common weak layers of ‘persistent weak layers’ (PWL) such as facets, depth hoar, surface hoar.

Intermountain Climates tends to be in between the two.

The comprehensive paper by Karl Birkeland and Cary Mock on snow climatology of the Western US can be downloaded here.

We have classic ideas of where these snow climates lie, but depending on the year, the storm pattern, etc. you can see a more maritime climate in Colorado or a more continental climate in Wyoming.

Birkeland and Mock came up with an avalanche index using the Relative Size of avalanches. (The size of individual avalanches (on the 1–5 U.S. scale) recorded was squared, and the daily totals were summed to construct an avalanche index.)

If you used this scale alone as a measure of stability, the problem would that many small avalanches (direct-action size 1’s) could yield the same avalanche index as one large delayed action size 4, yet the consequences are quite different.  That is a major reason why avalanche centers have moved to including Avalanche Problems in their forecasts. Check out this article: Avalanche Problems

Avalanche problems get to the heart of my answer.  Can you say that a small windslab pocket in high-angle alpine terrain is more unstable than a buried PWL at treeline?  Both can kill you in the wrong circumstances.  Both could be easily triggered at the height of their instability.  Yet storm snow instabilities are relatively more common in maritime climates and deep instabilities are more common in continental climates.  How the backcountry traveler approaches an unstable snowpack depends upon the avalanche problems that they will encounter that day.  Unstable shallow storm slabs can often be avoided by keeping slope angles comparatively low. Unstable deep slabs are harder to manage, as they are more likely to be remotely triggered – a far greater distance from avalanche paths is dictated and slope angles need to be dialed even farther back than with storm snow instabilities.

Slide3_0

SO WHAT?  In general, we are more puckered in continental climates than in intermountain and maritime climates.  The key is recognizing when a given mountain range is behaving differently.  When the Ski It If It’s White’ Range has a cold shallow snowpack in the early season, then the fact that it is generally classified as maritime range doesn’t mean much if anything.  Looking at stability (weighing the probability of triggering) and the avalanche problems encountered (implying the likely size and consequences of an avalanche) is a more wholistic approach and give you criteria for management.  How you manage the avalanche problem will often dictate the terrain that you choose – which is the next hurdle that avalanche centers are trying to communicate to their constituencies.

Check out this article that Chris Lundy wrote:  Different ways for different days

by Don Sharaf & Sarah Carpenter

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