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Understanding Alpha and Runout Angles

Two data points to help keep it real and safe in avalanche terrain.

By

Billy Haas

In Alaskan terrain, being mindful of runout and alpha angles is a wise mindset. Photo: Bill Haas

In Alaskan terrain, being mindful of runout and alpha angles is a wise mindset. Photo: Bill Haas

One of the first avalanche buzz words I heard as a younger avalanche student was the term Alpha Angle. It sounded important and fancy, so I latched on to it and tried to incorporate it into any avalanche path discussion I could to sound smart. Little did I know that at the time, I had…

Responses

  1. Whit

    Thank you for writing this up. I personally love nerding out on this stuff. I have wondered if avys run at lower slope angles in Continental vs Maritime snowpacks – if the 30-45deg average slope angle varies a little with location. Seeing that AAI page where it shows Maritime snowpacks having a higher alpha angle than Continental & Intermountain makes a ton of sense, even though a couple of degrees could easily be measurement (user) error in the field.

    1. Billy Haas

      Whit,

      Alpha Angles are certainly in the category of less precise measurements, and that is why they are often used by professionals as starting points for a more detailed assessment.

      With regards to starting zone slope angles, there is definitely some variance in snowpack type and regions, but it mostly has to do with the mechanics involved in the typical avalanche problems in those regions. Any mountain range can really have any type of avalanche problem, but there are definitely regional patterns. Low start zone angles are usually seen with dry slab avalanches (but also possible with wet slabs), and can be exacerbated by certain properties of the failure plane (bed surface, weak layer, and slab). A dense slab sitting on buried surface hoar sitting on a slick crust could be an example of a situation where you could see slab release on a slope below 30 degrees. It’s uncommon but not impossible. So slope angle and avalanche release are a matter of the properties of the failure plane, which common patterns will certainly vary region to region. This is also applies to variances in how avalanches flow. Certain avalanche types/characteristics flow slower, some faster, some longer, some shorter. Terrain plays a big factor here too. This all adds up to lower or higher Alpha Angles.

  2. Nolan Novotny

    Just out of nerdy curiosity: how does one think of alpha angles in steep valleys where the maximum extent of paths often runs uphill on the other side of the valley? Or is alpha angle just too rudimentary of a tool for these sort of situations and computational models are needed?

    1. Billy Haas

      Nolan,

      As you add complexity to terrain, assessments of max extent of paths can become harder to identify, and using a regional Alpha average to determine max extent may not be as reliable or applicable for certain paths or zones. Imagine a very steep valley that terminates in a gorge like feature, that would most likely have a very high Alpha Angle relative to other paths. However, plenty of paths that run up opposite sides of more open drainages (as opposed to paths that terminate in the valley bottoms) often have Alpha angles within “normal” ranges. For example in the Wasatch we have a moderate sized open drainage called White Pine that has a few large paths where the historical max extents run uphill on opposite sides of the drainage from the path. Most of the historical maximums (which are well defined by vegetation data) in that drainage have Alpha Angles ~19 degrees which is consistent with much of the terrain in that area.

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