Alpha Angle — How Far Can that Avalanche Go?

Post by blogger | November 26, 2014      

(Editor’s note: This post seems to be good grist for discussion, so I re-upped it today in anticipation of the coming (or already present) avalanche season here in the Northern Hemi. Comments appreciated.)

Some things in ski mountaineering take a high degree of mental acuity. Tech bindings might be one. For men, how to actually end up skiing with a woman instead of a group of guys is definitely another. Starting before 10:00 in the morning could be an item for improvement as as well.

But above all, sometimes it takes a while to wrap your mind around the fact that snow avalanches are predictable in a number of ways. Thus, while luck and chance play a part in any aspect of life, it is not just chance or luck of the draw that keeps you alive in avalanche country.

Avalanche slope profile side view showing alpha angle concept.

Avalanche slope profile side view showing alpha angle concept. This is a representative graphic of a fictional avalanche slope. Click to enlarge.

I’m making no claim here to any sort of engineering or geology expertise. This is written from one backcountry skiing layman to another, with the simple purpose of conveying a concept I’ve found useful and practical over the years for increasing avalanche safety. Any mistakes in terminology or facts are mine and mine alone and I’ll correct upon notification.

Avalanches behave according to natural physical laws involving mass under the influence of gravity and such. While it’s impossible down to inches to predict where an avalanche will stop moving (unless it goes up against a terrain feature), it is possible and quite easy to predict the maximum extent within a few hundred feet or less, depending on topography and some basic engineering concepts. Yep, we’re talking about alpha angle (also known by terms such as angle of reach). We’re talking about the learned skill of being able to recognize an avalanche slope and perhaps avoid the runout, even in situations such as a high altitude glacier with no vegetation clues.

Before we start with the details, if you don’t get this, here is the proof. All over the world, municipalities and land use planners in alpine regions zone or otherwise designate avalanche safe areas in alpine topgraphy. They don’t use voodoo to do this. They don’t use divining rods. They don’t just sit around and go “blah blah blah.” Instead, they have someone use a few simple techniques that give a very good idea of safe and unsafe areas. If necessary they refine this with engineering math, but anyone can do the basics.

Below depicts a huge avalanche path near McClure Pass, Colorado known as the Cleaver. (A good spring corn snow skiing destination, by the way.) The lower trim line is easily spotted, and is the result of a 100 year type of avalanche that occurred on January 12, 2005. How do we know it was a 100 year variety? It is because of the 100-year-old trees that were taken out.

Cleaver avalanche path, extent of 2005 'full path' slide is marked as one would shoot alpha

Cleaver avalanche path, extent of 2005 'full path' slide is marked as one would shoot alpha angle if he were standing at the bottom next to the trees that were still standing. I know from years of observations that this location would be quite safe during a normal avalanche in this path, but dangerous or deadly during the 100-year event due to wind blast and debris. Whatever the case, let's work with this spot and see what the angles are.

To get your alpha angles at home, just switch over to Google Earth.

Aha, using Google Earth yields 19.5 degrees. Considering the slide junk ran into the trees (we went up there and looked), max extent was probably closer to 18. There you have it. In this case perfectly predictable based on using 18 degrees as max extent of Colorado large slab avalanche. For larger paths you can get useful alpha angles at home for trip planning or curiosity. But smaller paths such as Sheep Creek are too sensitive to errors in exact location as well as anomalies of the elevation sampling grid to get anything useful out of Google Earth. You have to observe with your feet on the ground.

To get your alpha angles at home in Google Earth, first draw a path from the top of the slide path starting zone to the point where you want to catch an alpha angle. After the path is created, just click Edit/Show-Elevation-Profile then convert the percentage grade to degrees.

Here it is on USGS topo map. Google Earth worked better in this case.

Here it is on USGS topo map. Google Earth worked better in this case.

So, how do you observe alpha angle in the field? Simple, just sight with an inclinometer (or trained eyeballs) to the top of the estimated starting zone, and estimate where the sight line intersects behind you (or lie down on the snow and sight). That’s all you need to get an idea of where you are in relation to the path runout. Hopefully you do this when approaching the path from the bottom, as in “Hey Salley, looks like my inclinometer is showing 16 degrees to the top of that avalanche path above us, let’s stop for lunch here before we get any closer.”

While approaching an avalanche path from side, just hold the inclinometer up and tilt to align with terrain. Angles become pretty obvious once you do this. If it’s cloudy and you can’t see the avalanche path above you? Different story, and you’ll be depending totally on vegetation clues if they exist. If it’s all a total mystery due to visibility or strange terrain, that’s a fire alarm that in many cases would dictate a major route deviation or turning back (or very careful map reading).

It might go without saying, but should be emphasized that all this angle stuff is dependent on general probability of avalanches from a given exposure, based on avalanche forecasting factors. On a day with super low danger you might charge ahead with very little fooling around with judging safe zones. During a day with high hazard you might be figuring alpha angles numerous times as you progress your route. And to repeat, different regions and types of snowpack have different average alpha angles.

Beyond alpha angle, another sometimes esoteric concept is beta angle. This dictates the area of the path where the avalanche flows easily as opposed “thinking” about stopping. It’s somewhat arbitrary and not something I find that useful. But others disagree so it’s worth illustrating. See image below:

Beta angle in purple. It's the imaginary line drawn from start to where the path reaches 10 degrees angle.

Beta angle in purple. It's the imaginary line drawn from start to where the path reaches 10 degrees angle. In this case the beta angle is 28 degrees. I'd imagine geologists and avalanche pros use beta angle quite a bit for things like evaluating how violent a given avalanche could be, or as another way of looking at how steep an avalanche slope is. I supposed it could be used as a very non-conservative way of identifying a possible travel zone, or perhaps the spot where it becomes less likely you'll trigger the avalanche yourself. Comments on beta angle appreciated.

I’d like to emphasize a couple more things. First, yes, topography such as deep moats or vertical channeling can either reduce or extend flow, but smaller terrain features such as lower angled or shelf parts of the avalanche path are figured into the overall alpha. In other words, if you look up a slope from the bottom and shoot the alpha angle from where you are, either with an inclinometer or just by eye, do not let changes in path angle or shelves have too much influence on your decision making. The inclinometer doesn’t lie. Also, if you get a chance to observe powder slab avalanches it is amazing how quickly they stop once they start approaching the lower alpha angle region. You can feel it when you’re in one, it’s like someone putting on the brakes. Again, physics, not chance.

Okay, over to you guys. Commenters, a challenge. Imagine you are above timberline in Colorado, and you start heading towards an obviously loaded steep slope. Perhaps you’re going to Friends Hut from Aschcroft, a common route with several miles of above timberline terrain that passes under avalanche paths — many of which have no vegetation clues as to here the runouts are located. You know it’s an avalanche slope you’re looking at because in your Avy 1 class you learned that anything this steep with snow on it is an avalanche slope. But can you figure know how far away from the bottom of the slope you need to stay to get to the hut without unnecessary risk?

Real life avalanche route finding for a backcountry skier.

Here is a real life example. Small pocket avalanches can hurt you. This person had a wind slab flow around her feet and bury her skis, if she'd been higher up on the slope an injury could have resulted. As it was, she was scared but unharmed. This incident illustrates how important it is to judge your runout angles when you're in places where vegetation doesn't tell you where slide can go. The photo shows part of the route to Pearl Pass from Aspen side. At this particular spot, you can save a bit of energy by cutting up close to the wind roll, or you can drop perhaps 50 feet of vertical and swing around far enough away from the snow pillow to achieve an 18 or 19 degree alpha angle from the top of the avalanche slope to your track. Quite a few people have no ability to judge this without the use of an inclinometer. An experienced skier would tend to swing around lower, though it's easy to get lazy and I'll admit to that. This little thing actually taught me a big lesson about keeping up my guard and working better with groups. It's a perfect example of how alpha angle can be used to predict safe routes -- by experience or instrumentation.

Convert slope angles from percentage grade to degrees angle.

CAIC report on Sheep Creek avalanche.

Shopping for inclinometer slope meter angle gauge:

Inclinometers come in many different flavors. I prefer the non-electronic and inexpensive dial type, small enough to carry in a pocket. Good stocking stuffer for next December. But the Pieps electronic 30 Degree Plus is a cool option as well. (You can also use an angle gauge app for your smart phone, but then you’ve got the typical smart phone battery life and durability issues. At this time, we recommend most smart phones remain turned off and stored in a waterproof hard case if carried in the backcountry so they’ll work when you need them.)

Since every backcountry traveler should carry a compass, how about one with a built-in inclinometer slope meter? Below are a few compasses that work as clinometers, as well as some other slope meter options. (Note, slope angle gauges go by different names: Slope meter, clinometer, inclinometer, goniometer, or was that gonadometer? We tend to use the word inclinometer since it implies an incline, but we use other terms as well to mix it up.)

Silva Ranger is a classic that also includes a sighting mirror. If sighting mirror sounds like yet another geeky thing to learn about, know that the mirror doubles as a cover for the dial, and can of course be useful for a variety of purposes (checking your sunscreen, checking girls on the sly, checking makeup and so on) — as well as reading the slope meter from different angles. The cover can be removed if you want to reduce bulk and weight a bit.

If you’re planning on never geeking out with your compass but want a simple one that still functions as a slope meter, Sunnto MCA Challenger is a viable option. Costs less because it doesn’t have adjustable declination setting (if you don’t know what that means, then you don’t need it.) Again, the mirror allows you to read the slope meter from various angles. If you’re probably not going to navigate by compass but want one for a clinometer and simple direction finding, at around $33.00 this is probably the sweet spot for shopping.

If you really use your compass for navigation and want something that’s top-of-line and includes clinometer, check out Sunnto MC2G. This unit is especially nice if you’ll be traveling to the Southern Hemisphere as compasses are actually built slightly different depending on if you’re north or south of the equator.

If you plan on shooting a lot of alpha angles (perhaps you’re learning to eyeball avalanche paths, or teaching), a dedicated clinometer with larger more easily read scale is the ticket. You can pick one of these up at your local hardware store, but most I’ve seen have magnets that add cost and could interfere with electronics. Option to right is inexpensive and doesn’t have magnets. Set it on or tape it to a ski pole when you’re doing research. You have to read it from the side, but it’s much easier to read than the tiny clinometer scale on a compass. I found a really nice larger version of this with magnets which could perhaps be removed. If you’re doing avalanche research or accident reports, I’d recommend using a high-end electronic range finder. But if you’ve got the bucks, one of those could be fantastic for teaching.

Pieps 30 Degree Plus is beautiful.

Pieps 30 Degree Plus is beautiful.

One of the best clinometers that’s been created for avalanche safety work is the little Pieps 30 Plus that velcros to a ski pole or easily fits in your pocket. This little gem reads from the top or side and even has a thermometer. Weighs one ounce, tiny yet easy to read. Senses from horizontal to vertical (0 to 90 degrees in one degree increment with plus or minus one degree accuracy, meaning it’s going to be about the same as using an analog slope gauge in the field). Five year battery life. Give one to the newb in your life who’s trying to learn avalanche safety, and tape it to one of his ski poles. Sadly, the 30 Plus is hard to find and may even be out of production. If you find one, snap it up. While a bit pricey ($100 or so), definitely one of the cooler items to come along over the last few years. Check shopping availability here.


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76 Responses to “Alpha Angle — How Far Can that Avalanche Go?”

  1. Charlie May 3rd, 2013 9:12 am

    While ‘alpha angle’ is the accepted term for the concept, something like ‘runout angle’ may be more descriptive and easier to learn.

    This post would be even more useful if it can link to some studies that show measured alpha angles for tens to hundreds of avalanche paths. I suspect they exist, just don’t have time to look this morning.

  2. Charlie May 3rd, 2013 9:14 am

    Also, it’s easy to make your own (easier to read) inclinometer from a grade-school protractor, a piece of string, and something heavy, like a ~3/8″ nut. Super cheap, and just as good.

  3. Jack May 3rd, 2013 9:45 am

    The engineer in me loves this concept. Seems like it is simple enough to yield a result in the field when tired, stressed, etc.
    I’ve seen really good decision-making rubrics for sailing/voyaging decisions. The method is to pre-plan a set of criteria for a particular passage, based on sea conditions, forecast, wind strength, number, skill and tiredness of the crew, deployment of safety gear. The notion is by writing down Go/No Go/Seek safe harbor decisions *in advance* you mitigate the effects of “psychological momentum” The decision to run to safe harbor is difficult in the moment. Writing the criteria and decision down asks the question: “Ok, the decision we made last night was to abort if we hit these conditions. Why aren’t we aborting?”.

    I just offer this up as a thought. Improving group decision making is hard to do, which is one reason why boats have captains.

  4. Jay May 3rd, 2013 9:47 am

    I see the miitary pays attention to alpha angle stuff.

  5. Brian May 3rd, 2013 10:02 am

    Charlie, great idea. Maybe current avy maps could be updated with a colored scale similar to Lou’s above x-axis. Would really help for route planning. And using the Google method above would mean it could be practically automated from a computer.

  6. Brian May 3rd, 2013 10:09 am

    I’m curious about the resolution and error of the various available inclinometers. In the field we would need to reliably differentiate between 22 and 18 degree angles, according to the above argument. As much as I love the simplicity of the pendulum-style inclinometers for measuring slope angle, I can’t help but think the error in the narrower range of alpha angles would be too great.

  7. Charlie May 3rd, 2013 10:11 am

    Okay, curiosity kills cats. It looks like the most widely-cited paper on the subject is authored by McClung, Mears and Schearer.

    This paper quotes alphas of 22.6 ± 3.1 degrees for Colorado. (two links for paywall redundancy)

    In this context “± 3.1” is a single standard deviation. If avalanche alpha angles are “normally distributed”, in order to avoid 95% of slides, you’d need to broaden that to 22.6 ± 6.2 degrees. Particularly telling is the range of values that are quoted from Colorado. They observed Colorado alpha angles ranging from 15.5 to 30.7 degrees, with a sample size of 98 slides.

    Tread carefully if using 19 degrees as a gold standard.

    The paper above, and related works/presentations, are the best reference I’ve found on the subject. Anyone (CAIC ?) have better statistics?

  8. Charlie May 3rd, 2013 10:23 am

    @Brian – For precise/accurate results, I think I’d prefer a properly set-up protractor to my compass inclinometer, the larger scale is easier to read.

    A protractor is affected by pendulum swing, wind, and the quality of the pivot/knot/string at the center of the protractor. Compass inclinometers are affected by levelling, the damping liquid, and the quality of the pivot/bearing.

    I’m not sure that I’d trust any quick field angle measurement to better than two degrees anyway. If you think a degree or two is going to make a meaningful difference in your safety, I’d look real hard at the quality of the measurement (and back off by ~2+ degrees).

    One degree is <20 feet in error over 1000'. If you can estimate a start zone location that well, you've got a big future in Vegas.

  9. Brian May 3rd, 2013 11:26 am

    If my calculation is right, under a slope that drops 500′, an alpha angle that differs between say 20 to 22 degrees can mean a runout difference of over 100′. It’s been awhile since trigonometry so correct me if I’m wrong. In that case, a 2 degree difference would be meaningful. Is there a reliable way to measure such differences? Or, is a more qualitative go/no go measurement the best approach?

  10. Charlie May 3rd, 2013 12:08 pm

    You’re right – a shift in alpha angle from 22 to 20 degrees on a 500′ drop will move the horizontal distance travelled from 1238′ to 1374′ — 136′.

    The context in which I mean that 1 degree is < 20' over 1000' is transverse to the direction of measurement. Miss a bearing by 1 degree, travel 1000' in that direction, and you'll be 17' off the line you expected to travel.

    When alpha angles are small, a small error in measurement has big horizontal implication. Drawing a few triangles can make this qualitatively clear without resorting to trigonometry.

  11. Rob Mullins May 3rd, 2013 1:51 pm

    Lou speaks wisely “snow avalanches are quite predicable in a number of ways.” I think that anyone who has properly and understood the science of an aspect of avalanche potential does not need to apologize. Socially-appointed or self-appointed ‘experts’ are involved in a high percentage of avalanche accidents that I read about. In my mind and experience now over thousands of days on avalanche terrain, I am near 100% certain that I or my party will not get caught in an avalanche (I could die in an avalanche, yes, always that possibility). This is possible simply if one studies and understands the problem, and more importantly one exercises the self-discipline to stay out of an avalanche. That self-discipline includes judgement to behave far below the line that is too often pushed or passed, resulting in tragedy.

    A few winters ago there existed an historic avalanche hazard potential in the Cascades. I recall an emotional posting by well-know perhaps ‘experts’ who ventured through the furthest margin of the toe of an established avalanche path. This party was hit by an avalanche at the extreme toe of a previously known avalanche path- differentiated in the Cascades by lack of timber. The posting spoke of a close call and lost gear, and it is so scary (true) hazard potential because the ‘best’ got caught. LOL, the ‘best’, self-proclaimed experts, got hit by an avalanche within an established path during a period of well-defined historic avalanche potential. Duh!

    The clues and information are there. If one cannot get the clues and information, then, stay off of avalanche terrain

  12. Geoff May 3rd, 2013 2:22 pm

    Thanks for the article Lou. Question: how do you use the Alpha Angle once you have calculated it to determine whether you are potentially within the avalanche path? I.e, in your example, why was it safe to have lunch at a 16 degree Alpha Angle? Apologies if I missed something, but I reread the article several times & couldn’t find the answer to this question.

  13. Lou Dawson May 3rd, 2013 2:27 pm

    Geoff, you have to have a statistically relevant alpha angle in mind. For Colorado, 18 degrees works for being ultra conservative. In other words, you stay far enough away at the toe of the avalanche slope so you’re in the 18 degree zone. Farther away and you’ll be at 17, then 16, and so forth. Pretty simple, sorry my writing perhaps made it seem too complicated.

    To put it another way, you are predicting how far the avalanche could run based on the average angle shot from your position to the starting zone of the avalance, and positioning yourself accordingly. As alluded to in the blog post, many other factors come into play but the alpha concept is a really good tool, especially for beginners.


  14. Geoff May 3rd, 2013 2:32 pm

    Ah, thanks. I was wondering if that was the case, but I wasn’t sure if 18 degrees was specific to the particular example or was meant to be more universal. OK, follow-up question: so I noticed you mentioned in your reply that 18 degrees works for Colorado and is statistically relevant. So presumably that means 18 degrees might not be appropriate for say, a coastal snowpack in Washington? If so, how does one go about identifying an appropriate alpha angle for the area in which they are travelling? Thanks again.

  15. Chris May 3rd, 2013 2:36 pm

    I think you mean acuity. Acutance is a reference to contrast in photography film.

  16. Lou Dawson May 3rd, 2013 2:37 pm

    Geoff, yep, there are different numbers for different snow climates. I believe Tremper relates some of those in his book, but if you do a Google image search you’ll find tons of diagrams, some of which show numbers for different climates.

    When I’m backcountry skiing in Colorado, during average snow conditions (not particularly dangerous) I usually use 22%, but I wouldn’t camp there, just passing through. And if things seem more hazardous I drop back to 19 or so. Bear in mind that these numbers are subject to the accuracy of how you measure the angle, and again are intended to be used as a guideline for route finding.

  17. Lou Dawson May 3rd, 2013 2:41 pm

    Chris, thanks, I used the word on purpose, as it alludes to “edge effects” and that’s exactly what you’re doing with alpha angle, playing around with the edge of the danger zone! I was playing around with words and did look it up. But acuity would work. I’ll stick with acutance, reminds me of the old darkroom days. Lou

  18. Geoff May 3rd, 2013 2:43 pm

    Thanks, that clarifies things greatly. I really appreciate your blog in general, but your avalanche articles have been very informative.

  19. Lou Dawson May 3rd, 2013 2:44 pm

    Thanks Geoff, working hard on everything!

  20. Rob S. May 3rd, 2013 3:15 pm

    With apologies to those who hate electronic gadgets in the BC….there’s a pretty cheap iPhone app called “Theodolite” that allows you to make accurate inclination measurements (as well as compass heading) while sighting through the phone’s camera. This has the added bonus that you can capture a picture of the slope with the theodolite data overlaid (including GPS location and time). Not relevant to the real-time decision you are making regarding route selection, but it does create a record that allows you to go back and reassess after the fact.

  21. Bobby May 3rd, 2013 3:57 pm

    Does not the volume of snow in the starting or run zone factor in at all? How could there not be a difference between an October 2′ deep slab with no base vs. a March 30′ deep slab with a 150″ base? Is it that the Alpha Angle assumes worst case scenario for a typical snowpack? I’m struggling with this one.

  22. Niceroad May 3rd, 2013 4:11 pm

    Interesting but too far remote from what avalanches actually do in the field. This is just a single variable being studied. What about the importance of 1) the mass of snow involved in the avalanche, i.e. deeper instabilities produce larger avalanches that reach further; 2) wet versus dry snow avalanches; and 3) the importnace of previous avalanches in smoothing out the terrain by filling irregularities?

  23. Lou Dawson May 3rd, 2013 5:01 pm

    The concept is there is a maximum extent of how far a slide can run. It’s one of those things that’s too simple so it seems complicated. Look at it this way: An avalanche has to stop somewhere. It turns out that where it stops is somewhat predictable. Yes, amount of snow that falls obviously has an influence, but more influence in that with _less_ snow the slide might not run all the way to the conservative alpha angles.

    Thus due to the alpha angles we use for route evaluation and planning are statistically the ones for the _longest_ or _longer_ runs of the avalanche. Sort of worst-cast scenarios to one degree or another.

    Another way of looking at it is, could an avalanche run forever, all the way to the sea? I suppose it could if it was infinitely large and began to flow like a glacier. But avalanche scientists have found that for a given climate, there are some pretty definite stopping points that are consistent and can be statistically obtained. This stopping point is expressed by alpha angle.

    I don’t know how else I can explain it.

    As for this concept being too remote from what avalanches actually do, it’s actually the opposite. It’s quite useful and works super well for finding safe routes, setting up camp, and so forth.

    It is also a way of expressing what our gut tells us when we look at an avalanche path and say, “I think we’ll go around that way because it’s probably safe.” How do we know that (assuming we don’t have vegetation for clues)? We simply intuit the alpha angles. Beginners can measure them.


  24. Niceroad May 3rd, 2013 5:47 pm

    Interesting. Thanks for the explanation Lou!

  25. Eric V May 3rd, 2013 5:50 pm

    How far can an avalanche travel? This one went about 1/4 to 1/2 mile down a relatively flat canyon bottom INSIDE a ski area boundary. See:

  26. Lou Dawson May 3rd, 2013 5:55 pm

    The longest one on record was in Alaska, I recall, in the Kenai Mountains? Went for miles due to glide.

    Remember that alpha angle is specific to a type of slide and region, and I’d imagine doesn’t work for that well for glide avalanches or wet ones that get channeled and essentially flow like a river.

    I calculated the alpha angle for the Hood avalanche, 16.5 degrees. That’s really low. From the photos, it looks like it flowed as much as fell. Vertical according to the web page in 3,900 vert and it ran 2.5 miles. Wow.

  27. Lou Dawson May 3rd, 2013 6:19 pm

    This is a good essay on alpha angle:

    Jim Frankenfield says the concept isn’t useful for backcountry travel, I disagree but some of you guys might find his take supports your position about this being so much blather.

    What’s interesting is his discussion of how alpha angles are derived. Average for Colorado according to his take is about 22 degrees, meaning that using 18 as conservative is very appropriate.

  28. Tom May 3rd, 2013 6:47 pm

    Thanks Lou. For those that like books, there is also a pretty good discussion of “runout angles” in the book “Staying Alive in Avalanche Terrain”. Chapter 3, Terrain Management, pages 76 -79. A preview is available if you sign in to Amazon.

    According to the book (and trigonometry) at an angle of 19 degrees, the horizontal distance form the crown of the avalanche to the end of the runout area is 2.9 times the vertical fall of the avalanche path. (see page 77).

    library search:

    FYI, preview is available on Amazon with sign in:

  29. Tom May 3rd, 2013 6:51 pm

    you can play with the the trigonometry here:

  30. Charlie May 3rd, 2013 7:04 pm

    If you’re using a smartphone app, it’s worth checking to see if the system is properly zeroed at degree-level precision. There’s no requirement that the board that the camera’s attached to is the same one as the accelerometer.

    Furthermore, the sensors are aligned to the chip package at ~1 degree precision. The mounting of the chip to the circuit board need only be incorrect at the ~100 micron level to disturb things by a degree.

    Smartphones are great tools – just be aware of their limits.

  31. Lou Dawson May 3rd, 2013 7:08 pm

    The geek level here is rapidly increasing (grin)!

  32. See May 3rd, 2013 8:04 pm

    Thanks for the useful information. I only wish it were not occasioned by such a grim event.

    I have installed the Theodolite app and I intend to use it to develop my sense of slope/alpha angle. I may not use it in the bc, but it looks to be useful as a tool for calibrating my eyeballs.

    Sorry if my harping on the risk/culture issue is tiresome, but I think the recent Manaslu tragedy might be relevant to the discussion.

    Avalanche knowledge/education/experience are (of course) key; but I will reiterate that I think there is also a cultural issue to be considered.

    What was the alpha angle of camp 3 on Manaslu? Why were so many people in that position?

  33. Billy May 4th, 2013 1:36 am

    Lou, thanks for your always insightful posts!

    I’m a geomatics engineer by trade so I’m pretty familiar with all things related to positioning, distance/angle measurements, mapping, etc…

    A few things to note here without any particular order:

    Topo maps on google and most other sources (i.e. Garmin, etc…) are on the whole quite good but they’re pretty poor in mountainous area unless the topos were generated using LiDAR data. Many of the topos in the Canadian Rockies can be off by as much as 200 metres (~600ft)!

    There are a number of reasons for this. One major reason is because Google and other map providers take a lot of their topo data from old government survey data. Some of which can be 50+ years old. Newer topos generated from LiDAR can be very accurate (sometimes down to centimetres/inches) and IFSAR (interferometric synthetic aperture radar) data from providers such as intermap can provide meter level accuracy. Older topos generated from photogrametry (stereo orthophoto) is generally much poorer in accuracy and can range from 10s of metres to 100+ metres.

    Another major source of topo errors is due to the nature of how height is defined. Generally when people give an elevation, they reference to sealevel but sea level in Colorado isn’t the same as sea level in Seatle. This has to do with how mass (i.e. mountains) affects the gravitational pull of the earth which in turn can make sea level (i.e. equipotential surface where water remains ‘flat’ and does not flow away due to gravity) go up and down by up to 10s of metres. For this reason, when you measure height with GPS (i.e. for mapping), you’re not measuring the height above sea level which is defined by the local gravitational pull of the earth. Rather, you’re measuring the height above an theoretical ellipsoidal surface (i.e. WGS84) which is not aligned to sea level.

    If all of the above sounds overly complicated, just remember:
    Topo maps are generally not very accurate and can have errors of up to 200m in mountianous areas.

    Electronic angle measurements:
    Someone mentioned about apps and their accuracy for measuring angles. My experience has been that you can usually get accuracy to within 1 degree (often better) with a run of the mill smart phone.

    Granted the accelerometers need to be aligned with the reference edge of the device (i.e. side of your phone) but if it was off by 1 degree, you would only be about 1 degree off. For the most part though, the placement of accelerometers onto your phone is quite accurate and can be done to better than 1 degree in accuracy. Any misalignment of the sensor itself relative to the smartphone edge or PCB board reference direction can also be calibrated out. Whether this is done or not depends on the manufacturer but it’s safe to assume that if you buy a highend phone (i.e. iPhone, HTC One, Galaxy S3, etc…) this is done for you.

    How the smartphone measures angles is generally based on accelerometer outputs in the 3 axis. There are errors that can be caused by non-orthogonality of the sensor axis (i.e. not 90 degrees apart). In general however, these can be calibrated out. Same rule applies to the above where if you have a decent phone, it’s generally done anyways.

    What is a bigger problem is your pointing accuracy. Depending on how well you’re able to point/sight your target, this can lead to accuracies worse than 1 degree.

    All in all, a compass with a clinometer can typically give you ~1-2 degree accuracy overall (assuming you’re sober). Suunto has some higherend clinometers that are about ~1 degree accurate but at the end of the day, your smartphone do as good of a job as a pricy Suunto clinometer.

  34. Billy May 4th, 2013 1:55 am

    See May,

    I’ve heard first-hand from Greg Hill that one of the major reason why there was such a big tragedy on Manaslu at Camp 3 was because people got piled up in one place. As Greg put it, one person tires out and drops his gear and the next person following naturally will do the same. After seeing Greg’s photo of the mountain and the relative position of all the camps, I hesitate to even consider the location of Camp 3 as a real camp aside from the fact that many tents were pitched on that location on the mountain. Camp 3 was a horrible location from an avalanche safety point of view and given the huge amounts of snow that was dumped on Manaslu leading up to the tragedy, it really is quite bewildering that anyone would camp there.

    Greg was positioned lower down the mountain at a safer location primarily because they were in a smaller group (with the dynafit team) that was able to move fast and could leapfrog over the crowds much higher. These slower groups relied on more closely spaced camps. The site where Greg camped could only accomodate ~3 tents and would not have been suitable for the larger groups that were slower and higher up the mountain.

    Another major factor to the huge loss of life was that people just simply didn’t wear their beacons or bring shovel/probes. According to Greg, it seems that the Himalyas just lack the culture of snow safety that has built up over the years in North America. Greg mentioned that even many of the dynafit team didn’t have basic avalanche gear (no probe, shovel, beacon) and after the avalanche wiped out the camp, he was the only one that actually had a rescue shovel. No one else on the mountain at other camps that day had avalanche shovels.

    One of the primary reasons why Greg and 2 other members of the team left after the avalanche was because Greg thought that he broke too many of his own personal rules regarding snow safety while on Manaslu with the dynafit team and was not comfortable with continuing on with the expedition. To be clear, in no way am I saying the dynafit team were negligent or unsafe and I am not saying that Greg was implying this either. All I can say is that the situation described above was from Greg himself and it is what it is. We all have our own definition of what is safe and although there are generally accepted ways of assessing risk when it comes to snow safety, at the end of the day, we need to make our own decisions on whether to proceed or not. In Greg’s case, after the avalanche, he felt that he has overstepped his own personal boundaries and decided to go home.

  35. Lou Dawson May 4th, 2013 4:28 am

    I’d say the greater percentage of “climbing” now done on the 8,000 meter peaks is “bewildering.” Overall, a big strange mess.

    As for avy safety stuff, best practice as always is for the group to agree on a style before they leave the trailhead (or the airport, as the case may be). If they’re all in agreement, their choice. My recollection is that avalanches are one of the if not _the_ top hazards on the 8,000 meter peaks, but most are so big that a shovel and beacon are of no importance to the outcome. Hence, the choice by probably most climbers to not carry.

    RE choices for camps and such, my understanding is that when you’re on an 8,000er you in many cases assume a heightened level of physical risk and are willing to do things you might not otherwise accept.

    Out of curiosity, what was the alpha angle of the Manaslu avalanche camp location?

  36. Lou Dawson May 4th, 2013 4:43 am

    See, it really is quite effective to train your eyes by using an inclinometer to shoot angles. In my own experience I’ve found that I need a tuneup now and then, but generally do pretty well in evaluating an avy slope from below by just holding a ski pole up and sighting along it, then holding the pole in position and moving my head to the side so I can see what angle the pole is positioned in. Rough, yes, but better than just looking up the slope. As I’ve alluded to in the blog posts, after a while you can use your instincts to figure out where the safe zone is at the base/runout of an avalanche path, but training your eye by using inclinometer is very effective, and in my view an essential thing for beginners who want to learn the craft of avalanche safety route finding. As I’ve said before, sometimes I’ll carry an inclinometer in my pocket and use it quite a bit, especially in unfamiliar areas. I’ll also use the one on my compass.

    I’d prefer to have one all the time — which is one reason the Pieps Vector beacon is really nice, as it provides an inclinometer (along with rudimentary GPS). I’ve got a Vector here in play, but have been concentrating on ski testing so can’t say much more about beacons at this time. I’ll do more work with it this summer, as it’s pretty easy to test and evaluate beacons without snow on the ground, but a bit difficult to do so with skis!

  37. Lou Dawson May 4th, 2013 5:43 am

    For those of you who might want to play around with inclinometer and alpha angles while out ski touring, if you’re shooting an angle up or down a slope, be sure you shoot on the fall line, if you’re oblique to the fall line your measurement will get thrown off enough as to be possibly be pretty much useless. Lou

  38. Lou Dawson May 4th, 2013 5:48 am

    RE Manaslu, one has to wonder, how many people camped there actually knew they were in an avalanche runout zone? I’m certain some of them did, but how about various guided clients who were relatively clueless, and folks like that? Interesting to consider… anyone know the alpha angle from Manaslu Camp 3 up to the start of the avalanche? It was triggered by a serac fall, so normal alpha angle rules of thumb might not apply, but would be interesting to know.

  39. Tom May 4th, 2013 4:45 pm

    Billy, Thanks for the information. Just curious, is there a useful way to determine the source of a paper or electronic topo map ( LiDAR , IFSAR, etc.) ?

  40. bill h May 5th, 2013 1:10 am

    Tom… most maps will have some metadata associated with them if you dig hard enough.. popular paper maps (such as latitiude 40 series in colorado) usually will have some fine print about where the data was sourced etc. and web maps may require a little more digging.. not as easy in all the web-based applications such as imbedded google/google earth maps etc.

    But all that aside, you ‘generally’ aren’t going to find much LiDAR and IFSAR coverage for an area unless someone with some deeper pockets had a specific reason to fly it for LiDAR. Most instances of LiDAR-based DEM’s that are specific to avalanche modeling projects have been done for in-bounds academic purposes (i.e. at Jackson Hole etc). regardless, take home point: you’re won’t find many general coverage maps based on those super accurate remote sensing methods for awhile yet until costs come down.

    following up on Billy’s comments re smart phone inclinometers… they certainly are nice… but I would still vote for the plain old compass-based inclinometer with folding mirror… it weighs a few ounces, hangs off your neck just inside your first layer, and doesnt run out of batteries on a full-day or multi-day tour in cold weather… doesnt require a second person to read while you are sighting up the slope etc, and i’ve heard compasses are battery-free devices which are actually useful for other navigational tasks, but i can’t confirm 🙂

    added bonus: you can shave with the mirror, assess your sunburn, check out that stinky-but-kinda-cute tele chick on the sly, or admire your dead-sexy goggle tan every time youre checking angles. If you rocked out in a ski town in the late 80’s/early 90’s whilst I was just a middle school lad, I’m sure you can think of a few other things to do on a mirror too.

    Anyhoo, just like any other backcountry travel tip/know-how: if you think alpha-angles are useful, great; use them. Lots of folks in the professional avy-forecasting and safety field seem to agree

    If you dont, just keep arguing about it on, that certainly seems like a useful pasttime. No one here (especially Lou from what I can tell) is suggesting that one should base all route and travel decisions purely on alpha angle… the only thing that has been reiterated over and over again in every thread: another tool in the toolbox; learn how to use it, then learn when to use it, and then use it in the context of all the other information available to you when touring.

    At the very least, Lou has suggested two easy and useful times to think about alpha angles: skirting around the runout of avy paths in an alpine cirque which doesnt have easily definable vegetation clues for path runout distance, and helping to assess a suitable safezone to wait in for your partner after skiing a slope.

    Until you dial in your internal assessment of 30, 35, 37 degrees etc, you maybe already use an inclinometer regularly to measure slope angles on things you want to ski, so why not whip it out for a few other tasks as well?

    Its not meant to be the end-all-be-all of avalanche safety, so all the engineers in the audience can relax. But as Lou says, there are statistically definable characteristcs of slides, and if you don’t believe it, thats fine. But any community or agency that does building zoning on slide return probabilities (East Vail, City of Juneau, Alta/Snowbird, CDOT, UDOT, ski area lift placement, etc) is basing it on this stuff, and apparently that means the science is good enough for the multi-billion dollar insurance adjusters at AllState and StateFarm.

  41. Lou Dawson May 5th, 2013 6:51 am

    Good comment Bill, thanks much for contributing!

    I’d emphasize that a big deal to me over the years in teaching and practicing avy safety has been for folks to not stop in danger zones after skiing a run. For many years, it was surprising and disconcerting to me how many people seemed to ignore the danger they were in and after making some turns they’d just stand in the middle of an avalanche runout, when moving a few more feet down and/or to the side could make their spot perfectly safe.

    I’ve seen this improve a bit in our area, perhaps because of some recent near misses and accidents people hear about. But it’s still a common danger producing behavior that in many cases is totally unnecessary. But I still see it. I even fight it myself in our own groups, as it’s just human nature to stop and exacerbate the danger by congregating.

    After a while, it came to my attention that a lot of folks stopping in inappropriate places actually don’t know they are in danger, as they’d never really learned to evaluate how far and where an avalanche can fall.

    Sighting alpha angle from where you’re standing is super instructive when it comes to figuring out if you’re in danger or not.

    I know, I know, this sounds like geeky overkill to folks who are super experienced and can evaluate slopes by eye. But if you’re new to the game or just want to be extra careful, alpha angle is beautifully effective.

  42. Lou Dawson May 5th, 2013 8:49 am

    Added shopping information for a variety of slope meter inclinometers, see bottom of post. Lou

  43. Art Judson May 5th, 2013 12:04 pm

    The best way to stay safe from avalanches is to avoid them altogether, but most people that spend time in avalanche zones don’t want to hear that. Next best is to minimize your time in avalanche areas. Lou has just touched on this. I’ve got to second it. A sure way to get killed by avalanche is to build your house in a path. Some do this, still. But I remember looking at a path in Japan named Mitsumata. It had a lot of vegetation in it. And was 300 meters long and 36 degrees in inclination. Then I was told by my friend Mikio Shoda that it killed 158 people in January 1918. They were all in their houses 30 minutes before midnight. This is Japan’s greatest avalanche disaster. We haven’t had anything that bad in the U.S. yet, but we did lose 96 people in a train hit by a slide in Wellington, WA. in 1910. It makes sense to spend as little time as possible in avalanche paths.

    So we don’t recommend climbing up paths from the bottom whether you stop to dig pits or not. You shouldn’t be spending a lot of time doing profiles and ECTs in starting zones either. Andre Roch told a friend of mine that what he’d learned from decades of avalanche research and years as a guide was to never ski avalanche paths. Pretty good advice most don’t like to hear. An inclinometer is useful for measuring alpha, safest in summer. And it’s useful if you’re committed to skiing slopes less than, say 20 to 30 degrees, which by the way are angles you can find enjoyable for skiing powder.

    I skied my first avalanche path in 1951 when I knew nothing about avalanches. And my last a couple of years ago. In between I spent a life time as a snow ranger, avalanche forecaster, avalanche scientist, and consultant. It’s interesting stuff. But you’ll live longer if you don’t spend unnecessary time in slide zones. Alpha by the way is good, but it remains a research topic.

  44. See May 5th, 2013 8:08 pm

    To what do you attribute your survival after 60 years of avalanche slope skiing?

  45. Lou Dawson May 5th, 2013 9:41 pm

    Art, wonderful to get your voice here! Thanks. Lou

  46. Art Judson May 5th, 2013 10:27 pm

    This is for See. You’ve asked a fair question I find hard to answer in a forum like this. At first it was primarily luck. Then I trained under a guy who rode more avalanches than anyone I’ve ever known. His stories scared me. We used a lot of bombs and sometimes a recoilless rifle. And we skied off a carload of avalanches. The work was sometimes dangerous and sometimes I got caught so fear was a teacher. We seldom skied backcountry, mostly just paths we controlled or that others controlled. In this business I met a lot of people who knew a lot more than I did and who had bad consequences when they got caught. And you get to see the reports from the whole continent when people ran out of luck. No matter how much I knew or thought I knew there were always a lot of unknowns. You begin to respect such things, especially with a family. Thinking about the family made me more careful in my work. Sometimes I skied as a visiting fireman avalanche paths I did not like and sometimes we’d looked down paths that looked so bad no one wanted to ski them and we turned away, found an easier route down. I learned that over confidence brought trouble. So I steered away from it when I could. I was lucky and finally knew that there was too much I did not know and would never know. That helped keep me alive.

  47. Art Judson May 5th, 2013 10:52 pm

    Lou, I think we share the distinction of having been rescued by Harvey Carter. That was a long time ago.

  48. Scott D May 6th, 2013 9:21 am

    I wrote myself a note.
    Beta angle is the fall line slope that can slide.
    Alpha angle is where that avalanche slope ends up.

  49. See May 6th, 2013 8:45 pm

    Art Judson; thanks very much for sharing your insight and experience.

    Re. Manaslu: I’m still wondering what the alpha angel of camp 3 was, but (having scanned a paper by Art Judson) I recognize that the “volume and mass of the initial sliding snow” is significant. Serac fall equals large initial mass, but perhaps low frequency.

    Anyway, I am beginning to appreciate just how much I don’t know about avalanches. Luckily for me, my goals are relatively modest. I mostly enjoy just being in nature.

  50. Mike marolt May 6th, 2013 9:04 pm

    In the 7000 & 8000 meter peaks, yes I would agree with Lou that avalanches are a major concern. But it’s a different game compared to climbing in lower altitudes like colorado and Europe. You count time in lower peaks in a matter of hours. In the Hymalaya you count it by weeks or even months. So if a massive storm dumps a ton of snow, you simply hang out and wait for things to settle because you have time built into the process, a luxury that ironically isn’t part of the mind frame in lower peaks. “I have work tomorrow so I need to ski this peak….” That’s not part of the mindset at a basecamp generally.

    As for carrying avy gear, because you can hang out, you generally can avoid avalanches and don’t need it. The higher you go the more wind cleans things out and you generally find super hard snow. So I don’t know anyone that carries beacons and probes and have never seen them. I always carry a shovel. But it’s also a practical thing. You become so limited above 7000 meters that you simply can’t carry that gear. So you just have to be careful and avoid soft snow avy conditions. You have to take time which you have a bit more of on an expedition.

    Lou mentioned Manaslu and that peak is worth mentioning. From 1995 until last year, someone died in an avalanch every year but a couple. And they were caught in sections A to Z. It’s the harriest peak I have ever been on, and a testament to guide company greed that it has become a major sell as an entry level peak. When 12 people die in an avalanch while sleeping with no alternative safe spots, that peak may be easy, but it is not an entry level 8k peak and it is not a good place to run massive commercial operations. When the 12 people died at camp 7000 meters, there were 300 people on the peak. There is room for a thousand people at that camp so it was by the grace of God More were not there when it went. It’s a massive plateau. When we got there and looked at the massive ice cliff 3000 feet atop the 40 degree and more often than not loaded slopes leading into camp, we ran off that peak. It didn’t take but a quick look to understand that if the 500 foot ice cliff calved off, even not considering the loadd slope there was no chance. Not even debatable. There was def an alpha angle but it was well beyond camp down the peak into a massive cravass field.

    But in the massive glaciated regions it does become very difficult to calculate alpha. We were climbing Logan up in Canada and a massive serac broke off a nearby 16000 foot peak about 7000 feet above us. It ran over a mile onto flat glacier based on looking at a topo and the debris. We had to cross the path the next day and it cured me from walking anywhere near run outs for these features. When these mega massive avalanches run, they obviously don’t outrun physics, but the definitely outrun your minds ability to understand how far they can run.

  51. Shawn May 7th, 2013 8:17 am

    Sorry if this has already been discussed but I don’t have time to read all the comments yet. Does alpha angle work the same way for determining how far an avalanche will run up a slope on the opposite side of a gully at the bottom of an avalanche slope? Will it run up the slope until it reaches the 22 degrees as shot from where you are on the opposite slope to the top of the avy path?

  52. Lou Dawson May 7th, 2013 8:47 am

    Shawn, no that hasn’t been discussed in any sort of definitive way. Perhaps some of the engineers in the audience would care to comment. I do know from experience that in Colorado, using the 18 degree conservative alpha angle to define safe zone, one would not have to worry about being chased up the side of a valley to an even lower angle alpha. With the kind of vertical we’re usually dealing with in Colorado, alpha of your position in relation to top of avy path changes pretty fast as you gain elevation. Also, as I mentioned in the post, soft slab cold snow avalanches stop really fast once the slope angle gets low, so if they encounter an uphill they really screech to a halt (and yes, they do climb up stuff when they have enough residual energy to overcome the vert.)

  53. Art Judson May 7th, 2013 9:25 am

    According to Ron Perla, who has worked extensively with avalanche dynamics, it is possible to use alpha in cases where adverse grades exist, where the avalanche is running uphill. Andre Roch once told me of an Alaskan avalanche that fell from a 10,000 peak and ran completely over the top of a 4,000 foot peak in its runout.

  54. Chris Rogers May 7th, 2013 9:53 am

    A quick and dirty inclinometer is at the end of your arm. A fist held at arms length is 10 degrees. Seen from the “inside”, each finger (and thumb) is about 2 degrees. Wearing gloves errs on the side of caution.

  55. Mdibah May 7th, 2013 6:11 pm

    Just a plug for the Pieps 30 Plus–I’ve found it to be quite a handy little guy, and removing the barrier of even needing to open a jacket pocket to pull out a clinometer means you actually use it that many more times per day.

    + Very light, don’t notice the difference in swing weight between poles (and can be handy for separating left versus right poles). I would say that the weight is probably on par with the ratty duct tape stash people have on their poles–definitely not something you ever notice weight wise.
    + Stays put quite well–I have several years of rowdy lines / tree skiing / adventure skinning with it and haven’t lost or broken it.
    + Definitely a good “one-upper” sort of thing to have.

    — Thermometer is mostly worthless
    — Although the battery has a 5 year life span, it is NOT at all easily replaceable. Think apple products difficultly level. Ended up spending an entire afternoon trying to initially figure out where the battery cover was, and finally resulted to cutting and gluing in order to replace the battery. Still absolutely mystified as to the why of this design design decision.

    Bottom line:
    I also have a Suunto PM-5 clinometer, but it usually gets left at home in favor of the 30+ (unless I’m going out for snow science reasons as opposed to going to for tour). Furthermore, you can’t forget to pack it (unless you have truly screwed up and forgotten your poles).

  56. Tom May 14th, 2013 11:41 am

    Hopefully we can learn from this.
    ” Sad News: 2 Skiers Found Dead at Blackcomb Ski Resort | Boulder Fell on Their Tent”

  57. Lou Dawson May 14th, 2013 11:53 am

    Terrible… really sorry to hear about that…

  58. Lou Dawson November 27th, 2013 10:17 am

    Hey all, as we get into the thick of our Northern Hemisphere season, thought I’d bring this post up to the top while we’re traveling and have limited blogging time.

  59. Kelly C November 27th, 2013 10:53 am

    Did you seriously just write that it takes a high degree of mental acuity to “actually ski with a woman?” Whoa, sexist much? I think you owe all of us accomplished, competent and savvy women skiers an apology. I am deeply offended. Shame on you DPS. I’m no longer a fan.

  60. Lou Dawson November 27th, 2013 11:12 am

    Kelly, I’m not sure you got the joke… or are you joking yourself?

  61. chase harrison November 27th, 2013 2:14 pm

    Hey Lou,
    Great post. If you have figured out the alpha angle and are being very conservative as far as giving yourself plenty of room between you and the run out could you still sympathetically propagate a slide form where you are in the alpha angle area all the way up into the starting zone or steepest part of the slope?

  62. Howie Schwartz November 27th, 2013 2:57 pm

    Lou, the alpha angle discussion is one of limited value for the vast majority of backcountry travelers. It is designed and used primarily by forecasters and consultants working to figure out the best place for a road or building or other structure. If one is trying to assess the max runout of a path for a 50 or 100 year cycle in the field, they should consider that it may not be a great day to be exposed to avalanche terrain at all. There are many factors that affect runout distance and the alpha angle is a statistically based rule of thumb to give some basis for prediction. Those statistics are based on less than 75 years of avalanche data in most North American snowpacks. There are many situations, even in CO, where avalanches outrun their alpha angles. Here is a famous old one from your home state:,1436206
    Vegetative clues are arguably a more accurate and easy to judge estimator of maximum runout extent in much of this part of the world, and they don’t require purchase of any special gadgets. When considering max runout extent for managing risk via terrain selection, it is wise to factor in a margin of error and not rely on a field measurement at a resolution of a degree or two.

    Thanks for the food for thought but I think it would be better to kill this thread before too many backcountry folks latch on. The blog says to “speak your mind” so there you have it. Happy snow travels – Howie

  63. louis dawson November 27th, 2013 5:02 pm

    Howie, I respectfully disagree. Many points of contention with your take, but for starters where does this assumption about vegetative clues arise? People ski quite often above timberline, on glaciers or in places that for some other reason do not have vegetative clues. The concept of alpha angle is incredibly useful in those situations. More, it really does work as a route finding tool as I indicate. One only has to know the correct conservative alpha angle for their region, then factor that in to their route finding either by instinct or yes, by instrumentation. It’s also very good as a learning tool. Most people new to learning about avalanches have little to no sense of how far they can run, or how quickly they stop (all depending much on slope angles). When you take them out in the field and start shooting angles, some folks are pretty surprised at how wrong their assumptions were.

    (above edited after I got out of trying to text in a bumpy car ride)

  64. Joe John November 28th, 2013 9:05 am

    Louis, I agree with you 100% in regard to Howie’s food for thought. Keep up the great informative and inclusive discussions especially regarding safety. You all have a great Thanksgiving.

  65. Howie Schwartz November 28th, 2013 9:29 am

    Respectfully, of course. I can appreciate your points, and I really do enjoy the debate.

    Obviously, vegetative clues only apply to paths that run into vegetated areas. This applies to a great many large paths in North America, but certainly there are also many where it can’t be used at all. There is a need to understand potential runout distances in poorly defined paths, but it is good to remember that in some types of terrain there are easier, faster, and quite accurate ways to assess runout without any special tools or measurements, and even from a distance. Without vegetation, historical data would be the best indicator of max runout if we had enough centuries worth of observation. But we really don’t.

    I am not saying the alpha angle concept has no practical value, just very limited as a route selection tool for most backcountry recreational tourers (who don’t comprehensively study the same paths for decades). There is still a lot to be learned about max potential runout of avalanches and there are many variables involved (including the shape of the path, the size, thickness, and characteristics of the slab, the characteristics of both the weak layer and the bed surface, the snow surface characteristics in the track, the shape, steepness, and roughness of the terrain in the track and runout zone, the size, shape and vertical drop of the start zone, etc). The alpha angle concept attempts to simplify all that down to a statistical rule of thumb. The data set that is based on is not very robust and we regularly see the anomalies of backcountry avalanches that surpass estimates based entirely on alpha angle. It is inherently oversimplified to designate different numbers of degrees to different ranges or states as those are arbitrary lines of division. Snowpack characteristics can be highly variable across a region, and throughout a season or longer period. Alpha angle guidelines don’t take into consideration smaller avalanche paths that can only produce size 2-3 avalanches at best, so it is dubious to use such data to assess these paths in this way. Measurement of alpha angle in context of backcountry skiing and riding is an example of high precision with low accuracy, which always seems misleading to me.

    Max runouts really matter in managing risk of traveling in and around avalanche terrain when avalanche danger is High-Extreme. Campsite selection is an example of a decision where consideration of max runout is important since vulnerability to avalanches is very high while lying down in a tent. A concern for the general public and reliance on this technique in that people will increase exposure in the terrain in such conditions while measuring these angles. Avalanche danger ratings consider avalanche size so when planning a tour, it is good to be realistic about potential runouts for the conditions. Complete avalanche terrain avoidance based on worst-case scenarios can cause novices to expose themselves to other unnecessary dangers and inefficiencies in some cases.

    Where we can agree is that it can be a good learning tool in the ways you point out. Anything that encourages people to look at avalanche terrain with a more critical eye, and more often, is good. Let’s just be careful about how we present these forecasting tools so they are not misconstrued. Thanks for your influential voice in the world of backcountry sport and for being open to the discussion. Cheers, Howie

  66. George November 28th, 2013 10:07 am

    Here is another inclinometer that can is super light weight and can be attached to a ski pole or probe pole.
    The Slope Shot inclinometer is designed for hunting but I am testing for avi avoidance. The trick is finding a secure way to affix the device to a ski pole or probe pole or shovel.

  67. Mike November 29th, 2013 9:24 am

    “Avalanches behave according to natural physical laws involving mass under the influence of gravity and such”

    So where does MASS factor into this equation? Slab type? Surface type? I’ve seen slide do a lot of weird thing over the years and don’t believe that all of it could have been calculated.

    However, I think that this method could be accurate enough under certain circumstances such as a typical slide path, which is unobstructed. But there needs to be qualifiers that must be met before one even begins to trust the accuracy of the calculations.

    But I’m with Howie on this one Lou.

  68. SR December 2nd, 2013 6:04 pm

    I think Howie is in essence noting the limitations of alpha angle from a statistical perspective, in that distributions for slides are non-normal in a technical sense, as with many other things in life. In terms of using fragile statistical tools, the question to me is whether you’re betting the bank on them, or using them as one tool among a variety of tools intended to make your decision-making overall robust.

    For instance, route-finding that relies solely on alpha angle, on a black day, might still have its issues. Route-finding using alpha angle as one consideration on a moderate day can by contrast be pretty robust. Howie is also correct that route-finding that assumes other hazards due to alpha angle constraints on a day with manageable avy hazard may create MORE risk — as with all tools, judgment can be an essential component.

    FWIW, as one tool, to me it’s pretty useful. The math is similar to the financial concept of value at risk, with all its limitations.

  69. Lou Dawson December 3rd, 2013 9:13 am

    SR, you get it, thanks. Perhaps I can clarify those things in the essay, but really, it should be pretty obvious that this is just one tool, not the end-all-be-all. Main takeaway is that you CAN know where safe zones are, usually quite easily. Whether you choose to stay in those zones is the other part of the equation. Whatever the case, informed decisions are always better. Lou

  70. gringo November 27th, 2014 2:41 am

    I’ll second the vote for the Silva Ranger, nice compass.

    My Grandpa always told me the mirror is so you can see who is lost.

  71. SR November 29th, 2014 8:50 pm

    Great bump, and good to read in conjunction with the Star Mountain, Wallowa Alpine Htst, and Sheep Creek events in terms of remote triggering, and trees or other perceived islands of safety within a slide path and inside the alpha angle not cutting it unless the trees or other islands of safety are either dense or otherwise not exposed to the same risk as the risk of the path. In terms of beta angle marking the zone within which remote triggering becomes more likely, this intuitively makes sense but I am curious what data is out there. I’ve seen beta angle mainly used to model alpha angle for a path within a given area?

  72. Lou Dawson 2 November 30th, 2014 6:19 am

    SR, it’s said that one way they figure out alpha angle for a given snow climate is to average out many different avalanches. Others have told me that the concept is based on physics, i.e., the energy of the falling snow vs. its tendency to slow down and stop once the angle kicks back.

    I’m not sure where the data is or if that’s ever been done to any great degree. What I do know is that the concept works so long as you use a reasonable angle and don’t expect it to work for liquid flow avalanches, which can run more like moving water. 19 degrees works quite well for being conservative in Colorado, especially for things like camps and picnic spots where you have to be quite careful due to long exposure times. 18 degrees works well for route finding, traveling one-at-a-time of course. If you sight alpha angle at steeper than 19 degrees you need to be considering it’s likely a slide could hit you.

    In terms of beta angle, I think it’s more of a way for a geo engineer to ID an actual “avalanche path” as opposed to the runout zone. I’ve remote triggered plenty of avalanches from way outside the region defined by the beta angle.

    On the other hand, where beta is useful at least in concept is it defines whether you are positioned in the actual active avalanche, or in the runout. In smaller avalanches, you could still take an injury ride if you’re in the beta angle zone, but if you get caught while in the runout things are going to be much less violent due to a shorter and perhaps slower ride.

    Above all, accident reports and observed behavior indicate to me that many backcountry skiers seem to have a poor sense of where the true safe zones are. Traveling with an inclinometer for a while and playing with alpha angle can be a super educational tool to help with this. It’s a real eye opener for some people, especially those who have not seen many real live avalanches.


  73. Peter November 30th, 2014 6:22 am

    Lou, heads up! there is a bug in Google Earth that makes your Alpha Angle finder incorrect:
    Bug Report :

    I submitted it 1.5 years ago, they’re not fixing it.

    Basically, whoever wrote the code used the standard Rise/Run trigonometry formula, but forgot that using the Measure or Path features in Google Earth doesn’t give you the “Run” (the horizontal distance), it gives you the Hypotenuse.
    At very low slope angles, below ~10-12%, the error is very small, since the Horizontal distance and the Diagonal distance are almost the same. This is why anyone using Google Earth for hiking or traveling on paved roads doesn’t notice the bug.
    But, when you get to steep angles, it’s dangerous. For instance, if you use Google Earth to plot a course over a vertical cliff, check the Elevation Profile. It will show a 100% slope, aka 45deg.

    Here’s an example:
    I plotted a path over the largest cliff in Colorado, in Gunnison National Park. Notice that the Elevation Profile shows a 45deg slope. Yeah….that slope isn’t skiable.

    The remedy? Do NOT use Google Earth’s slope % to convert to degrees. Use Google Search to do the right formula, Find the elevation Drop and the length of the Path and enter “ARCSIN(Drop/Path) in Degrees” into the search terms, it will spit out the correct slope angle.

  74. SR November 30th, 2014 10:14 am This is a little dense but does get into a little of the stats.

  75. Lou Dawson 2 November 30th, 2014 11:13 am

    SR, that’s excellent, thanks! They seem to conclude some very conservative angles based on “100 year” slides. Good for mapping development, roads, that sort of thing. Lou

  76. Mike Marolt December 2nd, 2014 8:51 am

    Again, great info…

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