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Yesterday, while skiing out of bounds on the south west canyons of Mt. Hood, I experienced a small and extremely mild quake. This, combined with the melting conditions caused an extremely small avalanche in the canyon region we where in.

This was extremely mild in the spectrum of what is shown on TV or from what I've seen in other videos. A snow field of a bout 100 sq.m, shifted about 100 m down hill. And while mild, this was still my first ever experience in this type of situation.

Im a fairly experienced skier and we were already traveling downhill at about 35 mph +/- (we were tracking with a racing GPS app), when the snow "gave out from under". In normal "powder" and "good" snow, it normally feel like im floating on the snow. In this situation, it felt like I was being pushed from behind, while sinking under. Maneuvering was next to impossible and the only option to accelerate was to "tuck". While initially I accelerated a little, the snow caught me and my group rather easily and rapidly. Gladly, no one was hurt, but my questing are as follows:

1) If all free falling objects accelerate at the same rate (this was on a fairly steep mountain section), why did we get "trapped" into the avalanche, when our acceleration already had 35 mph +/- accelerating it? Did the thicker snow breaking and shifting somehow create more friction between my ski and the snow?

2) Most modern ski's today have alot more "nose curve", also known as rocker, in the front and back, in order to break through thicker snow with more ease enter image description here The skis I was riding have what most in the ski world would call "excessive rocker". Why was it so easy for snow to sink me down, when my ski's design is for 2+ ft of snow and I was forcefully trying to "float" above the collapsing snow? This avalanche was tiny in the spectrum of perimeter and mass traveled., And also only lasted about 3 seconds.

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    $\begingroup$ You sank for the same reason you sink in water: there is nothing to stop you. The snow particles in an avalanche behave much more like a liquid than a solid, i.e. you can push them out of your way while you are sinking deeper. As long as your density is higher than that of the now flowing snow, you will sink. One could imagine a kind of avalanche "suit", like a large airbag, that would inflate and that could reduce your effective density by almost an order of magnitude. It would probably have to blow up to a cylinder of tw0 to three times your diameter. That may help to stay near the surface. $\endgroup$
    – CuriousOne
    Commented May 21, 2016 at 8:47
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    $\begingroup$ @CuriousOne Such "suits" exist in the form of backpacks (e.g. abs-airbag.com/en). The do not work on the basis of reducing density though, rather on increasing area I would say. And concerning the statement that snow behaves more like "a liquid than a solid": that really only applies to powder, which I assume was not the snow conditions present, since that wouldn't give an avalanche as described. $\endgroup$
    – Wolpertinger
    Commented May 21, 2016 at 9:36
  • $\begingroup$ @BillyJean: could you describe the snow conditions? i.e. the compactness of the snow on the top layer and how deep it was. If you need inspiration: en.wikipedia.org/wiki/Types_of_snow#On_the_ground . After that I will probably be able to answer your question. $\endgroup$
    – Wolpertinger
    Commented May 21, 2016 at 9:38
  • $\begingroup$ @Numrok: From the images I have seen avalanches to behave somewhat like liquids, even if they may glide in layers. The comparison is not perfect, of course, since the material is, at the very least, a non-Newtonian liquid. Put enough pressure on it and it will solidify quickly. That's the problem once one is under layers of the snow... then it becomes a trap. I wasn't aware that the airbag technology existed. That's interesting and it does give some hope that operating in alpine environments can be made safer. Cool. $\endgroup$
    – CuriousOne
    Commented May 21, 2016 at 9:46
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    $\begingroup$ It was quite cloudy. 1 c +/- . The snow was semi compact from the rain/snow mix conditions the last few days prior. We have received over 400'' up there this season. It snowed 5'' the night prior. We call it "dust on crust" here. Hardpack under, with a moderately wet/heavy top layer of about 5'' of snow. $\endgroup$
    – Billy Jean
    Commented May 21, 2016 at 9:48

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Tl;DR: It is mainly due to the skier's positioning on the ski and how being thrown off-balance by the avalanche affects it.

Concerning the first part of the first question:

1) If all free falling objects accelerate at the same rate (this was on a fairly steep mountain section), why did we get "trapped" into the avalanche, when our acceleration already had 35 mph +/- accelerating it?

there is probably a large psychological component here. You were moving across a stationary ground and suddenly that ground starts gliding, which it didn't before. So it is really the change in its movement that you felt.

There is also a physical component however: you state correctly that "all free falling objects accelerate at the same rat" and under this assumption it shouldn't make any difference that the ground started moving. But a free fall approximation is not very good for skiing, especially when you already are in motion.

Did the thicker snow breaking and shifting somehow create more friction between my ski and the snow?

The answer is of course no, on the other this question already hints at what actually happens. Skiing is a large dependent on your position and balancing on the ski, and the acceleration does too (1). Our case is different from racing, but similar concepts apply. In the "dust on crust" you describe you have to lean back slightly to float, even with your rockers. This comes at a cost of acceleration. When the avalanche starts moving you get thrown off balance a bit, leaning back even further relative to the ground. To accelerate, you get back in front first applying the pressure to the ski as before. This takes some time of course.

Why was it so easy for snow to sink me down, when my ski's design is for 2+ ft of snow and I was forcefully trying to "float" above the collapsing snow?

Same reason, due to the leaning back your ski doesn't float properly anymore.

(1) which is e.g. why you we saw Ted Ligety win almost every Giant Slalom a couple of years ago: he invented a better ski position

(2) aaaah, second best after deep powder in my opinion

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    $\begingroup$ Free fall does not work at all because of the friction between the skis and the snow. Further, even if the avalanche had not started immediately below their skis, it still would have caught them because it has a much higher terminal speed than most skiers can achieve. The snow in an avalanche is basically ice slipping across ice, so there is going to be much less friction than skis on snow. $\endgroup$
    – honeste_vivere
    Commented May 21, 2016 at 12:23
  • $\begingroup$ @honeste_vivere I did say it is not a good approximation here, so we can't use it. but how does any of what you said change the interpretation I gave? $\endgroup$
    – Wolpertinger
    Commented May 21, 2016 at 13:09
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    $\begingroup$ Oh, sorry, I did not mean to imply your answer was incorrect. I was trying to provide some additional info for your answer to the question about all things accelerating at the same rate. $\endgroup$
    – honeste_vivere
    Commented May 21, 2016 at 13:12
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    $\begingroup$ Also air resistance --- this is why "tucking" is faster than skiing standing up. $\endgroup$
    – rob ♦
    Commented Jul 7, 2016 at 15:02

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