This is a video about avalanches, what they are, what causes them, how destructive ones can be prevented, and what to do if you're ever caught in one.

To actually feel the force of the avalanche on your body. There's kind of nothing that can prepare you for that.

For this video, we followed around the ski patrollers at North America's biggest ski resort, Whistler-Blackcomb to trigger avalanches and film them up close.

If you've ever driven through the mountains in winter or have gone skiing at a resort, you were kept safe by the knowledge gained by snow scientists, avalanche forecasters, and ski patrollers.

If you Google the deadliest avalanche, the first result will be from May, 1970, when the town of Yungguy and 10 nearby villages were destroyed.

An estimated 30,000 people were killed, but the avalanche was triggered by a 7.9 magnitude earthquake, and the avalanche caused a landslide which buried the town.

So it wasn't really avalanche destruction.

The deadliest snow avalanches happened in December of 1916 during World War I.

Some of the fighting took place in the Dolomites, mountain range in northern Italy.

There thousands of Italian troops were battling Austro-Hungarian soldiers, and it had been a particularly snowy winter.

Over 12 meters of snow had fallen on the mountain range in the first week of December.

On the 13th of December, a single avalanche wiped out the Austrian barracks near Mount Marmolada killing at least 275 people.

But its flow on effects were far more devastating.

Both sets of troops realized they could use snow as a weapon, so soldiers on both sides fired artillery shells into the mountain above their enemy's camps, deliberately triggering avalanches.

Over the next few days, between 2000 and 10,000 soldiers died buried underneath the snow.

But most deadly avalanches aren't triggered by earthquakes or artillery shells.

They're triggered by skiers, snowboarders, and snowmobilers recreating in the backcountry.

93% of fatal avalanches in the US were triggered by the weight of the victim or someone in the victim's party.

So how could something so massive and destructive be triggered by the weight of just a single human?

This is Bruce Tremper. He literally wrote the book on avalanches, the book, which inspired us to make this video.

I was a a hardcore ski racer, but when I was done racing, then I started building lifts for the Bridger Bowl Ski Patrol, and I got caught in my first avalanche.

My job for that day was tightening all the bolts at the base of each chairlift tower. 'Cause we were doing our load test that day.

So then they'd warn me, okay, once you get done with this tower, whatever you do, don't cross that big avalanche path below it.

And I made some snide remark like, "Yeah, yeah, yeah, I know about all that stuff. Don't worry about me."

When I got done tightening those bolts, then I realized I made a really serious error because I was just wallowing up to my chest and I almost needed to take out a shovel and dig like a tunnel through the snow to get back up to the ridge and it was just exhausting work.

And then I got cocky. I thought, "Well, I've heard about these ski cuts. I can just build up my speed and get to the other side 'cause that's where I want to be anyway."

Like an idiot, that's what I did.

The first thing that happens when an avalanche brakes is it just starts moving and the first thing you do is kind of flop over on the snow. 'Cause you lose your balance.

Like somebody pulls the rug out from underneath you and you fall uphill.

And there I was just kinda laying in the snow, getting dragged down the mountain, you know, I was caught. I couldn't get out of it and luckily I was able to grab a small tree, but the snow was just beating me to death, and just about snapping off my neck as it's going by me on this tree.

Luckily a lot of that snow went by and that's why I didn't die that day.

But then it finally, the tree finally snapped off or something happened and I was tumbling down the avalanche path.

Luckily when I got to the bottom, I was only buried up to my chest. I was in the tail of the avalanche and the avalanche dies from the tail first.

So that slows down while the front part keeps moving.

You know, I had a lot of time to think about that for the rest of the day. Tightening the rest of those bolts and thinking, holy smokes, I should, I could have died. I should have died and I didn't.

You were tightening bolts, you were caught in avalanche and then you went back to tightening bolts that day. You like, you went back to work.

Oh yeah, yeah.

Good job.

Well, somebody had to do it. I was the guy, that was my job. You know, that's how I was raised by my father. You get the job done.

At the most basic level, an avalanche happens when the crystalline bonds between the snow crystals break and the force of friction holding the snow on the mountain isn't enough to counteract the pull of gravity.

Avalanches are categorized by size on a scale of one to five.

One would not be large enough to bury a person, but size five would involve a 100,000 cubic meters of snow traveling, many kilometers causing destruction on a massive scale.

The first thing to note is that snow isn't uniform.

The snow pack is built up one storm at a time, so it's layered and these layers aren't all the same and they change over time, due to the conditions they're exposed to, the air temperature, humidity, sun, wind, and rain all affect the structure of the snow.

For example, after fresh snow has fallen, if there's a warm sunny day, the snow will melt and then refreeze overnight creating a sheet of ice.

The most important condition is the temperature gradient in the snowpack, but we'll come to that.

The snowpack preserves a record of everything that has happened to it.

If you dig a pit into the snow, you can read the different layers to understand its past.

Nine different avalanche problems are recognized, but there are two main types, slab and loose.

There are two types of loose avalanches, dry loose, and wet loose.

The snow in dry loose avalanches is well, loose. The snow isn't all stuck together. So after it's triggered, it fans out.

Sometimes loose avalanches are also known as point release avalanches.

Dry loose avalanches are commonly known by skiers as sluff, and they usually aren't a serious hazard, though they can drag a skier off a cliff.

It's funny because like in the skiing community, we've just said like this one kind of avalanche isn't an avalanche, we just call it something else.

I had a run in with sluff yesterday, a Helbronner, and you just feel like this wall of snow pushing you from the back.

Honestly, it's kind of fun, it's fun. It's like, it's kind of like surfing where it's dynamic and it's moving and you have to keep moving with the terrain because you know that yeah, the sluff his behind you and if you're not moving fast enough, it's gonna get you.

Another type of avalanche is wet loose.

It typically happens around spring on like south facing aspects. The sun heats up the snow and makes it more dense and slushy and runny.

The sun's starting to come out. I'm not currently too worried about wet loose right now, but I will be in about an hour.

But the more deadly avalanches are slab avalanches where the top layer of snow becomes stuck together like a cohesive slab.

When the avalanche is triggered, the snow releases together in big chunks.

The prevalence of slab avalanches depends on the angle of the ski slope.

Under 25 degrees, there isn't likely to be a slab avalanche because gravity just isn't strong enough to pull the snow down the mountain.

Any slope steeper than 50 degrees is also unlikely to have dangerous avalanches.

The slope is so steep that it's hard for much snow to accumulate. Regular small sluffs and slab avalanches prevent the buildup of larger, more dangerous slabs.

75% of dangerous slab avalanches occur between 34 and 45 degrees.

For comparison, a black diamond ski run is typically around 30 degrees and a double diamond is around 40 degrees.

In other words.

The best conditions for skiing are also the best conditions for avalanches.

What's really terrifying about slab avalanches is you can trigger them remotely.

You could be skiing or walking on a slope that is far less than 30 degrees and trigger an avalanche on a steeper slope above you.

And slab avalanches are fast. They can reach speeds up to 120 kilometers per hour.

Friends of mine have been killed that way. You just don't realize how far these cracks can propagate through the snow.

Slab avalanches have three parts to them, bed surface, which is the lower part of the snow. Then there is a weak layer and the overlying slab

The bed surface and the slab are strong layers. That is they're made out of snow that sticks to itself. The kind of snow that you can make into a snowball.

The shape of the snow crystals in strong layers tends to be rounded.

In contrast, the weak layer consists of snow crystals that don't readily cohere to one another. These tend to be more angular or faceted crystals.

But there are a few different weak layer types. One of the most worrying is known as surface hoar.

It's not made outta snow that falls from the sky. It grows on the surface of the snow.

On cold clear nights, the surface snow radiates a lot of its heat away becoming even colder than the air.

Since warm air holds more moisture than cold air, that moisture will crystallize at the top of the snow creating surface hoar.

Surface is formed by condensation. It's the snow equivalent of dew. It's really angular and it doesn't want to stick to itself.

Usually surface hoar is then broken down by wind or sun melting it.

But if a snowstorm occurs, just after these crystals have formed, they can become buried.

And in that case, they create a very nasty, weak layer, which is just perfect for avalanches.

Another weak layer to worry about are facets, snow crystals that used to be round, but that became angular in the snowpack.

Facets develop when there's a strong temperature gradient in the snow.

A thermal gradient of less than one degree per 10 centimeters does not produce facets. Anything more than that can, which is why the temperature inside a snow pit is measured.

If there's a large thermal gradient, there could also be a weak layer

Right above and right below crust, there can be really wild temperature gradients.

So we had it down 10 centimeters, it was minus 5.5 and down 20 it was minus 3.7. So that's two degree difference.

The bottom of the snow is warmer than the top of the snow. So the warmer snow near the ground, sublimates turning straight from solid water into water vapor.

This water vapor rises up through the snow pack and then encounters the colder snow crystals.

At this point, it refreezes and forms faceted crystals, which like surface hoar, don't stick together creating a weak layer.

The bigger the change in temperature, the faster the snow crystals change and the faster they change, if they change quickly, it can promote faceting which promotes instability.

The other important factor in slab avalanches is the stiffness of the slab.

Since the snow is cohesive, when a force is applied and the interface between the slab and the weak layer starts to slip, that force, and hence the slippage propagates.

While stiffer slabs might resist initial triggers better, their failure leads to more propagation, leading to larger avalanches.

This also results in more remote triggers.

Avalanches are common after storms. The extra weight of the snow exerts an additional load onto the snow pack, which can trigger the weak layers deeper in the snowpack to slide.

The vast majority of avalanches occur during or right after a storm, in other words-

The best conditions for skiing are also the best conditions for avalanches.

Strong winds also increase the risk of avalanches. The wind can pick up and transport the snow from open to more sheltered areas.

As the snow accumulates, the wind pressure compacts these snow particles as they're deposited, which creates a dense, cohesive layer.

These newly deposited layers of snow are called wind slabs.

Wind slabs can be quite unstable, especially shortly after formation, if they've formed over a weak layer.

Since they're more cohesive, these slabs can break free as large chunks during an avalanche making them particularly hazardous.

Another concern are cornices. Cornices form when the wind blows snow over the top of a ridge and it piles up on the other side hanging over the edge.

Cornices can weigh many tons and when they fall onto a slope, they can trigger massive slab avalanches.

One of the ways that backcountry skiers test the stability of the slope they're about to ride is by dropping a cornice onto it.

If the slope didn't slide under the weight of a cornice, it's unlikely to slide under the weight of a skier.

So how are avalanches prevented in ski resorts?

For this video, Veritasium producer Peter, went out with the ski patrol team at Whistler-Blackcomb, which is North America's largest ski resort.

Dude, I'm pumped, really pumped, really anxious, really stoked, should be a good day.

There are sections in ski resorts that are avalanche terrain, slopes that are greater than 30 or so degrees or under avalanche terrain.

To keep skiers safe, before ski areas open, the ski patrols do avalanche control work.

Fundamentally, it boils down to them triggering avalanches in a safe way before there are any skiers on or under those slopes.

This is done often enough so that the snow doesn't get a chance to build up too much. So the avalanches that are triggered are smaller.

There are a few ways that this is done, but the most common is by using explosives.

Explosive charges with a two minute fuse are lit and then detonated on the slopes where they are likely to produce avalanches.

Oh boy, really glad I put on those goggles.

Sometimes this is done by throwing the chargers out of a helicopter, but a lot of the time this is done on skis.

There are even special trams where the chargers are attached to a carabiner and then shuttled across to a hard to access slope.

As the charge detonates, the shockwave breaks the weak layer leading to an avalanche.

Yes, we got a slide, that wasn't too bad. It wasn't great.

It was something.

Despite doing the shoot the day after one of the biggest storms of the season, we weren't particularly lucky with getting large avalanches.

We shot all morning and only got this relatively small slide. So I was getting worried and then we were able to get this shot.

(upbeat music) Oh my god, look at that, oh.

Avalanche control work is also done on roads with basically the same idea, trigger avalanches before they get a chance to become big and destructive.

For example, the Trans Canada highway connecting the east and west coast of Canada goes through Rogers Pass, a mountain pass with 3000 meter toll peaks on either side of it.

The road is right next to a mountain that's actually called Avalanche Mountain.

Here Parks Canada and the Canadian Army conduct avalanche control by firing artillery shells at one of the 270 preset targets on the nearby mountains, releasing small avalanches before they get a chance to become big and destructive and damage the highway.

Deaths from avalanches do occasionally occur in bounds at resorts, but they are very rare.

Due to the diligent work of ski patrollers, most avalanche injuries and deaths occur in the back country.

It's kind of crazy 'cause as a skier, you hear so much about avalanches. (speaker speaking in a foreign language)

You realize it's deadly and people die every year. (speaker speaking in a foreign language)

But to actually like feel the force of the avalanche on your body, like there's kind of nothing that can prepare you for that. (snow thudding)

Every time I hit the ground I would just like dig my hands in my feet in like try to like claw myself down onto the ground while also like trying to protect my head.

I knew also like the moment the snow stopped, if I was still in the snow, I would be buried. I wouldn't be able to move, I would be counting on my buddy to find me and dig me out and save me. (exhales heavily) (speaker speaking in a foreign language)

This wasn't a huge avalanche, this was a size two, so like big enough to bury a car but not like a house.

So I was actually able to stop on the slope before it flattened out where the snow would accumulate and I would be buried. (speaker speaking in a foreign language)

Yeah, it was intense, really intense and I felt really stupid because it was the first mountain I ever skied on as a kid was this mountain, like this very slope and so I was just so comfortable with it and like didn't think twice about the fact that it was avalanche terrain and the snow or, I mean we did check the snow up top, but I don't know, I wasn't like fully tuned into to the dangers that are present.

Like the moment you step into avalanche terrain, something like that can happen.

So the three things that you need to bring when you go back country skiing is a beacon, which is this thing, a probe, which is this big, long stick that ends up collapsing. So it does fit in your backpack and shovel.

If you're buried in a slab avalanche, it's almost impossible to dig yourself out.

During an avalanche, the snow gets mixed around and friction heats it up so it melts a little bit and then when the avalanche stops, it refreezes, setting like concrete.

And that's why if you're fully buried, you need someone else to find you and dig you out.

This is why in 1968 the avalanche beacon was invented.

The beacon is a transceiver, so it is both a transmitter and a receiver of electromagnetic waves at 457 kilohertz.

So right now it is send mode. So this is the mode it would be on if I'm skiing in the backcountry and if there's an avalanche, what would happen is it'll keep sending out those electromagnetic waves in all of these directions.

Then my buddies would change from send mode to search mode and start looking for that signal.

And what that looks like is that Aaron is two meters away from me and her beacon is on and it's saying, cool, there's a person two meters away from me, so.

Around 25% of avalanche fatalities are from trauma hitting a tree or a rock or being pushed off a cliff. The rest are from asphyxiation.

If you're buried in an avalanche and your face is under the snow, there's a small air pocket for you to breathe.

The snow is initially porous, but the heat from your body and breath melts the snow, which refreezes forming an impermeable bubble.

As you breathe, the concentration of CO2 in that bubble increases and the amount of oxygen decreases until you die from asphyxiation.

If your friends find and dig you out in the first 10 minutes, you have an 80% chance of survival.

15 minutes in, that chance drops to 40%, 30 minutes in, you have just a 22% chance of survival. So time is key.

There is another invention that could save your life, which is an avalanche airbag.

If a skier triggers an avalanche, they can pull on a cord, which activates the airbag. The airbag rapidly inflates.

Historically this was done with compressed air, but newer versions use battery powered fans.

The airbag increases the skier's buoyancy, making it less likely for them to be buried deeply in an avalanche.

An added benefit is that if the skier is buried as the airbag deflates, it leaves a larger air pocket providing more time to be rescued.

Avalanche airbags decrease the chances of death by nearly half.

Around 30 people in the US die in avalanches each year, plus an additional hundred in Europe.

Most of them are backcountry skiers, snowboarders or snowmobilers, and they very likely triggered the avalanche that killed them.

Avalanches are beautiful, majestic, and completely terrifying.

If you go into the backcountry in winter, please carry a shovel, a probe, and a beacon, but also check the avalanche forecast and make good decisions when heading out to avalanche terrain.

Because the best way to survive an avalanche is not to be caught in one.

We know so much about avalanches today, thanks to people like Bruce, people who followed their curiosity and learned through discovery.

But thankfully, you don't have to go into avalanche terrain to experience this kind of learning.

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