These Are The Avalanches To Worry About

Veritasium
3 May 202425:03

Summary

TLDRThis video delves into the world of avalanches, exploring their nature, causes, and the devastating impact they can have. It emphasizes the importance of prevention and preparedness, especially for those who venture into the backcountry. The video follows ski patrollers at Whistler-Blackcomb, highlighting their crucial role in triggering controlled avalanches to ensure safety. It discusses the science behind avalanches, including the formation of weak layers like surface hoar and facets, and the conditions that make them hazardous. The video also covers the tools and techniques essential for backcountry skiing, such as carrying a beacon, probe, and shovel, and the life-saving potential of an avalanche airbag. It concludes with a reminder of the beauty and terror of avalanches and the importance of knowledge and caution when in their path.

Takeaways

  • 🏔️ Avalanches are a natural phenomenon that can be extremely dangerous and destructive, with the potential to bury entire towns and cause significant loss of life.
  • ⛷️ Recreational activities such as skiing, snowboarding, and snowmobiling can trigger avalanches, with 93% of fatal avalanches in the US being triggered by the weight of the victim or someone in their party.
  • 📚 Bruce Tremper, an expert in the field, has contributed significantly to knowledge about avalanches, emphasizing the importance of understanding the conditions that lead to their formation.
  • 🔍 Snowpack structure is crucial in avalanche formation; layers within the snowpack can vary due to temperature gradients, wind, sun, and other factors, creating weak layers that may lead to an avalanche.
  • 💥 The deadliest avalanches can be caused by natural events like earthquakes or by human actions, such as artillery fire during wartime, which can deliberately trigger massive avalanches.
  • 🚨 Ski patrols and avalanche forecasters play a critical role in safety, using their knowledge to manage risk and prevent avalanches from causing harm in ski resorts.
  • 💥 Slab avalanches are particularly dangerous as they can be triggered remotely and can reach high speeds, making them fast-moving and hard to escape.
  • 🧭 Backcountry travelers are advised to carry essential safety gear, including a beacon, probe, and shovel, and to check the avalanche forecast before venturing into potentially unstable terrain.
  • ⏳ Time is critical in avalanche survival; the chances of survival decrease significantly the longer a person is buried, highlighting the importance of quick rescue efforts.
  • 🎒 The use of an avalanche airbag can significantly increase survival chances by providing buoyancy and creating a larger air pocket when a skier is buried.
  • 📈 Learning and understanding the science behind avalanches can help prevent them and improve safety measures, both for professionals and recreational enthusiasts.

Q & A

  • What are the main causes of avalanches?

    -Avalanches are primarily caused by the weight of the snow itself, which can be triggered by natural factors like earthquakes or human activities such as skiing, snowboarding, or snowmobiling. They can also be triggered remotely due to the propagation of cracks through the snow.

  • What is the deadliest avalanche on record?

    -The deadliest snow avalanche on record occurred in December 1916 during World War I in the Dolomites mountain range in northern Italy, where an estimated 2,000 to 10,000 soldiers died after deliberately triggered avalanches buried troops.

  • Why are avalanches more likely to occur in certain slopes?

    -Avalanches are more likely to occur on slopes between 34 and 45 degrees because the gravitational pull is strong enough to initiate movement, yet not so steep that snow cannot accumulate and form a cohesive slab.

  • How do ski patrollers prevent avalanches in ski resorts?

    -Ski patrollers prevent avalanches by conducting controlled avalanche releases using explosives or other methods before ski areas open. This is done to ensure that the snowpack does not accumulate to dangerous levels and to trigger smaller, more manageable avalanches.

  • What are the three main components of a slab avalanche?

    -A slab avalanche consists of a bed surface, which is the lower part of the snow; a weak layer, which is where the snow crystals don't readily stick together; and the overlying slab, which is a cohesive layer of snow that can break free as large chunks during an avalanche.

  • What is the role of temperature gradient in the formation of weak layers in the snowpack?

    -A significant temperature gradient in the snowpack can lead to the formation of facets or surface hoar, which are types of weak layers. These layers consist of snow crystals that are angular or faceted and do not readily cohere to one another, making them prone to sliding and potential avalanches.

  • What are the essential items one should carry when going backcountry skiing?

    -When going backcountry skiing, it is crucial to carry a beacon (transceiver), a probe, and a shovel. These items are essential for locating buried individuals and digging them out in the event of an avalanche.

  • How does an avalanche airbag work and what is its purpose?

    -An avalanche airbag is a device that, when activated, inflates rapidly to increase the skier's surface area and buoyancy. This makes it less likely for the skier to be buried deeply in an avalanche. If buried, the airbag, as it deflates, leaves a larger air pocket, providing more time for rescue.

  • What is the survival rate for someone buried in an avalanche over time?

    -The survival rate for someone buried in an avalanche decreases significantly over time. If found and dug out within the first 10 minutes, the person has an 80% chance of survival. After 15 minutes, the chance drops to 40%, and after 30 minutes, it's just 22%.

  • What is the importance of checking the avalanche forecast before heading out into the backcountry?

    -Checking the avalanche forecast is crucial as it provides valuable information about the stability of the snowpack and the risk of avalanches. This allows individuals to make informed decisions about whether it is safe to enter avalanche-prone areas.

  • What is the role of wind in the formation of avalanches?

    -Wind plays a significant role in avalanche formation by transporting and depositing snow in sheltered areas, creating wind slabs. These slabs can be unstable, especially if they form over a weak layer, and can break free as large chunks during an avalanche, increasing the hazard.

  • How does the structure of the snowpack affect the likelihood of an avalanche?

    -The structure of the snowpack, which is composed of layers formed by different storms and weather conditions, greatly affects the likelihood of an avalanche. The layers can vary in stability, and the temperature gradient within the snowpack can lead to the formation of weak layers that are prone to sliding.

Outlines

00:00

🏔️ Understanding Avalanches

This paragraph introduces the topic of avalanches, explaining what they are, their causes, and the potential for destruction. It also discusses prevention strategies and the necessary actions to take if caught in an avalanche. The video features ski patrollers at Whistler-Blackcomb, North America's largest ski resort, who trigger and film avalanches up close. The importance of snow scientists, avalanche forecasters, and ski patrollers in keeping people safe is highlighted. The deadliest avalanches are mentioned, including the 1970 Yungguy disaster and the 1916 avalanche during World War I in the Dolomites, which led to thousands of deaths. The paragraph concludes with a discussion on how human activities, such as skiing and snowboarding, are responsible for triggering a significant number of avalanches.

05:02

🏂 Types and Triggers of Avalanches

The second paragraph delves into the mechanics of avalanche formation, where the bonds between snow crystals break and gravity overcomes friction. Avalanches are categorized by size, with size one being harmless and size five being highly destructive. The non-uniform nature of snow and its layers, influenced by various weather conditions, is explained. The paragraph identifies two main types of avalanches: slab and loose, with subcategories of dry loose and wet loose. The characteristics of each type are described, along with the conditions that make them more likely to occur. The paragraph also touches on the concept of weak layers within the snowpack, such as surface hoar and facets, and how they contribute to avalanche formation.

10:03

💥 Avalanche Control in Ski Resorts

This paragraph discusses the methods used to prevent avalanches in ski resorts. It details the work of ski patrol teams, such as those at Whistler-Blackcomb, who perform avalanche control by intentionally triggering smaller avalanches in safe ways before ski areas open. The use of explosives with fuses is highlighted as a common technique, along with other methods like helicopter deployment and tram systems. The paragraph also covers the importance of timing these control measures, especially after storms when the risk of avalanches is higher due to the additional weight of fresh snow. The role of wind in creating unstable snow layers, known as wind slabs, and the formation of cornices, which can trigger large avalanches, is also explained.

15:04

🚨 Survival Strategies in Avalanche Terrain

The fourth paragraph focuses on survival strategies for those caught in an avalanche. It emphasizes the importance of carrying essential gear such as a beacon, probe, and shovel when venturing into the backcountry. The paragraph explains the function of the beacon as a transceiver for electromagnetic waves, aiding in locating buried individuals. The risks of trauma and asphyxiation during an avalanche are discussed, along with the critical nature of time in rescue efforts. The introduction of the avalanche airbag as a life-saving device is highlighted, noting its effectiveness in reducing the risk of death. The paragraph concludes with a reminder to check avalanche forecasts and make informed decisions when entering avalanche-prone areas.

20:07

📚 Learning from Avalanches and Beyond

The final paragraph shifts the focus to the importance of learning from experiences and the curiosity-driven knowledge acquisition exemplified by individuals like Bruce Tremper. It encourages viewers to pursue knowledge and explore their interests through interactive learning platforms like Brilliant, which offers lessons across a wide range of subjects. The benefits of such an approach are outlined, including the development of critical thinking skills and the ability to apply concepts to real-world scenarios. The paragraph concludes with a promotional offer for Brilliant, encouraging viewers to take advantage of a discounted premium subscription.

Mindmap

Keywords

💡Avalanche

An avalanche is a rapid flow of snow down a slope, caused by the force of gravity overcoming the frictional resistance between the snow and the slope. In the video, avalanches are the central theme, with discussions on their causes, prevention, and survival techniques. An example from the script is the description of an avalanche triggered by Bruce Tremper, which serves as a pivotal moment in understanding the power and unpredictability of these natural events.

💡Ski Patrollers

Ski patrollers are professionals responsible for the safety of ski resorts, including avalanche control. They are highlighted in the video for their role in triggering avalanches in a controlled manner to prevent larger, uncontrolled ones from endangering the public. The script mentions following ski patrollers at Whistler-Blackcomb to film avalanches, emphasizing their expertise and importance in mountain safety.

💡Avalanche Control

Avalanche control refers to the deliberate triggering and management of avalanches in a safe manner to reduce the risk of larger, spontaneous ones. This concept is central to the video, which discusses various techniques such as using explosives to trigger controlled avalanches. An example from the script is the use of explosives by ski patrols to prevent the buildup of hazardous snow conditions on slopes.

💡Snowpack

A snowpack is the accumulated layers of snow on the ground, which can vary in structure and stability over time due to environmental factors. The video explains how the characteristics of the snowpack influence the likelihood of an avalanche. The script discusses how the snowpack preserves a record of its history, which can be read by digging a pit and analyzing the layers.

💡Weak Layer

A weak layer in a snowpack is a stratum of snow that is less cohesive than the layers above and below it, making it prone to failure and slide under certain conditions. The video emphasizes the role of weak layers in slab avalanches. An example from the script is the discussion of surface hoar, a type of weak layer that can lead to avalanches if buried by subsequent snowfall.

💡Avalanche Beacon

An avalanche beacon, also known as a transceiver, is a device used by backcountry travelers to locate victims buried under snow. The video highlights the importance of carrying a beacon as part of a safety kit. The script describes how the beacon operates by sending out and receiving electromagnetic signals to detect buried individuals.

💡Avalanche Airbag

An avalanche airbag is an inflatable device designed to increase a victim's visibility and buoyancy in the event of an avalanche, reducing the chances of being buried. The video discusses the airbag as a life-saving invention. The script mentions that pulling a cord on the airbag activates it, which can significantly increase the chances of survival during an avalanche.

💡Avalanche Forecast

An avalanche forecast provides information on the likelihood and severity of avalanches in specific mountain areas. The video stresses the importance of checking these forecasts before venturing into the backcountry. The script advises viewers to make good decisions based on the avalanche forecast to avoid dangerous situations.

💡Slab Avalanche

A slab avalanche occurs when a cohesive layer of snow, or slab, breaks free from the underlying weak layer and slides down the slope. The video delves into the mechanics of slab avalanches, which are often more dangerous than loose avalanches. The script uses the example of Bruce Tremper's experience to illustrate the sudden and powerful nature of slab avalanches.

💡Survival Rate

Survival rate in the context of avalanches refers to the probability of a person being rescued and surviving if buried by an avalanche. The video provides statistics on survival rates based on the time it takes to be rescued. The script states that the chances of survival decrease significantly as time passes, emphasizing the urgency of rescue efforts.

💡Backcountry Skiing

Backcountry skiing involves skiing in uncontrolled, mountainous terrain outside of designated ski resort boundaries. The video discusses the risks associated with this activity, particularly the danger of triggering avalanches. The script mentions that most avalanche fatalities occur in the backcountry, highlighting the need for caution and preparedness.

Highlights

Avalanches are natural disasters that can be extremely destructive, caused by a combination of factors including snow weight, temperature gradients, and human activity.

Whistler-Blackcomb, North America's largest ski resort, employs ski patrollers to trigger controlled avalanches to ensure guest safety.

The deadliest avalanche on record occurred in 1970 in Yungguy, triggered by a 7.9 magnitude earthquake, resulting in an estimated 30,000 deaths.

During World War I, over 12 meters of snow fell on the Dolomites in northern Italy, leading to a single avalanche that killed at least 275 people and triggered further avalanches that claimed thousands more lives.

In the US, 93% of fatal avalanches are triggered by the victim or someone in their party, highlighting the importance of understanding and respecting the power of these events.

Bruce Tremper, an expert on avalanches, shares his personal experience of being caught in an avalanche, emphasizing the unpredictable and dangerous nature of these events.

Avalanches are categorized by size on a scale of one to five, with size five involving over 100,000 cubic meters of snow and causing massive destruction.

Snowpack is not uniform and is affected by various conditions such as air temperature, humidity, sun, wind, and rain, which can alter its structure and stability.

Slab avalanches are particularly dangerous as they involve the cohesive top layer of snow releasing together in large chunks, which can be triggered remotely.

The angle of a ski slope plays a significant role in the likelihood of a slab avalanche; 75% of dangerous slab avalanches occur on slopes between 34 and 45 degrees.

Avalanche control in ski resorts involves triggering avalanches in a controlled manner before skiers are allowed on the slopes, using methods such as explosives.

Avalanche control is also conducted on roads, such as the Trans Canada highway, to prevent large and destructive avalanches from damaging infrastructure.

Avalanche beacons, probes, and shovels are essential equipment for backcountry skiing, aiding in rescue efforts and increasing survival chances.

The use of an avalanche airbag can significantly increase a victim's chances of survival by providing buoyancy and a larger air pocket for breathing once buried.

Checking the avalanche forecast and making informed decisions are crucial for anyone venturing into avalanche terrain.

Brilliant, the sponsor of this video, offers interactive lessons to help individuals learn and apply critical thinking skills to real-world problems.

The importance of respecting the power of nature and being prepared with the right knowledge and tools when engaging in outdoor activities in avalanche-prone areas.

Transcripts

00:00

- This is a video about avalanches, what they are,

00:03

what causes them, how destructive ones can be prevented,

00:07

and what to do if you're ever caught in one.

00:10

- To actually feel the force of the avalanche on your body.

00:14

There's kind of nothing that can prepare you for that.

00:19

- [Derek] For this video, we followed

00:21

around the ski patrollers

00:22

at North America's biggest ski resort, Whistler-Blackcomb

00:25

to trigger avalanches and film them up close.

00:29

If you've ever driven through the mountains in winter

00:31

or have gone skiing at a resort, you were kept safe

00:34

by the knowledge gained by snow scientists,

00:37

avalanche forecasters, and ski patrollers.

00:44

If you Google the deadliest avalanche,

00:46

the first result will be from May, 1970,

00:49

when the town of Yungguy

00:51

and 10 nearby villages were destroyed.

00:53

An estimated 30,000 people were killed,

00:57

but the avalanche was triggered

00:58

by a 7.9 magnitude earthquake,

01:02

and the avalanche caused a landslide which buried the town.

01:06

So it wasn't really avalanche destruction.

01:09

The deadliest snow avalanches happened in December of 1916

01:14

during World War I.

01:16

Some of the fighting took place in the Dolomites,

01:18

mountain range in northern Italy.

01:21

There thousands

01:22

of Italian troops were battling Austro-Hungarian soldiers,

01:26

and it had been a particularly snowy winter.

01:29

Over 12 meters

01:31

of snow had fallen on the mountain range

01:33

in the first week of December.

01:36

On the 13th of December,

01:37

a single avalanche wiped out the Austrian barracks near

01:41

Mount Marmolada killing at least 275 people.

01:46

But its flow on effects were far more devastating.

01:49

Both sets of troops realized they could use snow

01:53

as a weapon, so soldiers on both sides fired artillery

01:57

shells into the mountain above their enemy's camps,

02:01

deliberately triggering avalanches.

02:03

Over the next few days, between 2000

02:06

and 10,000 soldiers died buried underneath the snow.

02:12

But most deadly avalanches aren't triggered by earthquakes

02:15

or artillery shells.

02:17

They're triggered by skiers, snowboarders,

02:20

and snowmobilers recreating in the backcountry.

02:23

93% of fatal avalanches in the US

02:26

were triggered by the weight

02:27

of the victim or someone in the victim's party.

02:30

So how could something so massive

02:33

and destructive be triggered by the weight

02:35

of just a single human?

02:38

This is Bruce Tremper.

02:39

He literally wrote the book on avalanches, the book,

02:42

which inspired us to make this video.

02:44

- I was a a hardcore ski racer,

02:47

but when I was done racing, then I started building lifts

02:50

for the Bridger Bowl Ski Patrol,

02:52

and I got caught in my first avalanche.

02:54

My job for that day was tightening

02:57

all the bolts at the base of each chairlift tower.

03:00

'Cause we were doing our load test that day.

03:03

So then they'd warn me, okay, once you get done

03:05

with this tower, whatever you do, don't cross

03:07

that big avalanche path below it.

03:10

And I made some snide remark like,

03:12

"Yeah, yeah, yeah, I know about all that stuff.

03:15

Don't worry about me."

03:16

When I got done tightening those bolts,

03:18

then I realized I made a really serious error

03:22

because I was just wallowing up to my chest

03:24

and I almost needed to take out a shovel

03:26

and dig like a tunnel through the snow to get back up

03:29

to the ridge and it was just exhausting work.

03:33

And then I got cocky.

03:35

I thought, "Well, I've heard about these ski cuts.

03:38

I can just build up my speed and get to the other side

03:41

'cause that's where I want to be anyway."

03:43

Like an idiot, that's what I did.

03:48

The first thing that happens when an avalanche brakes

03:51

is it just starts moving

03:53

and the first thing you do is kind of flop over on the snow.

03:55

'Cause you lose your balance.

03:56

Like somebody pulls the rug out from underneath you

03:58

and you fall uphill.

04:00

And there I was just kinda laying in the snow,

04:02

getting dragged down the mountain, you know, I was caught.

04:06

I couldn't get out of it

04:08

and luckily I was able to grab a small tree,

04:10

but the snow was just beating me to death,

04:13

and just about snapping off my neck as it's going

04:16

by me on this tree.

04:18

Luckily a lot of that snow went by

04:20

and that's why I didn't die that day.

04:23

But then it finally, the tree finally snapped off

04:26

or something happened

04:27

and I was tumbling down the avalanche path.

04:32

Luckily when I got to the bottom,

04:33

I was only buried up to my chest.

04:35

I was in the tail of the avalanche

04:38

and the avalanche dies from the tail first.

04:40

So that slows down while the front part keeps moving.

04:44

You know, I had a lot of time to think about

04:46

that for the rest of the day.

04:48

Tightening the rest of those bolts

04:49

and thinking, holy smokes, I should, I could have died.

04:54

I should have died and I didn't.

04:56

- You were tightening bolts, you were caught in avalanche

04:58

and then you went back to tightening bolts that day.

05:01

You like, you went back to work.

05:03

- Oh yeah, yeah.

05:04

- Good job. - Well, somebody had to do it.

05:06

I was the guy, that was my job.

05:09

You know, that's how I was raised by my father.

05:12

You get the job done.

05:15

- At the most basic level,

05:16

an avalanche happens when the crystalline bonds

05:19

between the snow crystals break

05:21

and the force of friction holding the snow on the mountain

05:24

isn't enough to counteract the pull of gravity.

05:28

Avalanches are categorized

05:29

by size on a scale of one to five.

05:31

One would not be large enough to bury a person,

05:34

but size five would involve a 100,000 cubic meters

05:38

of snow traveling,

05:40

many kilometers causing destruction on a massive scale.

05:43

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

05:47

The snow pack is built up one storm at a time,

05:51

so it's layered and these layers aren't all the same

05:55

and they change over time,

05:58

due to the conditions they're exposed to,

05:59

the air temperature, humidity, sun, wind,

06:03

and rain all affect the structure of the snow.

06:06

For example, after fresh snow has fallen,

06:09

if there's a warm sunny day, the snow will melt

06:12

and then refreeze overnight creating a sheet of ice.

06:16

The most important condition is the temperature gradient

06:19

in the snowpack, but we'll come to that.

06:22

The snowpack preserves a record of everything

06:25

that has happened to it.

06:26

If you dig a pit into the snow,

06:28

you can read the different layers to understand its past.

06:35

Nine different avalanche problems are recognized,

06:38

but there are two main types, slab and loose.

06:42

There are two types of loose avalanches, dry loose,

06:46

and wet loose.

06:47

The snow in dry loose avalanches is well, loose.

06:52

The snow isn't all stuck together.

06:54

So after it's triggered, it fans out.

06:57

Sometimes loose avalanches are also known

06:59

as point release avalanches.

07:02

Dry loose avalanches are commonly known by skiers as sluff,

07:05

and they usually aren't a serious hazard,

07:08

though they can drag a skier off a cliff.

07:11

- It's funny because like in the skiing community,

07:13

we've just said like this one kind

07:15

of avalanche isn't an avalanche,

07:17

we just call it something else.

07:18

I had a run in with sluff yesterday, a Helbronner,

07:21

and you just feel like this wall

07:23

of snow pushing you from the back.

07:25

Honestly, it's kind of fun, it's fun.

07:27

It's like, it's kind of like surfing where it's dynamic

07:30

and it's moving and you have to keep moving with the terrain

07:33

because you know that yeah, the sluff his behind you

07:37

and if you're not moving fast enough, it's gonna get you.

07:39

- [Derek] Another type of avalanche is wet loose.

07:43

- It typically happens around spring

07:45

on like south facing aspects.

07:47

The sun heats up the snow and makes it more dense

07:51

and slushy and runny.

07:53

The sun's starting to come out.

07:54

I'm not currently too worried about wet loose right now,

07:57

but I will be in about an hour.

08:00

- [Derek] But the more deadly avalanches

08:02

are slab avalanches

08:03

where the top layer of snow becomes stuck together

08:06

like a cohesive slab.

08:08

When the avalanche is triggered,

08:10

the snow releases together in big chunks.

08:13

The prevalence of slab avalanches depends

08:16

on the angle of the ski slope.

08:18

Under 25 degrees, there isn't likely to be a slab avalanche

08:21

because gravity just isn't strong enough

08:24

to pull the snow down the mountain.

08:26

Any slope steeper than 50 degrees is also unlikely

08:30

to have dangerous avalanches.

08:32

The slope is so steep that it's hard

08:34

for much snow to accumulate.

08:36

Regular small sluffs

08:37

and slab avalanches prevent the buildup of larger,

08:40

more dangerous slabs.

08:43

75% of dangerous slab avalanches occur

08:46

between 34 and 45 degrees.

08:49

For comparison, a black diamond ski run is typically

08:53

around 30 degrees and a double diamond is around 40 degrees.

08:57

In other words.

08:59

- The best conditions for skiing are also the best

09:02

conditions for avalanches.

09:03

- [Derek] What's really terrifying about slab avalanches

09:06

is you can trigger them remotely.

09:08

You could be skiing

09:09

or walking on a slope that is far less than 30 degrees

09:13

and trigger an avalanche on a steeper slope above you.

09:17

And slab avalanches are fast.

09:19

They can reach speeds up to 120 kilometers per hour.

09:24

- Friends of mine have been killed that way.

09:26

You just don't realize

09:27

how far these cracks can propagate through the snow.

09:31

- [Derek] Slab avalanches have three parts to them,

09:34

bed surface, which is the lower part of the snow.

09:37

Then there is a weak layer and the overlying slab

09:41

The bed surface and the slab are strong layers.

09:45

That is they're made out of snow that sticks to itself.

09:47

The kind of snow that you can make into a snowball.

09:50

The shape of the snow crystals

09:51

in strong layers tends to be rounded.

09:54

In contrast, the weak layer consists of snow crystals

09:57

that don't readily cohere to one another.

09:59

These tend to be more angular or faceted crystals.

10:03

But there are a few different weak layer types.

10:07

One of the most worrying is known as surface hoar.

10:11

It's not made outta snow that falls from the sky.

10:14

It grows on the surface of the snow.

10:17

On cold clear nights, the surface snow radiates

10:20

a lot of its heat away becoming even colder than the air.

10:23

Since warm air holds more moisture than cold air,

10:26

that moisture will crystallize at the top

10:29

of the snow creating surface hoar.

10:31

Surface is formed by condensation.

10:34

It's the snow equivalent of dew.

10:37

It's really angular and it doesn't want to stick to itself.

10:42

- Usually surface hoar is then broken down by wind

10:45

or sun melting it.

10:47

- [Derek] But if a snowstorm occurs,

10:48

just after these crystals have formed,

10:50

they can become buried.

10:52

And in that case, they create a very nasty, weak layer,

10:55

which is just perfect for avalanches.

10:59

Another weak layer to worry about are facets, snow crystals

11:02

that used to be round,

11:03

but that became angular in the snowpack.

11:06

Facets develop when there's a strong temperature

11:09

gradient in the snow.

11:10

A thermal gradient of less than one degree

11:12

per 10 centimeters does not produce facets.

11:15

Anything more than that can, which is why the temperature

11:19

inside a snow pit is measured.

11:21

If there's a large thermal gradient,

11:22

there could also be a weak layer

11:25

- Right above and right below crust,

11:26

there can be really wild temperature gradients.

11:29

So we had it down 10 centimeters, it was minus 5.5

11:32

and down 20 it was minus 3.7.

11:36

So that's two degree difference.

11:38

- [Derek] The bottom of the snow

11:39

is warmer than the top of the snow.

11:41

So the warmer snow near the ground,

11:43

sublimates turning straight from solid water

11:46

into water vapor.

11:47

This water vapor rises up through the snow pack

11:50

and then encounters the colder snow crystals.

11:53

At this point, it refreezes and forms faceted crystals,

11:57

which like surface hoar, don't stick together

12:00

creating a weak layer.

12:02

- The bigger the change in temperature,

12:03

the faster the snow crystals change

12:07

and the faster they change,

12:09

if they change quickly, it can promote faceting

12:12

which promotes instability.

12:14

- [Derek] The other important factor in slab avalanches

12:17

is the stiffness of the slab.

12:19

Since the snow is cohesive, when a force is applied

12:22

and the interface between the slab

12:23

and the weak layer starts to slip, that force,

12:25

and hence the slippage propagates.

12:29

While stiffer slabs might resist initial triggers better,

12:31

their failure leads to more propagation,

12:34

leading to larger avalanches.

12:36

This also results in more remote triggers.

12:41

Avalanches are common after storms.

12:43

The extra weight

12:44

of the snow exerts an additional load onto the snow pack,

12:48

which can trigger the weak layers deeper

12:50

in the snowpack to slide.

12:52

The vast majority of avalanches occur during

12:54

or right after a storm, in other words-

12:57

- The best conditions

12:58

for skiing are also the best conditions for avalanches.

13:01

- Strong winds also increase the risk of avalanches.

13:04

The wind can pick up

13:06

and transport the snow from open to more sheltered areas.

13:10

As the snow accumulates,

13:12

the wind pressure compacts these snow particles

13:14

as they're deposited, which creates a dense, cohesive layer.

13:19

These newly deposited layers of snow are called wind slabs.

13:23

Wind slabs can be quite unstable, especially shortly

13:26

after formation, if they've formed over a weak layer.

13:29

Since they're more cohesive,

13:31

these slabs can break free as large chunks

13:33

during an avalanche making them particularly hazardous.

13:36

Another concern are cornices.

13:40

Cornices form

13:41

when the wind blows snow over the top of a ridge

13:44

and it piles up on the other side hanging over the edge.

13:48

Cornices can weigh many tons

13:50

and when they fall onto a slope,

13:52

they can trigger massive slab avalanches.

13:56

One of the ways that backcountry skiers test the stability

13:58

of the slope they're about to ride

13:59

is by dropping a cornice onto it.

14:02

If the slope didn't slide under the weight of a cornice,

14:05

it's unlikely to slide under the weight of a skier.

14:09

So how are avalanches prevented in ski resorts?

14:12

For this video, Veritasium producer Peter, went out

14:14

with the ski patrol team at Whistler-Blackcomb,

14:17

which is North America's largest ski resort.

14:19

- Dude, I'm pumped, really pumped, really anxious,

14:22

really stoked, should be a good day.

14:25

There are sections in ski resorts

14:27

that are avalanche terrain, slopes that are greater than 30

14:30

or so degrees or under avalanche terrain.

14:33

To keep skiers safe, before ski areas open,

14:36

the ski patrols do avalanche control work.

14:38

Fundamentally, it boils down

14:41

to them triggering avalanches in a safe way

14:43

before there are any skiers on or under those slopes.

14:47

This is done often enough

14:48

so that the snow doesn't get a chance to build up too much.

14:52

So the avalanches that are triggered are smaller.

14:54

There are a few ways that this is done,

14:57

but the most common is by using explosives.

15:00

Explosive charges with a two minute fuse are lit

15:03

and then detonated on the slopes where they are likely

15:06

to produce avalanches.

15:10

- Oh boy, really glad I put on those goggles.

15:15

- Sometimes this is done

15:16

by throwing the chargers out of a helicopter,

15:19

but a lot of the time this is done on skis.

15:26

There are even special trams where the chargers are attached

15:29

to a carabiner and then shuttled across

15:31

to a hard to access slope.

15:34

As the charge detonates,

15:35

the shockwave breaks the weak layer leading to an avalanche.

15:44

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

15:51

It wasn't great.

15:52

- It was something. - It was something.

15:54

- [Peter] Despite doing the shoot the day after one

15:56

of the biggest storms of the season,

15:58

we weren't particularly lucky with getting large avalanches.

16:02

We shot all morning

16:03

and only got this relatively small slide.

16:06

So I was getting worried

16:09

and then we were able to get this shot.

16:12

(upbeat music)

16:31

Oh my god, look at that, oh.

16:43

Avalanche control work is also done on roads

16:45

with basically the same idea, trigger avalanches

16:48

before they get a chance to become big and destructive.

16:54

For example, the Trans Canada highway connecting the east

16:56

and west coast of Canada goes

16:58

through Rogers Pass, a mountain pass

17:00

with 3000 meter toll peaks on either side of it.

17:04

The road is right next to a mountain

17:06

that's actually called Avalanche Mountain.

17:09

Here Parks Canada

17:11

and the Canadian Army conduct avalanche control

17:14

by firing artillery shells at one

17:16

of the 270 preset targets on the nearby mountains,

17:20

releasing small avalanches

17:21

before they get a chance to become big

17:24

and destructive and damage the highway.

17:27

- [Derek] Deaths from avalanches do occasionally occur

17:30

in bounds at resorts, but they are very rare.

17:33

Due to the diligent work of ski patrollers,

17:35

most avalanche injuries

17:37

and deaths occur in the back country.

17:42

- It's kind of crazy 'cause as a skier,

17:44

you hear so much about avalanches.

17:47

(speaker speaking in a foreign language)

17:48

- You realize it's deadly and people die every year.

17:51

(speaker speaking in a foreign language)

17:53

- But to actually like feel the force

17:56

of the avalanche on your body,

18:03

like there's kind of nothing that can prepare you for that.

18:08

(snow thudding)

18:16

Every time I hit the ground I would just like dig my hands

18:19

in my feet in like try

18:20

to like claw myself down onto the ground

18:22

while also like trying to protect my head.

18:34

I knew also like the moment the snow stopped,

18:36

if I was still in the snow, I would be buried.

18:39

I wouldn't be able to move,

18:40

I would be counting on my buddy to find me

18:44

and dig me out and save me.

18:46

(exhales heavily)

18:55

(speaker speaking in a foreign language)

18:59

- This wasn't a huge avalanche, this was a size two,

19:01

so like big enough to bury a car but not like a house.

19:06

So I was actually able to stop on the slope

19:09

before it flattened out

19:10

where the snow would accumulate and I would be buried.

19:12

(speaker speaking in a foreign language)

19:16

- Yeah, it was intense, really intense

19:18

and I felt really stupid

19:21

because it was the first mountain I ever skied

19:23