Avalanche – Snowslides in the Mountains
An avalanche is a large mass of snow that moves quickly down a mountain. It breaks loose from the area around it and collects more and more snow as it pushes downward.
Avalanches occur in mountainous regions, for example the Alps, Himalayas or the Rocky Mountains. They can reach speeds of over 100 km an hour and destroy forests and villages that are in their way. They can block roads and train tracks and make areas unreachable from the outside. Avalanches kill over a hundred people in Europe and North America every year, burying them below tons of snow.
Although the danger of avalanches is high throughout the winter months there are weather and snow conditions in which avalanches occur more often. The composition of snow is an important factor. Once the snow is on the ground temperature changes its structure so that different layers of snow can form on the slope.
Avalanches occur on slopes that have an angle of 30 to 60 degrees. On slopes that are not so steep there is not enough gravity to let the snow break off from its surroundings. On steeper slopes the snow usually breaks off more quickly.
Avalanches can be triggered by many factors. In some cases an increasing amount of new snow can start an avalanche. Strong winds can transport snow to the leeward side of the mountain. There, the snow stays soft and does not bind with other layers. In other cases, skiers, snowboarders or other alpine tourists can trigger an avalanche.
As snow falls it builds up layer upon layer. After a certain time the layers harden and bond with one another. If they don’t the snow profile becomes unstable and soft layers can slide on top of harder ones.
Avalanche in the Himalayas
Parts of an avalanche
An avalanche is usually divided into three areas.
- The starting point is where snow breaks off and begins to slide. It is mostly in higher regions of a slope.
- The track of an avalanche is the path that it follows downhill, which can be hundreds of metres long. As the avalanche moves towards the valley it gathers more and more snow.
- The runout zone is where the avalanche slows down and finally comes to a stop.
Types of avalanches
- Powder snow avalanches occur when more and more powder snow accumulates and starts moving like a cloud in the sky at speeds of up to 400 km an hour.
- Wet snow avalanches are triggered when snow starts becoming warmer and wetter. These avalanches spread sideways as they move downhill. Snow hardens so that buried victims hardly have a chance of breathing.
- Slab avalanches are the most common. They occur when layers of snow don’t cling together so that skiers or snowboarders trigger the top layer which slides down on the other harder ones.
Avalanche Warning and Protection
In areas where avalanches are likely to occur, people have taken action to protect themselves and their surroundings. Avalanches can be set off on purpose through explosions. In some places rakes are set up on slopes to slow down and split up the moving snow. Trees, fences, rocks or concrete barriers can also to slow down moving avalanches. Most alpine countries have avalanche warning systems that tell tourists when and where it is dangerous to move around in the mountains.
In most countries the danger of avalanches is displayed on a risk scale ranging from one to five. One is a low risk situation with stable snow conditions while five displays a very high risk of avalanches with unstable snow conditions.
Snow fences in Switzerland
There are some safety precautions you can take in order to avoid avalanches.
- Be aware of your surroundings and check weather conditions
- Always takes someone with you, never go alone.
- Take a shovel, a rescue beacon and an avalanche probe with you.
- Watch out for shady places where snow can accumulate.
- Examine and test the snow stability
When moving around in areas where avalanches can occur you should carry a few things with you:
- An avalanche beacon is a small transceiver. It can be switched on from transmit to receive in order to send a signal or find a person who is caught under the snow.
- Avalanche probes are poles that can be collapsed so that you can put them into your backpack. They let you find buried objects in the snow as far as 2 feet downwards.
- Small shovels are necessary for digging out objects and people.
If you can’t escape from an avalanche,
- get skis and poles off your body, because they can pull you down
- use swimming movements to get on top of the avalanche
- try to grab trees nearby to get away from the snow masses
- wait for rescue and stay calm. Heavy breathing will use up oxygen too quickly.
Rescue and search teams are trained to find people who are trapped under the snow and dig them out. Statistics, however, show, that 90% of the victims die if they are not dug out within the first half hour.
Avalanche danger scale
- accumulate = gather, collect
- although = while
- angle = space between two lines that cross each other
- avalanche probe = long thin object that is used to find something in the snow
- avoid =escape
- aware = alert, pay attention
- backpack = bag used for carrying things on your back
- bind = mix, connect
- block = here: to close off, so that you cannot use it
- bond = join
- break loose = break off
- breathing = taking air in and blowing it out
- bury = to put something under the ground
- certain = some
- cling = stick to
- collapse = fold together and make smaller
- collect = gather
- composition = makeup
- concrete barrier = here: fence or gate made up of stone and cement
- destroy = damage completely, tear down
- display = show
- divide = split
- downward = here: towards the valley
- factor = element
- fence = structure made out of wood or metal that keeps something away
- grab = get hold of with your hands
- gravity = the power or force that pulls you down to earth
- harden = to become harder
- heavy = hard
- however = but
- increasing = more and more
- layer = here: snow that is in the middle of other masses of snow
- leeward = the side that is protected from the wind
- likely = probably
- mountainous = areas with high ,mountains
- movement = to move parts of your body
- necessary = something you need
- occur = happen
- on purpose = here: not by nature, but by people
- once = as soon as
- oxygen =gas that is in the air and which we need to breathe
- path = route
- pole = long stick
- powder snow = soft and light snow
- protect = guard, defend, save yourself from something
- rake = an object that has teeth sticking out of it
- receive = get, accept
- rescue = someone who will save you
- rescue beacon= a radio signal that you use to let others know where you are
- risk scale = here: the numbers show how likely it is that an avalanche could break off a mountain slope
- safety precaution = something you do to protect yourself from something dangerous
- set off = here: start
- shady = out of the sunlight
- shovel = tool used for digging
- sideways = to the side
- slide = glide; to move smoothly over an even surface
- slope =side of a mountain
- snow profile = the combination of the different layers of snow
- speed = how fast something is
- steep = at a high angle
- structure = what it is made up of
- surroundings = the world around us
- switch on = turn on
- take action = do something
- throughout = during, in all of
- tracks = two metal lines along which trains travel
- transceiver = radio that can send and receive messages
- transmit = send
- trapped = if you cannot escape
- trigger = start, set off
- unreachable = you cannot get there
- unstable = not steady; here: danger of breaking off
- usually = normally
- valley = area of low land between two mountains, usually with a river flowing through it
- victim = here: person who is trapped in an avalanche
- village = very small town in the countryside