Avalanches
Each year, avalanches claim numerous lives. Skiers or
even backcountry hikers get covered in a wall of snow
in a valley or on a mountainside. Basically, avalanches
occur when there is more stress on the top layer of
snow than on the bottom layer. In winter and spring,
experienced and novice skiers alike frequently search
for fresh powder trails. On ski mountains, it is not
uncommon for skiers to deliberately ski outside the
marked boundaries of trails. Avalanches can travel at
up to 300 km/h and carry with them thousands of tons
of snow. Being trapped in an avalanche is the worst
fate imaginable. If you are buried underneath several meters of snow, you will run
out of oxygen and suffocate. After 35 minutes, a person has only a 30 percent
chance of survival.
Multiple Snow Layers
There are three main causes that contribute to avalanche conditions. On the
surface of a mountain is the snowpack. This is the snow that has fallen and
accumulated over the past weeks and months. Usually, there are several layers of
snow on the mountain. The first layer of snow is called the “bed layer.” Some of
this snow will melt away, leaving a second thin layer of ice particles on top of the
bed layer. It might take several weeks before another heavy snowfall. The danger
comes when the third layer of snow falls. If it is made of light, loose or fluffy
snow (not heavy wet snow), the danger starts. This third layer is called the “weak
or slab layer.” A weak layer of snow lying on top of ice and bed layers means that
an avalanche is more likely to occur.
The Trigger and Angle
A trigger is needed for the slab layer of snow to slip off. This means something
causes the slab to slide off the second thin layer of ice. This is the second cause of
an avalanche. The pressure from the weight of a skier can easily cause the top slab
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Anatomy of an Avalanche T-07
Moving Up R_B3 2013.12.26 10:11 AM 페이지36
Unit 04 37 | Avalanches
Passage 1 Science
layer to break off. Then an avalanche starts. The third cause of an avalanche
relates to slope angle. Most avalanches occur on slopes that have an angle of 25 to
60 degrees. If the slope is greater than 60 degrees, the snow will constantly slide
off, never allowing avalanche conditions to form. According to statistics, slab
avalanches account for 90 percent of all ski fatalities. Since it is very difficult to
know how much stress there is on snow slopes, most ski resorts have strict rules
about skiing out of bounds.
AvalanchesEach year, avalanches claim numerous lives. Skiers oreven backcountry hikers get covered in a wall of snowin a valley or on a mountainside. Basically, avalanchesoccur when there is more stress on the top layer ofsnow than on the bottom layer. In winter and spring,experienced and novice skiers alike frequently searchfor fresh powder trails. On ski mountains, it is notuncommon for skiers to deliberately ski outside themarked boundaries of trails. Avalanches can travel atup to 300 km/h and carry with them thousands of tonsof snow. Being trapped in an avalanche is the worstfate imaginable. If you are buried underneath several meters of snow, you will runout of oxygen and suffocate. After 35 minutes, a person has only a 30 percentchance of survival.Multiple Snow LayersThere are three main causes that contribute to avalanche conditions. On thesurface of a mountain is the snowpack. This is the snow that has fallen andaccumulated over the past weeks and months. Usually, there are several layers ofsnow on the mountain. The first layer of snow is called the “bed layer.” Some ofthis snow will melt away, leaving a second thin layer of ice particles on top of thebed layer. It might take several weeks before another heavy snowfall. The dangercomes when the third layer of snow falls. If it is made of light, loose or fluffysnow (not heavy wet snow), the danger starts. This third layer is called the “weakor slab layer.” A weak layer of snow lying on top of ice and bed layers means thatan avalanche is more likely to occur.The Trigger and AngleA trigger is needed for the slab layer of snow to slip off. This means somethingcauses the slab to slide off the second thin layer of ice. This is the second cause ofan avalanche. The pressure from the weight of a skier can easily cause the top slab51015202530Anatomy of an Avalanche T-07Moving Up R_B3 2013.12.26 10:11 AM 페이지36Unit 04 37 | AvalanchesPassage 1 Sciencelayer to break off. Then an avalanche starts. The third cause of an avalancherelates to slope angle. Most avalanches occur on slopes that have an angle of 25 to60 degrees. If the slope is greater than 60 degrees, the snow will constantly slideoff, never allowing avalanche conditions to form. According to statistics, slabavalanches account for 90 percent of all ski fatalities. Since it is very difficult toknow how much stress there is on snow slopes, most ski resorts have strict rulesabout skiing out of bounds.
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