HOW AVALANCHES WORK
PART I
THE LONGEST LIST OF THE LONGEST
STUFF AT THE LONGEST DOMAIN NAME AT LONG LAST
How do avalanches work?
Avalanches seem to be something of a mystery to many
people as they are things that are full of contrasts. Avalanches are a natural
part of mountain life but they are also one of the deadliest aspects of it as
well. They are beautiful and awesome to watch yet the destruction they leave
behind is sometimes some of the ugliest destruction on earth. There are two
different types of avalanches, loose snow avalanches and slab avalanches. Slab
avalanches are when there is literally a slab of very strong and heavy snow
rushing down the slope and it is this type that is most common to claim victims.
Those who enjoy winter sports such as skiing, snowboarding, and snowmobiling
through mountainous areas need to always be aware of the dangers of avalanches
and truly understand how they work, what creates them, and how they take so many
lives each year. It is only with this understanding that one can hope to survive
an avalanche, should one occur while they are enjoying the fine mountain air. Of
course, one would much rather prevent an avalanche from occurring rather than
survive one and although they may seem to strike without warning, the truth is
that many avalanches are started by the same people who the avalanches later
bury. Knowing how they work is key in preventing this scenario from happening.
Understanding SnowAvalanches are
generally made up entirely of snow with the occasional rock, boulder, or ice
pack getting in the mix of things as well. There are also other kinds of
avalanches, such as rock avalanches which are made up almost entirely of rock
but generally when people refer to avalanches, they are referring to snow
avalanches. It is also snow avalanches that kill the most people so to
understand them, one must first understand snow and its properties.
It’s true that no two snowflakes are identical but
they all generally have a similar appearance. Snowflakes are generally
six-pointed figures and depending on conditions such as humidity and
temperature, they will have different shapes and sizes. When a lot of snow has
fallen and begins to accumulate on the ground, it begins to form what is called
a snowpack. Every snowpack is made up of different layers and the stability of
the layers forms the stability of the snowpack. The common six-pointed snow
crystals form the most stable layers but layers which contain substances such as
rime and graupel create very unstable layers. Rime is formed when snow crystals
in the air meet with air that is very, very cool. This rime collects and forms
graupel deposits in the snowpack and layers within a snowpack that contain
graupel are extremely unstable.
Once a snowpack has formed on the ground, it
continues to change as the weather and temperature also change. If the surface
of the snowpack thaws and refreezes repeatedly, a layer of ice will begin to
form on top of the snowpack. This continual thawing and refreezing also causes
ice particles to form on the surface of the snowpack but these ice particles
often have spaces in between them, instead of forming one solid layer of ice.
These small, multiple ice particles however, actually create an even more
unstable layer within the snowpack and make it difficult for the next layer of
snow to form a new, stable layer. The dew point in the air just above the
surface of the snowpack also contributes to its changing properties. If the air
does reach this point, a substance called hoar can develop around the edges of
the snowpack. Hoar looks like feathery crystals and it will not attach itself to
snow crystals, disallowing other snow crystals to collect and form as part of
the snowpack.
Other changes within the snowpack are due to the
temperature gradient, which is the difference in temperature between the top and
bottom layers. The bottom layers will always be warmer than the top layers
because they absorb heat from the ground. The temperature of the top layers will
depend on the air that surrounds it, meaning that if it’s very cold outside, the
top layers of snow will be very cold and if it’s warm, the snowpack surface will
be as well. The individual snowflakes within the snowpack will all begin to
change depending on the temperature. When there is a large difference in the
temperature gradient of a snowpack, such as when the air is very cold therefore
making the top layers very cold but the bottom layers are quite warm, this can
cause the different snowflakes to form facets. Facets have flat surfaces, making
it almost impossible for other surfaces to adhere well to it. Snowflakes can
begin to develop many facets and once this happens, a depth hoar is created. A
depth hoar is a very dangerous property of a snowpack as they are extremely
unstable.
However, temperature gradients can also help snow
packs form more stable layers. Of the difference of temperature is consistently
low, or the temperatures are so cold that they fall to sub-freezing temperatures
but remain consistently cold, this causes rounding. Rounding enable snowflakes
to compress together more tightly and adhere to each other better. There is also
water vapor among the snowflakes and as they are being squeezed together with
water vapors constantly being exchanges, the vapor begins to form bridges and
connect to other snowflakes. This creates a very stable and strong layer. It is
these stable and unstable layers that both help to form avalanches.
How Avalanches are Formed
Avalanches need three components to form. These are
snow, a slope, and something to cause them. Unstable layers within the snow
packs consisting of ice, graupels, depth hoar, and faceted crystals also help
with the formation of avalanches. If the unstable layer is close to the surface
of the snowpack, it is called a sluff. A sluff is shaped in the form of a ‘V’
and consists of snow that is loose and powdery. These are considered to be
avalanches but they are not considered to be of the dangerous sort and they can
pass by people and objects leaving them undisturbed.
It is when the unstable layer is found deeper
within the snowpack that the avalanche becomes far more dangerous. This is when
slab avalanches are formed and the slabs consist of a layer that breaks entirely
free from the snowpack and rushes down the mountain. It’s much like how the snow
falls down a car’s windshield as it melts. The weak layer that causes the slab
avalanche is sometimes called the failure layer and the strength of the
avalanche will depend on the strength of the slab and the how deep within the
snowpack the weak layer was. Slabs that are very stiff and have compressed
tightly together will form large chunks of snow while looser snow will create
smaller chunks. If the snow found within the slab is very wet, it will move much
slower than snow that is dry but wet slabs are much more powerful.
Mountain slopes that are at a 25 to 60 degree angle
from the ground have prime conditions for an avalanche but it is those with a
slope between 35 and 45 degrees that have the most avalanches. Any slope that
has a smaller slope than 25 degrees is generally not steep enough to create an
avalanche and mountains with slope that exceed the 60 degree angle mark are so
steep that they create continuous sluffs of snow rather than one large slab
avalanche. There are also two different types of mountain slopes: leeward and
windward. It is generally the leeward slopes that see the most avalanches occur
on them.
And the third component to the avalanche is the
trigger. Unlike many people believe, avalanches are not usually started by loud
noises. There are some natural causes of avalanches such as falling trees, icy
overhangs, or a sudden change in temperature but the avalanches that cause the
most fatalities are those that people create. People cause the avalanche when
they move over the slab with the weak layer, causing it to move and therefore
moving the entire slab and the slabs around it. The slabs then begin to break
apart and fall, following quickly behind the skier or other outdoor enthusiast.
Once the avalanche has begun it has three parts to it. The starting zone is the
first part and that is where the slab breaks away from the rest of the snow. The
track is the second part of the avalanche and this is the path it makes as it
rushes down the mountain. The runout is the third and final part of the
avalanche and this is when the snow and debris it’s collected along the way slow
down and come to a stop. This is the point that the snow sets and it becomes
very hard. People have often compared it to concrete because it is so strong and
when a victim is buried under it, this firmness is fatal as it’s nearly
impossible to dig one’s way out and so, a rescue team is often the only hope.
Next:
Avalanche Prevention and Control
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