Snow is surprisingly hot.
And this heat
can kill.
Avalanches kill about 150 people every year.
But one of the biggest mysteries is
how the shape of a tiny snowflake
can cause something as deadly and powerful as an avalanche.
Okay, it might sound unbelievable,
but let's start from the beginning.
You might imagine a typical
snowflake like this, but there are
actually over 35 different types of snowflakes.
As a snowflake falls, it can
change its shape and size multiple times.
And all this is possible because snow
exists almost always near its melting temperature.
What does that mean?
Well, it means it's considered a hot
material. So it's always undergoing some
kind of phase change, like from solid to liquid,
solid to vapour or liquid to vapour.
So what's this got to do with avalanches?
Well, all these changes to
the shape of a snowflake mean it can
become much more dangerous once it hits the ground.
So when we look out at the snow, it seems
like it's just a uniform blanket.
But that's not the case.
If we look inside the snowpack, we see
that it's formed of many discrete layers.
We might see some layers
that look like those beautiful snowflakes
that fall out of the sky.
Other layers might be small and rounded
and well bonded, and then we might have
some weak layers that tend to be large,
angular grains that are pretty loose.
And that's the thing.
The shape of a snowflake or a crystal can
make a layer either strong or weak.
And the most dangerous avalanches have this weak layer.
One way to imagine it is
like a row of wine glasses all stacked on
top of each other. They might be
relatively stable by themselves, but as
the layer of snow above gets thicker and
thicker, as dense as a row of bricks
being stacked on top of the glass,
eventually that weak layer will break.
This is when we get the avalanche.
Of course, there are a few other
ingredients that contribute.
The angle of the slope is important.
About 39 degrees is the avalanche sweet spot,
a thick layer of snow on top.
And also we need some type of trigger.
And usually that's us – regular humans who
trigger 90% of all avalanche disasters.
But still, the most important ingredient of all
is that invisible, weak layer,
the hidden killer below the surface.
Avalanche workers are tracking weak layers
throughout the season. And if an
avalanche occurs, the first question
you're always asking the other avalanche
worker is, Which weak layer did it run on
and how deep was that weak layer buried?
So what is it that creates these deadly weak layers?
It's heat.
So weak layers form in the snowpack due to
temperature gradients. And oftentimes
large temperature gradients are present
in the snow because the ground surface is
almost always near zero degrees C,
whereas the air temperature above at the
top of the snowpack can be significantly
colder than that.
Those temperature differences cause the movement
of water vapour, and as it moves,
it forms these large angular crystals
that don't bond easily to each other and
therefore become weak layers.
So there are three main types of weak
layer that cause the most concern for
avalanche experts.
Depth hoar.
Surface hoar.
And near surface facets.
First on the list is depth hoar or sugar snow.
Its snow grains grow like inverted cups
that are stacked up on top of each other.
You can almost see them just with your
naked eye. You put your hand into the
snowpit wall and it just comes pouring
out like sugar.
And you have these big sparkly grains.
And this is a type of weak layer that's
responsible for most of the avalanche
fatalities.
Depth hoar forms at the ground snow
interface. That generally means
that the entire snowpack is engaged in the avalanche.
Surface hoar is the pretty one and you
might even recognise this one.
The conditions that form surface hoar are
the same sorts of conditions that put
frost on your car windshield.
So if you find yourself in the morning
scraping the frost off of your car
windshield with your scraper, you
probably could find surface hoar on the snowpack.
These snow grains look like bird feathers
sticking up out of the snow surface.
The problem is that when we have all
these crystals lined up, it resembles a
house of cards and acts like one too.
So when more and more weight piles on top,
the house of cards collapses and
there's an avalanche.
The final one is near surface facets,
a.k.a. recycled powder or loud powder.
Because it tends to be very fun to ski.
You almost sound like you're skiing
through little crystals because you can
hear the snow breaking.
But that breaking is also what can make it
dangerous. These are small angular
crystals of about a millimetre in size,
and they tend not to bond well with
crystals around them.
That's why they're loose and sugary and
also why they form weak layers.
So that's why understanding more about
these weak layers is at the cutting edge
of avalanche science.
It all comes down to the strange
properties of these beautiful, delicate
little snowflakes and how their shape can
be formed and morphed by the heat around
them.