How do these massive oceans of snow and ice stay on a mountain in the first place?
When an avalanche starts, is there anything we can do?
The power of nature is devastating, as pictures and video of the recent earthquake and avalanche
in Nepal illustrate, but avalanches don't just happen during earthquakes, and, in fact,
are far more common than you think; but not for the reasons you may think.
An avalanche is a falling mass of snow, ice, and rock that tumbles rapidly down a mountainside.
They can be dangerous, and deadly.
These massive clouds of rock and frozen water can reach 80 miles per hour (129 kph), sweeping
trees, buildings and even massive steam trains out of the way.
Dry snow avalanches can reach speeds of more than 200 miles per hour (322 kph).
Avalanches are formed because of the physics inherent in the natural process of snowpack.
Mountains alter how air flows over the surface of the planet.
When moist air hits a mountain it must either go around, or up and over -- but to do either
it must CHANGE.
If it goes up, the air cools, and moisture condenses into rain or snow.
As the tallest mountain in the world, Everest collects a lot of cold wind and precipitation
as the air moves around and over it -- and as the snow falls, it creates the conditions
for an avalanche.
Snow crystals, as you'd know if you're a subscriber to DNews -- if you aren't come join us!
-- come in a few shapes.
Most are six pointed water crystals, which lock together nicely.
When snow comes in contact with super-cooled air, like it can on mountains, the snowflakes
form rime or, like, a frost on the edges -- sometimes forming a gravel-like snow called graupel.
Different snow falls in different conditions, forming specific layers on the mountainside.
The danger in an avalanche, aside from the speed, is the sheer volume and mass of the
speeding snow.
In April of 2014, a MASSIVE formation weighing 31 and-a-half million pounds sloughed off
an Everest glacier, killing 16 sherpas.
And stories like this are relatively common in many mountain-ranges across our planet.
Plus, once the snow stops falling, it sets up like concrete and the weight compresses
the snowpack, meaning death often occurs from injury or suffocation, aside from hypothermia.
Cartoons made me believe sound causes avalanches, but while 90 percent of avalanches are caused
by a victim or a member of the victim's party, it's not due to sound.
Avalanches are ACTUALLY caused simply by weight of the snow thanks to gravity.
The snow is in three layers, the oldest most packed-in snow on the bottom is compressed
and solid -- that's called the bed surface.
The middle layer is a weak layer of snow which initiates the slide -- called the weak layer;
or failure layer; and then on top is the slab.
Slabs are new snow, or compressed and compacted chunks of snow and ice which break off and
begin the avalanche.
When a slab sits on a failure layer, at JUST the right slope -- between 35 and 45 degrees
-- the slab will break off.
Slabs range from 10 centimeters to several meters thick.
This also means that avalanches are more common after a new, heavy snow -- which can add a
lot of weight to an already-loose slab.
Though that is not the whole story in Nepal -- where a 7.8 magnitude earthquake shook
the snow off the mountain.
And for more background on the geology of that region and why Nepal is prone to earthquakes,
watch this DNews episode from Julia:
Ultimately, avalanches and earthquakes are going to happen and once they do, there's
nothing to stop them.
But the more we understand them, perhaps the better prepared we can be when these disasters
strike again.