Avalanche! It is something those of us in the US Upper Midwest do not have to worry about when skiing locally or regionally. Also, our friends who ski on the East coast only hear the word in a local context when talking about Tuckerman Ravine in New Hampshire.
As a lover of snow and snow sports I have made an effort to learn more about avalanches, the conditions leading to them, and how to mitigate the risk of being seriously hurt (or worse) by an avalanche.
You do not have to go up or a down a mountain to be affected by avalanches. Being close to a mountain puts you at risk. If you move into an avalanche runout zone you could be swallowed up by one or even crossing terrain adjacent to avalanche sensitive zones could trigger an event.
Avalanche! What are they?
Remember, snow is water and what does water on a hill want to do? Right, it wants to go down the fall line and get to the lowest possible spot it can (which would be any one of our oceans). Put simply, an avalanche is water flowing down a hill in a raging torrent.
Avalanche! The Forces
There are two main forces at play when it comes to avalanches.
Gravity, we all understand this force. Gravity is the force that pulls all things downhill. It is responsible for driving us down the hill on our skis and driving the snow downhill when it avalanches. Also, it is the force that causes an upper layer to crush down on weak ones. Gravity is a constant force and for the purposes of our discussion it does not vary.
The other force at play is friction. Friction, is the force responsible (along with the cohesive tendency of water) for holding snowflakes to other, for holding the snow to the surface of the mountain, and for holding one layer of snow on to the other. The frictional forces work against gravity.
However, the frictional forces at work in a snowpack vary. Why is this? This is because the snow changes in response to its environmental conditions. The changes in the snow
affect how much friction it presents to snow on top or below it.
Avalanche! The Snow
As we all know, snow is a variable beast. It falls out of the sky in many different forms. Not only is snow a varying creature, it is a living creature. As it is on the ground it ages and changes form.
There are three forms of snow avalanche researchers blame for many avalanches. They are termed faceted snow, wind-blown snow, and hoar (which can be formed on the surface or at depth). Two factors work together to rot the snow from its virgin form into the rotten snow responsible for avalanches. These two factors combine together to be called the temperature gradient.
Avalanche! Temperature Gradient
Calculate the temperature gradient by dividing the difference in temperature between the air and the earth (assume the earth’s temperature to be 0° C) by the depth of the snowpack. So, let us assume the difference in the earth and air temperature is 20° C and the depth of the snow is 20 cm (nearly 8″). This means the temperature gradient is 1° C/cm of snow which means the snow is rotting. Snow heals when the temperature gradient is less than .1 ° C/cm and rots when the temperature gradient is higher!. That is, if the temperature gradient is higher than 1 ° C and the snow depth is less than or equal to 10 cm.
This fact leads to a counter-intuitive fact. I always had the notion that cold temperatures would make for better stronger snow. However, cold air temperatures result in higher snow pack temperatures gradients which as we discussed above results in rotting snow.
Gravity plays a big role in avalanches, as it is the force that propels the snow down the slope. The slope of the hill the snowpack is on has to be just right. Too slight and the force of gravity is not strong enough parallel to the hill, too much and the snow rarely is able to stick in sufficient quantities to be dangerous. The goldilocks zone for avalanches is roughly 30° through about 50°. That is, the slopes many of us skiers and snowboarders strive to be on.
Avalanche! A Trigger
A trigger is something applying a force to the snow that overcomes the frictional forces holding the snowpack together and to the mountain. When that occurs the break spreads out from the trigger point (the trigger point and the location of the break are not always together) and the top layers of snow break free form the weak underlying layers and they slide.
The types of triggers we often see are snowmobilers, skiers, hikers, and snowboarders doing their thing on the snow. Sometimes it is a ski-patrollers bomb or shell, occasionally we see avalanches triggered by a cornice breaking off, and sometimes the snowpack simply triggers from its own weight.
Avalanche! Some Good Reads and Videos
I have found the following very informative when it comes to understanding avalanches: