How to

6 June 2016

The Science Behind Avalanches


It’s every skier’s nightmare, a colossal, rolling wall of snow hurtling down the mountainside, burying everything in its path. They rest on a hairpin trigger, and many unfortunate souls have actually been the conjuror of their own demise. There’s something oddly beautiful about them, their fluidity and their power, but there’s nothing majestic about the wave of destruction they bring.

It might not seem immediately obvious what actually causes an avalanche, and from a certain point of view it might just look like random chance, or the result of snow piling unevenly on a steep gradient. It’s more complex than that, and it has a great deal to do with the actual composition of the snow.

When it’s compressed heavily enough (like on a mountain), the lowest snow forms what’s known as snowpack. With snowpack, the crystals are so compressed that they begin to interlock, but when cooled water comes into contact with airborne crystals, they form graupel, which is rounded, and far easier to unsettle. When snow like this collects on a steep surface it can just roll off in relatively harmless clouds of ‘sluff’. When the loose layer is deep in the snowpack though, then you’ve got a problem.

If it starts moving, it causes what’s known a slab avalanche, which is essentially just a huge mass of packed snow thundering downhill, dislodging more as it goes. Sometimes the slabs will largely stay together, other times they will fragment, but the end result is the same – carnage.

They tend to form on leeward slopes, and such is their sensitivity that the trigger can be anything from a change in the weather to something coming off a tree, but in many cases, it’s a person’s weight passing over the snow. The loud noises thing is at least partially true, but we’re talking about a deafeningly loud noise here, something no human would be capable of making, not even Brian Blessed. When it does happen, it’s usually an explosion.

The really dangerous thing about avalanches isn’t so much the actual downward path of the snow, but the way it behaves when it settles again. The weight and pressure cause it to compact and harden in such a way that anyone trapped beneath is either crushed, or can’t get out. If you ever find yourself trapped under a drift, here’s an important tip – spit and see which direction the dribble moves in, many a disoriented victim has dug themselves deeper into the snow because they thought they were facing in the other direction.

Avalanches can hit basically any time, so preventive measures have to be taken. Sometimes the answer is to actually cause smaller avalanches. By doing this, the consistency and build of the snowpack can be monitored and controlled. There’s also ski checking, which involves actually skiing along high fracture lines to see if it triggers anything. It sounds terrifying, but it’s always done in groups with a partner at a safe distance who can get help organise if an avalanche does start.


The worst avalanche ever recorded was in Peru in 1970. It was brought about by a massive coastal earthquake, causing the northern slope of Mt. Huascaran to collapse and come crashing down. The avalanche hit speeds of over 100mph, and picked up 2 million cubic metres of ice. It completely buried two towns in almost 100 metres of snow, rock and debris. It is thought to have killed more than 20,000 people. 


Callum Davies

Callum is a film school graduate who is now making a name for himself as a journalist and content writer. His vices include flat whites and 90s hip-hop.