How to make an avalanche

The long-suffering towns of the Alps know the power of snow too well. For centuries, before control techniques were perfected, they were plagued by periodic avalanches. More than 2000 people died in 1618 when tons of snow slid into Plurs, Switzerland, and the Chiavenna Valley of Italy. Avalanches can strike anywhere there's enough snow: disastrous ones struck sites in Peru, China and India last century, and in 1836 eight people were killed at the British town of Lewes. North America's deadliest avalanche occurred on March 1,1910, when a train stranded for five days near Wellington, Washington, was finally hit by a wall of snow, killing 96 passengers.

Conditions are ripe for an avalanche when a light coating of early-season snow warms and refreezes over a long period (days to weeks) of relative dryness. Water vapour from the still warm ground seeps up through the snowpack, and the interlocking snowflakes morph into rounded crystals that can't keep the snowpack as firmly bonded. From this point or the base can disintegrate in a loose-snow avalanche, which typically starts small and – like a snowball – gains momentum. For a slab avalanche, you need a wet, heavy snowpack on top of this loosened base. The slightest trigger – perhaps a skier, or (increasingly, these days) a high-powered snowmobile – can set a 3m/10ft -deep slab of snow sliding downhill at speedsthat can top 160kph/100mph. Slopes of 30°-45°are the most vulnerable, as steeper slopes tend to shed their snow in more manageable bits.

When fully buried by an avalanche, less than half of all victims survive for more than half an hour, as the snow melted by their breath gradually freezes into an icy mask that cuts off oxygen supply. Shrewd outdoors people swim toward the surface of an avalanche, as if riding a wave; if burial seems certain, they quickly form a breathing space with their arms.They also carry digital beacons that send GPS signals to help searchers find them if they're trapped in a snowbank. Finally, if they must cross a vulnerable snowfield, they do so singly rather than in a pack.