France has seen five deadly Alpine avalanches so far this winter, claiming 12 lives, eight of them in the past week.
The string of tragedies is explained partly by weather conditions but also probably by ill-advised risk-taking, experts said.
- What causes avalanches? -
Most avalanches are the result of a combination of weather and geological factors.
In general, an avalanche results from fresh heavy snowfall that fails to stick to snow already on the ground.
This was the case for Monday's avalanche, which killed five members of the French Foreign Legion.
The incline of the spot where the avalanche originates is a main factor determining its size -- that avalanche was some 400 metres (1,300 feet) wide and slid some 250 metres.
Another scenario is that of violent wind following a moderate snowfall at low or very low temperatures. The wind carries large quantities of snow that can then break up depending on the slope and the state of the ground underneath.
That was the case in the avalanche that occurred last Wednesday, killing two high school students and a Ukrainian tourist, experts say.
"That's a typical case of a wind slab, that is a slab of snow formed by very strong wind," Dominique Letang, director of the National Agency for the Study of Snow and Avalanches (ANENA) told AFP.
At temperatures below -15 Celsius (5 Fahrenheit), far lighter snowfall can lead to an avalanche less than a metre thick.
Spring thaws bring another type of avalanche, with snowlides depending on how fast the snowpack melts.
- Why so many? -
The instability of the Alps' snow cover is the result of an unusually mild autumn, according to Letang.
"Snow fell at the end of November onto warm ground then began to change," he said, adding: "The base level of the snow is not sticking together. It's something that is neither sticking to the ground nor the snow that is falling on top of it."
ANENA appealed for vigilance three days before both of the latest tragedies, with an avalanche risk level of three on a scale of five in both areas.
At level three, a single skier can unleash a snowslide, while ski areas are closed when the top level is reached.
Each year between 500 and 1,500 avalanches are recorded in the French Alps.
Avalanche corridors are identified in most European countries, and in skiing areas explosives are used to set them off artificially.
- Are the Alps more dangerous this winter? -
Dozens die each year in avalanches in France's popular ski resorts. At least 45 people died in snowslides during the 2014-15 winter season in France, according to ANENA, more than double the previous year's 20.
ANENA data show that between 1971 and 2011, avalanche accidents averaged 21, with an average of 30 fatalities.
Letang said that while more and more people are using areas that have not been cleared as safe, the surge has not been matched by an increase in fatalities.
- How can the deaths be minimised? -
Group members are advised to spread out, leaving some 20 metres between each other, to help avoid setting off a snowslide.
"People should observe a big safety distance when the snow cover is unstable," Letang said. "Even if you won't avoid the avalanche, it would carry off one person, not 13" as in Monday's accident.
France has seen five deadly Alpine avalanches so far this winter, claiming 12 lives, eight of them in the past week.
The string of tragedies is explained partly by weather conditions but also probably by ill-advised risk-taking, experts said.
– What causes avalanches? –
Most avalanches are the result of a combination of weather and geological factors.
In general, an avalanche results from fresh heavy snowfall that fails to stick to snow already on the ground.
This was the case for Monday’s avalanche, which killed five members of the French Foreign Legion.
The incline of the spot where the avalanche originates is a main factor determining its size — that avalanche was some 400 metres (1,300 feet) wide and slid some 250 metres.
Another scenario is that of violent wind following a moderate snowfall at low or very low temperatures. The wind carries large quantities of snow that can then break up depending on the slope and the state of the ground underneath.
That was the case in the avalanche that occurred last Wednesday, killing two high school students and a Ukrainian tourist, experts say.
“That’s a typical case of a wind slab, that is a slab of snow formed by very strong wind,” Dominique Letang, director of the National Agency for the Study of Snow and Avalanches (ANENA) told AFP.
At temperatures below -15 Celsius (5 Fahrenheit), far lighter snowfall can lead to an avalanche less than a metre thick.
Spring thaws bring another type of avalanche, with snowlides depending on how fast the snowpack melts.
– Why so many? –
The instability of the Alps’ snow cover is the result of an unusually mild autumn, according to Letang.
“Snow fell at the end of November onto warm ground then began to change,” he said, adding: “The base level of the snow is not sticking together. It’s something that is neither sticking to the ground nor the snow that is falling on top of it.”
ANENA appealed for vigilance three days before both of the latest tragedies, with an avalanche risk level of three on a scale of five in both areas.
At level three, a single skier can unleash a snowslide, while ski areas are closed when the top level is reached.
Each year between 500 and 1,500 avalanches are recorded in the French Alps.
Avalanche corridors are identified in most European countries, and in skiing areas explosives are used to set them off artificially.
– Are the Alps more dangerous this winter? –
Dozens die each year in avalanches in France’s popular ski resorts. At least 45 people died in snowslides during the 2014-15 winter season in France, according to ANENA, more than double the previous year’s 20.
ANENA data show that between 1971 and 2011, avalanche accidents averaged 21, with an average of 30 fatalities.
Letang said that while more and more people are using areas that have not been cleared as safe, the surge has not been matched by an increase in fatalities.
– How can the deaths be minimised? –
Group members are advised to spread out, leaving some 20 metres between each other, to help avoid setting off a snowslide.
“People should observe a big safety distance when the snow cover is unstable,” Letang said. “Even if you won’t avoid the avalanche, it would carry off one person, not 13” as in Monday’s accident.
