By Issy Ronald, CNN
(CNN) — The whole mountain rumbles when an avalanche rolls off it. The swirling, whirling mass approaches like a steam train, picking up thousands of tons of matter on its descent, throwing clouds of snow into the air as its gathers speeds of up to 130 kph (80 mph).
They are one of the most dangerous phenomena in the mountains. An avalanche in California killed nine skiers on Tuesday, including six close friends.
And this winter in Europe has proven particularly deadly.
According to the European Avalanche Warning Services (EAWS), at least 99 people have died since October 2025, mostly in the Alps — the mountain range that serves as the continent’s skiing hub and sprawls across several countries including France, Italy, Switzerland and Austria.
In the French Alps alone, avalanches have killed 28 people since the first fatal accident of the winter on December 26, according to EAWS. That is a huge increase from the average eight deaths typically seen at this point in the season, according to France’s National Association for the Study of Snow and Avalanches (ANENA). In Italy and Switzerland, the avalanche death tolls are also much higher than normal.
And, showcasing their awesome, frightening power, one avalanche derailed a train in Switzerland on Monday, injuring five on board, while another swept through a refuge on a French mountainside earlier this month, shattering windows and dumping snowdrifts inside the building’s kitchen.
‘A conveyer belt’
Avalanches require a combination of three things – snow, a slope steeper than 30 degrees and a trigger, such as fresh or melting snow, a person, an animal or wind.
Of course, these are present in the Alps every winter but specific conditions have made this winter more dangerous than others, explained Stéphane Bornet, director of ANENA.
After the first snowfall in November, he said, there was a long period of atmospheric high pressure where the snow on the ground evolved into “angular grains” — which look a bit like large sugar crystals and don’t bond effectively with surrounding layers.
On its own, this type of snow isn’t dangerous. But, if covered by fresh snow, “these fragile layers can now be triggered and act a bit like ball bearings, like a conveyor belt that allows the avalanche to slide quite easily,” Bornet said.
And, over the last few weeks, several meters of fresh snow have fallen across Europe, part of a weather pattern driven by an unusual southerly jet stream that has dumped huge amounts of precipitation across the continent, causing flooding in lower altitude areas.
At the same time, “the snowpack is being fed by the wind, which means we have a large accumulation of snow on the ground,” Bornet added.
Such conditions prompted several regional avalanche forecast services to issue a severe level 4 warning for much of the last two weeks, with some areas even reaching the most severe level 5 warning, at least for a few days.
Still, these dangerous conditions are unusual, but “not extraordinary,” noted Christine Pielmeier, an avalanche forecaster at the WSL Institute for Snow and Avalanche Research SLF in Switzerland.
They can occur roughly every five to 15 years, she told CNN, emphasizing that this number is only a statistical guide, so the cond