More than 90 deaths this season: Are we seeing more avalanches?

It is not unusual for avalanches to be in the news in the northern hemisphere at this time of year, the height of the annual ski season.

But Tuesday's deadly incident in California, which took the lives of eight skiers and left one more unaccounted for, and the difficult situation in much of the Alps - where there have been more fatalities than usual this year - has put a spotlight on avalanches and how prepared winter sports enthusiasts should be for them.

The two situations would seem to be very different. California has been coming out of a so-called snow drought and experts do not consider the latest snowfall exceptional.

But droughts followed by intense snow can cause problems - the new snow is unable to bind to old snow underneath, which can either be very hard or non-existent, and therefore remains loose.

Europe, however, has seen two major storms in a week and a large amount of heavy snow combined with strong winds, leading to high avalanche warning levels across an unusually large part of the Alps. Three Britons were among dozens killed in incidents so far over the season.

One Alpine resort suggested that the current conditions could be the worst there this century.

Recent snows have also led to evacuations of several communities in Switzerland and northern Italy, as well as power outages and a train derailment at Goppenstein in the south-western Swiss canton of Valais.

But is there a connection between events in Europe and California?

Is climate change to blame?

Climate change might seem an obvious culprit for the unstable weather conditions that have brought them about.

"There is evidence that climate change will lead to sharper, more intense precipitation followed by long periods without," Simon Mason, senior scientist at SEI US, told the BBC.

This would seem to be the case in Europe, which has recently had several seasons with lower than average snowfall, and some dry periods in the early part of this season.

"However, since many factors control avalanche risk and precipitation is only one controlling influence, it is difficult to conclude how risks may evolve with future climate change," Mason said.

Climatologist Christoph Marthy told Swiss outlet SRF the current Alpine winter - with weather until recently dominated by high pressure and several long, dry periods - is exactly the opposite of a typical climate change winter.

Benjamin Zweifel, of the Swiss Federal Institute for Snow and Avalanche Research, told the BBC the storms were "only one piece of the puzzle".

"The main challenge this winter was a very weak snowpack [an accumulation of snow that compresses and melts seasonally] with very persistent weak layers, caused by long periods without new snow and low temperatures," he said.

While global warming affects weather patterns, the temperature itself could also have a direct impact on avalanches. According to the US Northern Rocky Mountain Science Center, wet snow avalanches - which are likely to become more frequent as temperatures rise - are dangerous, difficult to predict and relatively poorly understood compared with the dry snow equivalent.

"Wet snow avalanches are caused by weakening in the strength of the snowpack, often triggered by rain, abundant sunshine, or warm temperatures," the centre says in an article on wet snow avalanche research, adding their frequency was expected to increase with changing temperatures.

A 2021 research paper in the journal Frontiers in Physiology suggests wetter and warmer snow may also adversely affect a person's chances of surviving an avalanche, while thinner snow cover may increase the risk of blunt trauma injuries.

Broadly, there would seem to be four destabilising issues that are changing the dynamics of avalanches - switching between droughts and heavy precipitation, the precipitation itself, wetter snow and snow which is actually melting.

How are resorts mitigating the risk?

Blaise Agresti, a high-mountain guide at Chamonix, told France's Sud radio that there was another important factor in Europe's spike in avalanche deaths - the changing behaviour of skiers and higher levels of risk.

"People are increasingly practising off-piste skiing. Among skiers, 25% go off-piste," he said.

Way-marked pistes are generally protected by pre-emptive avalanche blasting, and their snow is flattened and compacted to further reduce the risk. If there is still a danger of avalanches, pistes can be closed off, as has happened during the recent warnings.

But while forecasting and blasting techniques have improved in recent years, it is not impossible for avalanches to hit pistes that have been declared safe. There is no such thing as zero risk, and it may be advisable to keep away from pistes beneath mountainsides covered with powdery-looking snow.

Sometimes resorts can detect a wider threat. The French resorts of Tignes and Val d'Isere even imposed a lockdown because of the avalanche threat, forcing residents and tourists to stay indoors.

In general, though, resorts will not prevent tourists from going off-piste so instead they offer advice. Another French resort, La Plagne, for instance, tells skiers to get information about conditions, make sure they have the necessary safety gear - avalanche transceiver, shovel and probe - know how to use them, and always go out with a professional guide and never alone.

The resort, which has one of the world's largest off-piste areas, holds weekly safety days in high season and has recently invested in a drone equipped with a thermal camera for use in avalanche detection and rescue operations.

Thomas Hager, a ski guide based in the Zell am See region in the Austrian Alps, told the BBC World Service's Outside Source programme that sometimes it was easy for snow sports enthusiasts to be complacent.

"Snow looks so calm. It's comparable to water," he said. "In an ocean you don't see undercurrents but local people know where the undercurrents are and where avalanches go off frequently. So please always talk with local people and listen to the avalanche forecast."

He also suggested it was vital to carry an air-bag, which is not required by law but could help keep avalanche victims close to the surface of the snow.

Though the recent spike in avalanche deaths in Europe is worrying, it is not exceptional - according to European Avalanche Warning Services, 95 have died so far this season, compared to 70 in the whole of 2024-24 and 87 in 2023-24. The 2020-21 season, when 131 died, and 2017-18, when the figure was 147, seem comparable.

Could it have been a lot worse?

But if you ask weather watchers about the situation, a different picture emerges. Meteo France's team in Bourg-Saint-Maurice near the Italian border, told the BBC the region went from below-average snowfall to "remarkable" above-average snowfall in under 10 days.

"Statistically speaking, we experience a situation like this once every eight years. This means a very significant avalanche risk," it said.

Meanwhile the Val d'Isere resort, in its Thursday weather bulletin, said a third wave of 30-50cm (12in-20in) of snow - after the two storms - would have to be taken very seriously and required "certain precautions".

"The situation has not been seen for a long time (perhaps since February 1999)," it added.

So have modern avalanche-warning systems and safety measures taken by resorts in recent years kept casualty figures down?

Benjamin Zweifel believes the situation could be a lot worse without these factors, and says he sees a "big effect", at least in Switzerland.

He notes, though, that France and Italy have both seen higher fatalities - 25 in each, compared to Switzerland's 13 - but says this could be linked to "even more challenging conditions" in the western and southern Alps.

The worst could be over, though. The weather is expected to improve from Friday, giving resorts the space to carry out more safety work and allowing the snowpack to settle.

However, sunny, south-facing slopes will lose some of their snow, potentially leading to greater risks for off-pisters.