Hot summer of 2026: what it means for the winter

During the summer, snow was in short supply in many places, even at high elevations. At the Weissfluhjoch test site used by the WSL Institute for Snow and Avalanche Research SLF, located 2,536 metres above sea level above Davos in the canton of Graubünden, it was entirely absent throughout July, August and September. “That was the first time in the measuring site’s 90-year history,” explains Christoph Marty, a snow climatologist at the SLF. Even during other notably hot summers – in 1947, 2003, 2015, 2018 and 2022 – snow was present for at least brief periods during these months. But the record doesn’t come as a complete surprise, says Marty: “The long-term trend towards fewer days of snow in summer is clear.” In the first 45 years of the measurement series, the median was 26 days; in the past 45 years, it has fallen to just 16 days.

However, that doesn’t necessarily mean that 2026/27 will be a low-snow season, as there is no clear link between a hot summer and the following winter, explains Marty: “There isn’t a consistent pattern.” He looked at the measurement series for Andermatt in the canton of Uri (1,440 metres above sea level) and the Weissfluhjoch test site. The winters of 2022/23 and 2015/16 were indeed characterised by low snowfall. In 2018/19 and 2003/04, however, the picture was mixed: in Andermatt there was as much snow as in other years, and at the Weissfluhjoch test site even more than usual. And after the hot summer of 1947, both locations saw large quantities of snow the following winter.

The picture is clearer when it comes to the onset of snow cover – the point at which a continuous winter snowpack is established. To assess this, Marty compared figures from the municipalities of Mürren (canton of Bern) and Davos, where, on average over the long term, continuous snow cover begins on 19 and 17 November respectively. “After hot summers, the onset of snow cover has always been later than usual,” he observed. In Mürren, the snow arrived between two and nineteen days later, and in Davos, between one and forty-seven days later. If it takes that long again, the municipality may not be covered by a continuous snowpack until 2027.

“Another key factor that’s often underestimated is the temperature of the water,” says Wolfsperger. If the streams and lakes supplying snow cannons and lances are too warm, the machines will be less effective at producing snow.

This isn’t just an issue for cross-country ski trails. “Some ski resorts will undoubtedly have insufficient water reserves, or reserves that are too warm,” explains the engineer. Elsewhere, air temperatures are simply still too high for snowmaking. Ideally, temperatures need to remain below −5 °C for an extended period for snowmaking to be effective, says Wolfsperger: “It takes 100 hours of cold weather to get the ski season under way – that’s enough to prepare the pistes.” What’s more, the cold doesn’t need to be continuous. Wolfsperger is optimistic about the chances, particularly for resorts at elevations of around 2,500 metres and above. The ground still being warm after the extremely hot summer will not pose a problem, he says: “The fact that slightly more machine-made snow will melt as a result won’t matter much.” At least, not on the ski slopes.

The timing of the onset of snow cover will also determine how much the hot summer ultimately affects permafrost (see box). Permafrost entered the warm season in spring 2026 in good condition, as the ground was relatively cool following the cold, low-snow winter of 2025/26. However, the summer heat was clearly measurable in the uppermost layers of the ground. Heat penetrates to depth only slowly, so it will take until 2027 to reach a depth of 10 metres. “Below a depth of 15 metres, seasonal temperature fluctuations aren’t measurable at all. We won’t see the effects of the heat from summer 2026 at that depth until winter 2027,” says SLF permafrost expert Jeannette Nötzli.

What happens next depends on how the winter gets under way. If continuous snow cover forms early in autumn, the ground will be unable to cool down. “The heat will then be trapped in the ground, allowing it to penetrate deeper and intensify the effects of the hot summer,” explains Nötzli. On the other hand, if snowfall comes late, giving the ground time to cool down, this may help to mitigate the impact of the hot summer. In permafrost below a depth of 10 to 15 metres, temperatures are influenced by long-term changes at the surface, while seasonal fluctuations are barely measurable. Here, it is not individual summers that matter, but the numerous consecutive warm years over the past decade. 2026 was simply the most extreme year so far. Together, all the hot summers have led to an appreciable warming of the permafrost. This can cause ice loss in the ground and allow water to penetrate deep into the permafrost. In turn, this reduces the stability of mountain slopes and can ultimately lead to rockfalls and large-scale slope failures.