In the mountains, summer 2025 had started under scorching auspices. The Alpine massifs experienced, from the end of June, temperatures far superior to normal, promoting an accelerated melting of residual snow and glaciers. But since July 10, a clear change of diet has taken place, with a switch to an unstable, fresh and disturbed time.
An exceptionally hot early summer, then a brutal reversal
The first weeks of meteorological summer were marked by particularly hot conditions at altitude. Several high mountain stations (such as the Agnel pass or the Midi needle) recorded values worthy of August, with temperatures exceeding 20 ° C at more than 2000 m altitude. This prolonged heat favored a rapid melting of the snowpack still present up to 3000 m, accelerating glacial ablation on all Alps. This context was brutally interrupted around July 10, with the arrival of a mass of polar air leading to a spectacular drop in temperatures. Several cold monthly records had been approached or broke, as in Bonneval-sur-Arc or in the High Tarentaise.
Back of instability and snow from 2700 m: a disturbed and dangerous time
Anomaly of air mass temperatures in the center-east of France © The weather channel
For the past week, an active train regime associated with a cold drop has maintained very unstable conditions on the Alps. Frequent, sometimes stormy showers occur every day, especially at the end of the afternoon. The rain-snow limit has lowered up to 2,700 m, or even 2600 m locally, with accumulations of snow several centimeters above 2,800 m, especially on the writings and the Mont-Blanc massif. We draw your attention to the risks linked to this instability: sudden thunderstorms, landslides, fresh snow flows or landslides are all dangers for hikers. The conditions at altitude are made difficult, even dangerous, including on classic mountaineering routes.
The 0 ° C isotherm is currently around 3000 m above sea level, which is not exceptional in the heart of summer. But this is accompanied by a strong nebulosity (fog at altitude) and precipitation.
A 0 ° C isothermal located around 3000 m, allowing snowfall from 2700 m © The weather channel
In this second half of July, the mountain offers a radically different face from that observed in early summer. Cold, humidity, fog and snow at high altitude recall how quickly the weather can evolve quickly. Vacationers must redouble their prudence: consult the mountain bulletins each morning, adapt their routes and never underestimate the risks linked to the storm or the sooty terrain. The mountain remains a demanding medium, even (and above all) in the heart of summer.