You are all set for the UTMB® race, and the only remaining uncertainty is the weather in Chamonix in August. Accurate forecasts months in advance are impossible, but reliable trends can be drawn from the region's historical data, and temperature variations by altitude can be anticipated.
UTMB® 2026 dates
UTMB® 2026 takes place from Monday 24 to Sunday 30 August, with the flagship race starting on Friday 28 August at 17:45. As every year, it is the last week of August that matters most from a weather perspective.
Average temperatures
In the valley, Chamonix sees an average high of around 26°C during the last week of August, with a daily average of around 21°C. On the cooler end, overnight lows average 10°C, with some years dipping close to 6°C. These figures apply to the valley: you should expect significantly lower temperatures at altitude on the course, with a drop of around 6.5°C per 1,000 metres. To put this in perspective, at the 2025 edition start, forecasts showed around 12–19°C in the valley, 4–9°C at 2,000 m and just 1–6°C at 2,500 m — a clear illustration of the gap between the Chamonix start line and the high points of the course.
Sky conditions and rainfall risk
On average, the last week of August in Chamonix is fairly sunny with some cloudy intervals, and historically it rains on only one day during the week — rarely continuously. These statistics should still be treated with caution: in high mountain terrain, all scenarios are possible. The 2025 edition is a prime example, with near-constant rain over several days and a felt temperature close to 0°C on certain sections of the course. It is therefore better to pack your mandatory kit with the assumption of deteriorating conditions built in, even when the averages look favourable.
Where to check the weather in real time before the race?
Historical trends help you prepare your kit in advance, but they are no substitute for a short-range forecast. In the days before your start, check the official UTMB® weather forecasts, updated regularly by the organisation based on evolving conditions across the massif.
Frequently asked questions
What is the weather usually like during the UTMB®?
The last week of August in Chamonix is generally mild, with days around 21°C and little rain. However, conditions can vary significantly from one edition to the next, with occasional prolonged rainfall or notably cool spells.
What temperatures can you expect at altitude on the course?
Expect around 6.5°C less for every 1,000 metres of altitude gained compared to the valley. At the highest points on the course, temperatures can drop close to 0°C, particularly at night.
Should you expect rain or snow?
Rain is possible at any time, and snow cannot be ruled out on the highest sections when temperatures drop. That is why the mandatory kit list imposed by the organisation includes waterproof and insulating gear, regardless of the forecast at the start. Find out more:
