You're all set for the UTMB race and the only remaining uncertainty is the weather in Chamonix in August. Even if it's difficult to make precise forecasts, it is nevertheless possible to identify established trends based on the historical weather data for the region from previous years and the averages we have been able to establish.
UTMB dates
In 2025, the UTMB will take place from Monday 25 to Sunday 31 August. Overall, year after year, the UTMB is always held during the last week of August. This is therefore the week that needs to be analysed from a statistical weather perspective.
Average temperatures
As for daytime highs, Chamonix sees an average maximum of 26°C during the last week of August. This suggests pleasant sunny days without the thermometer being excessively high. The average daytime temperature sits at around 21°C, which corresponds to agreeable, mild conditions for running. As for the lows, the average overnight minimum is 10°C, with some years seeing figures closer to 6°C. Of course, these values apply to the Chamonix valley, and significantly lower temperatures at altitude should be expected. You should subtract 6.5°C for every 1,000 metres of altitude gained — and sometimes the drop can be even greater!
Sky conditions
Overall on average, this period around late August in Chamonix tends to be sunny with some cloudy spells but without too much rainfall. On average, during the last week of August, it only rains one day in Chamonix and not continuously. We can therefore consider light showers as likely, without there being too great a chance of a completely overcast sky. Nevertheless, all this data should be taken with a pinch of salt, as historically speaking, all kinds of weather events can occur during this period. A very warm episode is just as possible as a rainy or cool one, though severe cold is unlikely. To find out more:
