Weather in Chamonix-Mont-Blanc in September

A good month. September works. It is not the peak, but nothing about it fights you. Highs average 18.8 °C, lows 8.6 °C, and rain falls on 13 days — nothing in the month is working against you.

September is the highest-scoring month of the twelve. The nearest rival is August, and the month at the other end of the year is January.

Rain on 13 days, 2 of them heavy. Enough to need a plan B, not enough to need a plan around it. Nothing specialised needed — this is the month you pack for the trip rather than for the weather.

What the averages hide

Averages flatten a month, and this one hides a few things: it never really gets hot: not one day in an average September reaches 30 °C. Across the twenty years measured, the hottest September day reached 28.8 °C and the coldest night fell to -1 °C, and the wettest single day dropped 35 mm — roughly 46% of the month's rain in one day.

September is statistically steady but not small: the year-to-year variation is modest in relative terms, yet the wettest September on record still delivered 128 mm against 40 mm in the driest. In a month this wet, even a tame spread is a lot of rain in absolute terms — the 76 mm average is a reasonable planning figure, not a promise.

Wind is light at 2.6 km/h, so the temperature on the page is close to the temperature on your skin, and humidity runs at 77%, high enough to notice on the warm days without dominating them.

How September sits in the year

The month is on the way down: 3.3 °C cooler than August, with October dropping another 3.9 °C. The first week and the last are not the same trip.

Daylight in September

12.6 hours of daylight, losing 1.6 hours across the month. That is between Chamonix-Mont-Blanc's extremes of 15.7 and 8.7 hours, so days are long enough not to constrain the plan.

An inland climate

With no coast nearby, the year swings a fair way — 28.9 °C between the warmest afternoons and the coldest nights, wider than any coastal city at this latitude would manage. At 1060 m the air is thin enough to matter — sun that burns faster than the temperature suggests, and nights that cool off quickly.

How busy it gets

Peak season. Expect the highest prices of the year and to be booking well ahead.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

September in Chamonix-Mont-Blanc versus nearby

If Chamonix-Mont-Blanc is one option among several, the nearby comparison is worth making. Annecy is close enough to be a coin toss — 3.3 °C warmer, 2 days less rain. Zermatt is the weaker option in September — 3.7 °C cooler, the same number of rain days.

So: September is the answer for Chamonix-Mont-Blanc. If it is booked out or priced out, August is the next best thing at 1 points behind, and January is the month to keep off the shortlist.

Every month in Chamonix-Mont-Blanc

See all twelve months in Chamonix-Mont-Blanc, or where else to go in September.