Du Léman au Mont Blanc: which way and when to ride
The best month for Du Léman au Mont Blanc is September: 64% of days were dry with highs of 10–30 °C. In September, riding Geneva → Chamonix-Mont-Blanc meant an average tailwind of 2.1 km/h (ERA5 2006–2025, daytime 9:00–18:00).
Weather by month
| Month | Good days | Rain days ≥ 1 mm | Rain, mm | Avg high | Days ≥ 30 °C | Days < 10 °C | Daylight, h |
|---|---|---|---|---|---|---|---|
| January | 2% | 31% | 150 | 1 °C | 0% | 97% | 9.0 † |
| February | 9% | 28% | 109 | 3 °C | 0% | 90% | 10.3 † |
| March | 23% | 34% | 136 | 6 °C | 0% | 74% | 11.8 |
| April | 41% | 39% | 117 | 11 °C | 0% | 44% | 13.5 |
| May | 42% | 55% | 190 | 14 °C | 0% | 22% | 14.9 |
| June | 48% | 50% | 150 | 19 °C | 1% | 4% | 15.7 |
| July | 54% | 44% | 151 | 21 °C | 2% | 1% | 15.4 |
| August | 61% | 37% | 135 | 20 °C | 2% | 2% | 14.1 |
| September | 64% | 34% | 116 | 16 °C | 0% | 9% | 12.5 |
| October | 59% | 26% | 114 | 12 °C | 0% | 28% | 10.8 † |
| November | 19% | 32% | 142 | 6 °C | 0% | 78% | 9.4 † |
| December | 2% | 32% | 177 | 2 °C | 0% | 98% | 8.7 † |
Best months are all months within 3 percentage points of the best one. † less than 11 hours of daylight — long stages get tight.
Wind by month
| Month | Wind Geneva → Chamonix-Mont-Blanc, km/h | Headwind > 15 km/h, Geneva → Chamonix-Mont-Blanc | Headwind > 15 km/h, Chamonix-Mont-Blanc → Geneva | Better way |
|---|---|---|---|---|
| January | −1.7 | 0% | 0% | either |
| February | −0.4 | 0% | 0% | either |
| March | +0.9 | 0% | 0% | either |
| April | +1.9 | 0% | 0% | either |
| May | +2.5 | 0% | 0% | Geneva → Chamonix-Mont-Blanc |
| June | +2.8 | 0% | 0% | Geneva → Chamonix-Mont-Blanc |
| July | +3.2 | 0% | 0% | Geneva → Chamonix-Mont-Blanc |
| August | +3.0 | 0% | 0% | Geneva → Chamonix-Mont-Blanc |
| September | +2.1 | 0% | 0% | Geneva → Chamonix-Mont-Blanc |
| October | +0.5 | 0% | 0% | either |
| November | −0.9 | 0% | 0% | either |
| December | −2.2 | 0% | 0% | Chamonix-Mont-Blanc → Geneva |
Along the route (sections of about 25 km)
| km | Heading | Tailwind in September, km/h |
|---|---|---|
| 0–25 | SE | +1.5 |
| 25–44 | E | +2.5 |
| 44–70 | SE | +2.5 |
| 70–95 | SE | +2.6 |
| 95–112 | NE | +1.4 |
How to read the numbers
Wind is the 10 m wind of a 25-km grid cell, daytime hours 9:00–18:00 local solar time; at saddle height in a sheltered valley it is weaker, on a coast or ridge stronger. A positive value is a tailwind. ERA5 slightly over-counts days with ≥ 1 mm of rain (by about 5–7 percentage points) and underestimates daily highs by 1–2 °C. Good day = less than 1 mm of rain between 9:00 and 18:00 (riding hours, local solar time) and a high between 10 and 30 °C; rain at night or in the evening does not spoil it.