GTJ à vélo: which way and when to ride
The best month for GTJ à vélo is September: 69% of days were dry with highs of 10–30 °C. In September the wind barely favours either direction — on average 0.4 km/h along the route (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 | 5% | 31% | 140 | 4 °C | 0% | 93% | 8.9 † |
| February | 16% | 30% | 108 | 6 °C | 0% | 81% | 10.2 † |
| March | 38% | 33% | 126 | 10 °C | 0% | 53% | 11.8 |
| April | 52% | 39% | 116 | 14 °C | 0% | 23% | 13.5 |
| May | 51% | 48% | 170 | 17 °C | 0% | 7% | 15.0 |
| June | 52% | 44% | 158 | 22 °C | 4% | 0% | 15.8 |
| July | 56% | 37% | 142 | 24 °C | 8% | 0% | 15.5 |
| August | 59% | 34% | 142 | 23 °C | 8% | 0% | 14.2 |
| September | 69% | 31% | 128 | 19 °C | 0% | 2% | 12.5 |
| October | 63% | 28% | 124 | 14 °C | 0% | 16% | 10.8 † |
| November | 27% | 33% | 141 | 8 °C | 0% | 65% | 9.3 † |
| December | 5% | 35% | 164 | 5 °C | 0% | 92% | 8.6 † |
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 Montbéliard → Culoz, km/h | Headwind > 15 km/h, Montbéliard → Culoz | Headwind > 15 km/h, Culoz → Montbéliard | Better way |
|---|---|---|---|---|
| January | −1.8 | 8% | 2% | either |
| February | −1.3 | 7% | 3% | either |
| March | −0.3 | 7% | 4% | either |
| April | +1.1 | 3% | 4% | either |
| May | +0.7 | 3% | 4% | either |
| June | +1.0 | 1% | 3% | either |
| July | +0.6 | 2% | 1% | either |
| August | +0.1 | 2% | 1% | either |
| September | +0.4 | 3% | 2% | either |
| October | −1.0 | 4% | 2% | either |
| November | −1.9 | 6% | 2% | either |
| December | −2.7 | 7% | 1% | Culoz → Montbéliard |
Along the route (sections of about 25 km)
| km | Heading | Tailwind in September, km/h |
|---|---|---|
| 0–25 | S | +0.6 |
| 25–50 | SE | +1.1 |
| 50–75 | S | +0.8 |
| 75–101 | SW | +0.2 |
| 101–127 | SW | −0.3 |
| 127–152 | SW | 0.0 |
| 152–177 | SW | −0.1 |
| 177–202 | SW | −0.5 |
| 202–227 | S | +0.3 |
| 227–253 | S | +0.5 |
| 253–278 | S | +0.3 |
| 278–304 | SW | +0.5 |
| 304–329 | SW | −0.6 |
| 329–354 | S | +1.2 |
| 354–372 | SE | +2.5 |
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.