Winter — December, January, February
The clearest skies of the year happen in cold, dry, high-pressure spells between Christmas and mid-February. On such a day, visibility can exceed sixty kilometres and the summit platform is open throughout. The trade-off is wind — the Atlantic lows that bring the warm, overcast days also bring sustained winds of 70 km/h or more, and the lift to the top is then stopped for safety. We tend to prefer late January over December: cleaner air, fewer visitors, early sunset at 17:00 for a short but reliable blue hour.
Spring — March, April, May
Spring is the most variable season. Three days of 40-km visibility can follow one day of 12-km haze and vice versa. Pollen from the Bois de Boulogne begins to lift in early April and can soften the colour of the view by a factor noticeable to the eye. The compensation is longer light hours — by late April the golden window stretches past 20:00 and the Champ-de-Mars is at its most photogenic.
Summer — June, July, August
Summer is the season with the longest daylight and, paradoxically, the worst visibility on average. Warm air holds more moisture, which scatters sunlight into haze. Mid-afternoon on an August day can produce visibility figures as low as 15 kilometres with no cloud in the sky. The best summer windows are the first hour after sunrise, when the air at summit level is coolest, and the hour after a convective shower, when rain washes dust out of the air column.
Autumn — September, October, November
Autumn is our favourite season for climbing. Cold-front passages between late September and mid-November leave the air clean behind them, often for twenty-four to forty-eight hours. Sunset moves from around 20:00 in early September to 17:00 in late October, and the geometry of the sunset is such that the golden light hits the north-east face of the tower end-on. This is why October produces the most of the warm, orange-lit tower photographs circulated online.
A rough month-by-month table
| Month | Median visibility | Open-platform days | Comment |
|---|---|---|---|
| Jan | 34 km | 26 | Clear and cold |
| Feb | 31 km | 23 | Variable |
| Mar | 28 km | 24 | Spring showers |
| Apr | 29 km | 26 | Pollen begins |
| May | 26 km | 29 | Milder, hazy |
| Jun | 23 km | 29 | Humid |
| Jul | 22 km | 29 | Heat haze |
| Aug | 22 km | 29 | Heat haze |
| Sep | 28 km | 27 | Transition |
| Oct | 32 km | 26 | Clear fronts |
| Nov | 30 km | 24 | Early sunsets |
| Dec | 33 km | 23 | Short days |
Figures are approximate and based on our own notes from 2021 through 2026, cross-checked against Montsouris station archives.