Siena weather in September
September in Siena: warm, rainy, with clean air — one of the best months to visit.
#2 of 12 months in Siena · #149 of 897 destinations to visit in September · comfort 91/100 (tier S)
Updated June 2026 · climate normals from ERA5 reanalysis
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Siena climate normals — September
| Daytime high | 26°C / 78°F | 🌤 warm |
|---|---|---|
| Nighttime low | 15°C / 59°F | |
| Rainfall | 88 mm / 3.5 in | 🌧 rainy |
| Rainy days | 8 days | |
| Humidity | 70% | 💧 humid |
| Dew point | 14°C / 56°F | |
| Sunshine | 10.4 h/day | |
| Daylight | 12.5 h | |
| Wind | 13 km/h / 8 mph | |
| Air quality (US AQI) | 41 | 🍃 good |
Monthly averages from ERA5 reanalysis. AQI is the mean US Air Quality Index for the month.
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Siena in September — FAQ
- Is September a good time to visit Siena?
- September in Siena: warm, rainy, with clean air — one of the best months to visit. It ranks #2 of 12 months for Siena on SeasonMap's comfort score (91/100).
- What is the weather like in Siena in September?
- In September, Siena averages a daily high of 25.5°C and a low of 14.9°C, with about 88.2 mm of rain over roughly 8.4 rainy days.
- What is the air quality in Siena in September?
- Siena has a mean US Air Quality Index of about 41 in September.