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San Francisco has an oceanic climate — mild temperatures year-round with persistent maritime moisture and more rain than continental cities. The advantage is stability: outdoor temperatures rarely create the extremes that make ventilation dangerous. The main challenge is humidity: on many days, outdoor relative humidity reaches 75–90%, and opening windows in warm, humid weather can increase indoor moisture rather than improving comfort. The key judgment is distinguishing temperature from comfort — 18°C at 85% humidity feels refreshing briefly but raises indoor dew point, risking condensation over time.
Summer evenings and nights are typically the best ventilation window in San Francisco: temperatures drop to 13–18°C, Atlantic or coastal breezes lower humidity, and outdoor air is consistently cleaner than during daytime traffic hours. Winter ventilation requires burst opening rather than continuous exposure: the combination of cold (1–7°C) and high humidity means extended open windows bring damp cold air that costs significant energy to reheat. OpenWindow.live accounts for both temperature and humidity in its decision, flagging high-humidity conditions where ventilation should be limited or done briefly.
In September, the warmest month in San Francisco, afternoons reach 20 °C (69 °F) on average and nights fall to 14 °C (58 °F). The coolest air arrives between 04:00 and 07:00 (15 °C (59 °F) on average): that is when opening the windows cools the home fastest.
In December, the coldest month, air the rooms for 5–10 minutes around 15:00, the mildest hour (13 °C (55 °F) on average). Average humidity is 85%, so short, wide-open airing keeps condensation and mould away without losing much heat.
Fine particles (PM2.5) are usually lowest around 07:00 (10 µg/m³) and highest around 20:00 (12 µg/m³).
| Month | Avg high | Avg low | Humidity | Coolest hours | PM2.5 |
|---|---|---|---|---|---|
| January | 14 °C | 8 °C | 79% | 06:00–09:00 | 13 |
| February | 14 °C | 8 °C | 79% | 06:00–09:00 | 14 |
| March | 14 °C | 9 °C | 78% | 05:00–08:00 | 11 |
| April | 16 °C | 10 °C | 77% | 04:00–07:00 | 11 |
| May | 18 °C | 11 °C | 75% | 04:00–07:00 | 10 |
| June | 18 °C | 12 °C | 77% | 04:00–07:00 | 10 |
| July | 18 °C | 13 °C | 81% | 04:00–07:00 | 9 |
| August | 20 °C | 14 °C | 81% | 04:00–07:00 | 9 |
| September | 20 °C | 14 °C | 80% | 04:00–07:00 | 10 |
| October | 20 °C | 13 °C | 76% | 06:00–09:00 | 11 |
| November | 16 °C | 10 °C | 79% | 06:00–09:00 | 12 |
| December | 13 °C | 8 °C | 85% | 06:00–09:00 | 15 |
Data: Open-Meteo historical archive, hourly averages 2021–2025 (air quality 2023–2025), local time.
How to air your home in winter in San Francisco without condensation and mould →
In San Francisco, the best ventilation times are typically early morning (5–8 AM) and late evening (after sunset) when outdoor temperatures drop below indoor levels.
Yes, in most weather conditions. Open windows can drop indoor temperature by 2–5°C in 30 minutes if outdoor air is cooler. Check the live recommendation above for tonight.
Above 80% humidity, opening windows can make indoor air feel sticky. Combine with dehumidifier or wait for drier hours.