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Seoul has a continental monsoon climate with four sharply distinct seasons and a rainy summer period that defines the annual rhythm. Winters are cold and dry (typically −5 to 5°C), springs warm rapidly, and summers bring both high heat — often reaching 32–38°C — and the summer rain season that raises humidity significantly. Autumn is typically the most comfortable season for ventilation: clear skies, lower humidity, and falling but still-comfortable temperatures after the summer heat.
The summer ventilation window in Seoul is compressed by the combination of heat and humidity. Early morning is the most viable time: from 5–8 AM, overnight cooling brings temperatures down to 22–27°C with lower humidity than the afternoon peak. The monsoon rain periods temporarily reset conditions — the hours following summer rainfall often offer the day's best air quality and most comfortable temperatures. Winter brings the opposite challenge: cold dry air is easy to ventilate briefly, but heating emissions — coal, gas, or wood — can spike outdoor PM2.5 on calm, cold days. OpenWindow.live's real-time AQI monitoring is particularly valuable in Seoul during winter inversions, when air quality can deteriorate rapidly without obvious visible cues.
In July, the warmest month in Seoul, afternoons reach 30 °C on average and nights fall to 23 °C. The coolest air arrives between 04:00 and 07:00 (23 °C on average): that is when opening the windows cools the home fastest.
In January, the coldest month, air the rooms for 5–10 minutes around 15:00, the mildest hour (2 °C on average). Average humidity is 60%, so short, wide-open airing keeps condensation and mould away without losing much heat.
Fine particles (PM2.5) are usually lowest around 15:00 (22 µg/m³) and highest around 21:00 (34 µg/m³).
| Month | Avg high | Avg low | Humidity | Coolest hours | PM2.5 |
|---|---|---|---|---|---|
| January | 2 °C | -7 °C | 60% | 06:00–09:00 | 33 |
| February | 5 °C | -5 °C | 63% | 05:00–08:00 | 29 |
| March | 12 °C | 1 °C | 67% | 05:00–08:00 | 34 |
| April | 18 °C | 7 °C | 66% | 04:00–07:00 | 32 |
| May | 22 °C | 12 °C | 70% | 04:00–07:00 | 31 |
| June | 27 °C | 18 °C | 74% | 04:00–07:00 | 29 |
| July | 30 °C | 23 °C | 82% | 04:00–07:00 | 25 |
| August | 29 °C | 23 °C | 82% | 04:00–07:00 | 29 |
| September | 26 °C | 18 °C | 80% | 04:00–07:00 | 23 |
| October | 19 °C | 10 °C | 77% | 05:00–08:00 | 24 |
| November | 12 °C | 2 °C | 72% | 05:00–08:00 | 27 |
| December | 3 °C | -5 °C | 64% | 06:00–09:00 | 27 |
Data: Open-Meteo historical archive, hourly averages 2021–2025 (air quality 2023–2025), local time.
How to air your home in winter in Seoul without condensation and mould →
In Seoul, 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.