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Tokyo's ventilation challenges are defined by its humid subtropical climate and extreme seasonal variation. The rainy season (tsuyu) in June–July brings weeks of 85–95% humidity — a period when opening windows brings in air that is warmer, wetter, and less comfortable than staying sealed. This is followed by the hot, humid summer peak in August (33°C, 75% humidity).
Outside the rainy season, Tokyo has excellent ventilation windows. Spring (March–May before humidity builds) and autumn (October–November) offer mild temperatures, low humidity, and clear skies. The Japanese concept of kazadōshi (wind-through ventilation) — creating cross-ventilation by opening windows on opposite sides of a room — maximises the benefit of these comfortable windows.
Air quality in Tokyo has improved dramatically since the 1960s, with typical AQI readings of 20–40. However, spring brings significant cross-border PM2.5 from China, and high-pollution days occasionally warrant keeping windows closed even in otherwise comfortable weather conditions.
In August, the warmest month in Tokyo, afternoons reach 33 °C on average and nights fall to 25 °C. The coolest air arrives between 04:00 and 07:00 (25 °C on average): that is when opening the windows cools the home fastest.
On the hottest nights the outdoor air may stay warmer than indoors — open only if it is cooler outside, and close again by 07:00.
In January, the coldest month, air the rooms for 5–10 minutes around 14:00, the mildest hour (9 °C on average). Average humidity is 62%, so short, wide-open airing keeps condensation and mould away without losing much heat.
Fine particles (PM2.5) are usually lowest around 15:00 (18 µg/m³) and highest around 21:00 (36 µg/m³).
| Month | Avg high | Avg low | Humidity | Coolest hours | PM2.5 |
|---|---|---|---|---|---|
| January | 9 °C | 0 °C | 62% | 05:00–08:00 | 28 |
| February | 11 °C | 1 °C | 59% | 05:00–08:00 | 23 |
| March | 15 °C | 6 °C | 68% | 04:00–07:00 | 26 |
| April | 20 °C | 10 °C | 71% | 04:00–07:00 | 27 |
| May | 23 °C | 15 °C | 75% | 03:00–06:00 | 23 |
| June | 27 °C | 19 °C | 79% | 03:00–06:00 | 27 |
| July | 32 °C | 24 °C | 76% | 03:00–06:00 | 28 |
| August | 33 °C | 25 °C | 76% | 04:00–07:00 | 20 |
| September | 29 °C | 22 °C | 79% | 04:00–07:00 | 21 |
| October | 22 °C | 14 °C | 77% | 04:00–07:00 | 25 |
| November | 17 °C | 9 °C | 73% | 05:00–08:00 | 33 |
| December | 12 °C | 3 °C | 66% | 05:00–08:00 | 36 |
Data: Open-Meteo historical archive, hourly averages 2021–2025 (air quality 2023–2025), local time.
How to air your home in winter in Tokyo without condensation and mould →
In Tokyo, 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.