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Lisbon has a subtropical Atlantic climate — warmer and sunnier than northern oceanic cities but tempered by consistent Atlantic airflows that moderate heat extremes. Summers are warm (25–30°C) and drier, winters mild (10–15°C) and wet. The combination of sunshine and Atlantic breeze makes for genuinely good ventilation conditions across most of the year. Spring and autumn — April through June and September through October — are the best seasons: outdoor temperatures of 16–22°C, moderate humidity, and clean air from Atlantic wind systems.
Summer management in Lisbon: on the hottest inland days, the morning ventilation window (6–9 AM) is effective, while late afternoons may be too warm for passive cooling unless a sea breeze is present. The Atlantic acts as a natural temperature regulator — coastal positions benefit from consistent onshore breezes that urban inland areas don't receive. Air quality in Atlantic-facing cities is among the cleanest in Europe, with persistent airflows flushing pollutants. Occasional peaks occur during extended calm anticyclonic periods in winter, when traffic emissions accumulate without dispersal. OpenWindow.live identifies these infrequent but genuine air quality events.
In August, the warmest month in Lisbon, afternoons reach 29 °C on average and nights fall to 19 °C. The coolest air arrives between 05:00 and 08:00 (19 °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 16:00, the mildest hour (15 °C on average). Average humidity is 80%, so short, wide-open airing keeps condensation and mould away without losing much heat.
Fine particles (PM2.5) are usually lowest around 16:00 (9 µg/m³) and highest around 23:00 (13 µg/m³).
| Month | Avg high | Avg low | Humidity | Coolest hours | PM2.5 |
|---|---|---|---|---|---|
| January | 15 °C | 9 °C | 80% | 07:00–10:00 | 13 |
| February | 17 °C | 10 °C | 79% | 07:00–10:00 | 15 |
| March | 18 °C | 11 °C | 77% | 06:00–09:00 | 11 |
| April | 20 °C | 12 °C | 74% | 05:00–08:00 | 10 |
| May | 23 °C | 14 °C | 67% | 05:00–08:00 | 9 |
| June | 26 °C | 17 °C | 70% | 04:00–07:00 | 10 |
| July | 29 °C | 18 °C | 65% | 05:00–08:00 | 9 |
| August | 29 °C | 19 °C | 67% | 05:00–08:00 | 10 |
| September | 26 °C | 17 °C | 71% | 05:00–08:00 | 9 |
| October | 24 °C | 16 °C | 75% | 06:00–09:00 | 11 |
| November | 19 °C | 12 °C | 80% | 06:00–09:00 | 12 |
| December | 16 °C | 11 °C | 83% | 07:00–10:00 | 12 |
Data: Open-Meteo historical archive, hourly averages 2021–2025 (air quality 2023–2025), local time.
How to air your home in winter in Lisbon without condensation and mould →
In Lisbon, 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.