Heatwaves, city by city
A heatwave in the sense of Perkins and Alexander (2013): at least three days in a row above the 90th percentile of the calendar day over 1991–2020. Marker colour compares the heatwave days of the season in progress with the normal at the same date.
- below normal (≤ −5 d)
- near normal (± 5 d)
- above (+5 to +15 d)
- far above (> +15 d)
ERA5 reanalysis (Open-Meteo), one grid cell per city; size: heatwave days of the season in progress.
Tokyo Japan · 35.69°, 139.69°
Show the data as a table
Show the data as a table
| Decade | Heat | Cold |
|---|---|---|
| 1950s | 3.2 | 39.2 |
| 1960s | 5.3 | 32.8 |
| 1970s | 3.2 | 29.6 |
| 1980s | 4.2 | 39.0 |
| 1990s | 8.8 | 16.6 |
| 2000s | 6.0 | 9.9 |
| 2010s | 14.5 | 9.8 |
| 2020s (6) | 42.0 | 6.8 |
| Period | Days | Max. departure (°C) |
|---|---|---|
| 2025-08-17 → 08-27 | 11 | 3.7 |
| 2022-06-24 → 07-03 | 10 | 6.2 |
| 2010-08-29 → 09-07 | 10 | 2.6 |
| 2025-07-21 → 07-30 | 10 | 2.6 |
| 2024-09-06 → 09-15 | 10 | 2.4 |
| 2024-07-22 → 07-31 | 10 | 2.1 |
| 2025-06-15 → 06-23 | 9 | 5.2 |
| 2023-07-24 → 07-31 | 8 | 4.3 |
| 2023-09-13 → 09-20 | 8 | 2.2 |
| 1979-06-20 → 06-26 | 7 | 4.7 |
| Period | Days | Max. departure (°C) |
|---|---|---|
| 1950-12-21 → 01-08 | 19 | 2.4 |
| 1954-01-24 → 02-09 | 17 | 8.0 |
| 1980-07-31 → 08-12 | 13 | 4.4 |
| 1981-01-03 → 01-14 | 12 | 5.7 |
| 1976-12-27 → 01-07 | 12 | 2.8 |
| 1984-02-01 → 02-11 | 11 | 9.7 |
| 1976-06-28 → 07-08 | 11 | 6.4 |
| 1988-07-22 → 08-01 | 11 | 4.4 |
| 1956-12-16 → 12-26 | 11 | 2.9 |
| 1967-02-11 → 02-20 | 10 | 11.9 |
| City | Heatwave days | Yearly normal | Trend (d/decade) | Latest heatwave | In progress |
|---|---|---|---|---|---|
| Rome Italy | 94 (2026) | 18.2 | +4.9 ± 1.1 | 2026-10-01 (4 d) | — |
| Cairo Egypt | 11 (2026) | 16.6 | +0.6 ± 0.7 | 2026-02-06 (5 d) | — |
| New Delhi India | 16 (2026) | 20.4 | -2.0 ± 1.0 | 2026-05-19 (3 d) | — |
| Lagos Nigeria | 49 (2026) | 9.6 | +5.3 ± 1.8 | 2026-10-14 (4 d) | — |
| Paris France | 81 (2026) | 15.8 | +3.2 ± 0.6 | 2026-10-03 (4 d) | — |
| Tokyo Japan | 27 (2026) | 9.9 | +3.3 ± 1.0 | 2026-07-29 (5 d) | — |
| Berlin Germany | 32 (2026) | 16.0 | +2.7 ± 0.5 | 2026-10-04 (4 d) | — |
| London United Kingdom | 66 (2026) | 16.6 | +3.4 ± 0.5 | 2026-10-13 (3 d) | — |
| Madrid Spain | 56 (2026) | 20.3 | +4.5 ± 0.6 | 2026-10-13 (4 d) | — |
| Mexico Mexico | 23 (2026) | 18.7 | +5.7 ± 1.1 | 2026-07-19 (3 d) | — |
Daily maximum and minimum temperatures (UTC days) of the ERA5 grid cell containing each city (Hersbach et al., 2020), read through the Open-Meteo historical API, since 1950. Thresholds: 90th percentile of Tmax and 10th percentile of Tmin of the calendar day, computed over 1991–2020 with a 15-day window centred on the day, 29 February sharing the window of 28 February. A heatwave is at least 3 consecutive days above the threshold (CTX90pct of Perkins and Alexander, 2013); a cold spell, at least 3 days below the Tmin threshold (the symmetric definition). Yearly indices: number of events, number of days (HWF), longest event, largest departure from the threshold. In the southern hemisphere the season runs from July to June.
Comparison with the nearest GHCN-Daily station with a continuous record (Menne et al., 2012), values flagged by the quality control left out.
The thresholds are relative to each city’s climate and to 1991–2020: a “heatwave” in London does not have Delhi’s temperature. With a recent base period, early decades naturally count fewer days; the rise of the last decades remains clear in almost every city.
A reanalysis grid cell is not a thermometer: the urban heat island and local effects (relief, coast) are smoothed, and ERA5 is less constrained by observations before the satellite era (before 1979), especially in Africa and South America. This page describes episodes, not their health effects, which also depend on humidity and nights.
- Daily ERA5 reanalysis at 21 reference places (Open-Meteo historical weather API)
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Zippenfenig, P. (2024). Open-Meteo.com Weather API (1.4.0). Zenodo. Hersbach, H., et al. (2020). The ERA5 global reanalysis. Q. J. R. Meteorol. Soc., 146, 1999–2049. Contains modified Copernicus Climate Change Service information.
· doi:10.5281/zenodo.7970649 · source page ↗
Licence : CC BY 4.0 (Open-Meteo data); ERA5: Copernicus licence, credit C3S/ECMWF; free API for non-commercial use within the fair-use limits · retrieved on Oct 25, 2026 · SHA-256 4bcb4b0ae4fd… - GHCN-Daily stations of the reference places (NOAA NCEI)
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Menne, M. J., Durre, I., Vose, R. S., Gleason, B. E. and Houston, T. G. (2012). An overview of the Global Historical Climatology Network-Daily database. J. Atmos. Oceanic Technol., 29, 897–910. Menne, M. J., et al. (2012). Global Historical Climatology Network - Daily (GHCN-Daily), Version 3. NOAA NCEI.
· doi:10.7289/V5D21VHZ · source page ↗
Licence : U.S. Government work, public domain (data from national services under WMO Resolution 40); cite Menne et al. (2012) · retrieved on Oct 25, 2026 · SHA-256 fb525098ce40… - Perkins et al. (2013)
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Perkins, S. E., & Alexander, L. V. (2013). On the measurement of heat waves. Journal of Climate, 26(13), 4500–4517.
· doi:10.1175/JCLI-D-12-00383.1
Licence : Published literature; cited - Hersbach et al. (2020)
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Hersbach, H., Bell, B., Berrisford, P., Hirahara, S., Horányi, A., Muñoz-Sabater, J., et al. (2020). The ERA5 global reanalysis. Quarterly Journal of the Royal Meteorological Society, 146(730), 1999–2049.
· doi:10.1002/qj.3803
Licence : Published literature; cited - Menne et al. (2012)
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Menne, M. J., Durre, I., Vose, R. S., Gleason, B. E., & Houston, T. G. (2012). An overview of the Global Historical Climatology Network-Daily database. Journal of Atmospheric and Oceanic Technology, 29(7), 897–910.
· doi:10.1175/JTECH-D-11-00103.1
Licence : Published literature; cited - Zippenfenig (2024)
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Zippenfenig, P. (2024). Open-Meteo.com Weather API (version 1.4.0). Zenodo.
· doi:10.5281/zenodo.7970649
Licence : Published literature; cited