Heatwave Days
The Warm Spell Duration Index (WSDI) - the number of heatwave days each year - has risen sharply worldwide since the 1970s, broadly in step with the long-term warming trend, though not at the same pace everywhere. This page tracks the WSDI since 1950 for the world, every continent, 160+ countries, US states and UK regions. An overlay shows where and how strongly ENSO (El Niño / La Niña) shifts the odds of heatwave days, independent of the warming trend.
Heatwave Days – Global
Warm Spell Duration Index (WSDI) - days per year in a 6+ day run above the local 90th-percentile temperature.
Heatwave Days (WSDI)
Source: World Bank Climate Change Knowledge Portal (ERA5-derived) (heatwave days), refreshed roughly 1-2x/year, ~7-8 months behind present. Land-area-weighted mean across 169 countries. · NOAA RONI (Relative Niño 3.4 index, ERSSTv5) - the same ENSO series shown on the ENSO Tracker.
How Heatwave Days Track Local Warming
How closely each place's heatwave-day trend lines up with its own local warming trend (r²). Pick a country, US state, or UK region above to see its figure here.
Most
Strongest linkLeast
Weakest linkBy Continent
Mean of each continent's placesThis is a statistical correlation (r²) between heatwave days and local warming, not formal detection-and-attribution science.
Map
Heatwave Days Map
Source: World Bank Climate Change Knowledge Portal (ERA5-derived), Warm Spell Duration Index (WSDI).
ENSO & Other Factors
Beyond the Warming Trend
Worldwide, 2025 averaged 17.8 heatwave days - the record is 27.5 days in 2024 (land-area-weighted across 169 countries, since 1950).
Global heatwave days track the long-term warming trend closely, and heatwave duration is increasing faster than the temperature trend itself - the longest, rarest heatwaves are lengthening fastest (Nature Geoscience). At the country level the picture is far less uniform. ENSO (El Niño / La Niña) is one of the clearest secondary influences, shifting the odds of a heatwave in a given region independent of the warming trend - toggle the ENSO overlay on the chart above to see how the two line up. Other regional factors (jet-stream blocking, soil moisture, aerosols, nearby marine heatwaves) matter too, though separating them out needs more than a statistical correlation.
A note on rigour: figures on this page are statistical correlations, not formal detection-and-attribution science (which needs climate-model counterfactual ensembles - see World Weather Attribution for that). Treat “how closely a country’s heatwave trend has tracked X” as description, not a formal attribution claim.
Explore
Explore Climate Data
Data Sources
The Data
Heatwave days per year (Warm Spell Duration Index) from 1950 to 2025, land-area-weighted across 169 countries.
View the Data - Global Heatwave Days (WSDI), Annual76 rows
| Year | Heatwave days (WSDI) |
|---|---|
| 2025 | 17.8 |
| 2024 | 27.5 |
| 2023 | 22.5 |
| 2022 | 13.6 |
| 2021 | 15.7 |
| 2020 | 16.1 |
| 2019 | 15.3 |
| 2018 | 11.6 |
| 2017 | 12.8 |
| 2016 | 17.3 |
| 2015 | 15.4 |
| 2014 | 7.3 |
| 2013 | 9.1 |
| 2012 | 6.9 |
| 2011 | 5.8 |
| 2010 | 13.1 |
| 2009 | 7.1 |
| 2008 | 5.3 |
| 2007 | 7.4 |
| 2006 | 5.5 |
| 2005 | 7.3 |
| 2004 | 4.2 |
| 2003 | 4.6 |
| 2002 | 5.9 |
| 2001 | 3.4 |
| 2000 | 2.7 |
| 1999 | 3.2 |
| 1998 | 7.8 |
| 1997 | 3.6 |
| 1996 | 2.3 |
| 1995 | 4 |
| 1994 | 5 |
| 1993 | 4 |
| 1992 | 4.2 |
| 1991 | 4.5 |
| 1990 | 6.9 |
| 1989 | 4.9 |
| 1988 | 5.1 |
| 1987 | 6.9 |
| 1986 | 3.5 |
| 1985 | 3.9 |
| 1984 | 3.5 |
| 1983 | 5.7 |
| 1982 | 2.9 |
| 1981 | 4.7 |
| 1980 | 4.8 |
| 1979 | 4.3 |
| 1978 | 2.9 |
| 1977 | 4 |
| 1976 | 2.3 |
| 1975 | 2.9 |
| 1974 | 2.4 |
| 1973 | 3.9 |
| 1972 | 3.7 |
| 1971 | 2.8 |
| 1970 | 3.4 |
| 1969 | 5.4 |
| 1968 | 3.6 |
| 1967 | 3.1 |
| 1966 | 6.3 |
| 1965 | 6 |
| 1964 | 5.6 |
| 1963 | 7.5 |
| 1962 | 4 |
| 1961 | 5.4 |
| 1960 | 4.4 |
| 1959 | 4.1 |
| 1958 | 6.8 |
| 1957 | 5.1 |
| 1956 | 3.1 |
| 1955 | 4.7 |
| 1954 | 5.6 |
| 1953 | 4.5 |
| 1952 | 4.8 |
| 1951 | 4.6 |
| 1950 | 4.2 |
Methodology & Sources
- Heatwave days - Warm Spell Duration Index (WSDI), the WMO/ETCCDI standard, from the World Bank Climate Change Knowledge Portal (CCKP), derived from ERA5 reanalysis. Annual-only by construction, refreshed roughly 1-2x/year as a whole-history replacement, currently running ~7-8 months behind present.
- Country/region aggregation - CCKP’s 0.25° ERA5 grid, area-weighted to country and region boundaries using the same weighting method as this site’s temperature and precipitation figures.
- ENSO overlay - the annual RONI (Relative Niño 3.4 index, ERSSTv5) value from this site’s own seasonal record, the same series shown on the ENSO Tracker, shown as its own stacked panel with a red (El Niño) / blue (La Niña) vertical line marking the peak of every moderate-or-stronger episode since 1950 - same idiom as the 4byo Climate Symphony page.
All datasets are updated as source agencies publish new readings.