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
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
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.
FAQs
Heatwave Days - Common Questions
What counts as a "heatwave day" here?
This page uses the Warm Spell Duration Index (WSDI), the WMO/ETCCDI standard heatwave measure: a day counts if it falls within a run of 6 or more consecutive days where the daily maximum temperature exceeds that calendar day's own 90th-percentile threshold. The threshold is calculated per calendar day (a +/-2-day window across a fixed baseline period), so a hot June day is judged against other hot June days historically, not against a January baseline - it is a genuine warm-spell measure, not just "hot in absolute terms".
Why are heatwave days a useful indicator of climate change?
Heatwave days are a directly observable signal of a warming climate at a given location, rather than an abstract global average. The IPCC's Sixth Assessment Report found it virtually certain that global warming has increased the frequency and duration of heatwaves worldwide, and the Warm Spell Duration Index used on this page is one of the WMO/ETCCDI's standard indices for tracking exactly that. How much a given place's heatwave-day count has actually shifted varies a lot - see the ranked list further down the page.
Why does the country figure sometimes look lower than I'd expect?
WSDI is computed on ERA5's 0.25° (~28km) reanalysis grid, then area-weighted to country boundaries - not from individual weather stations. That spatial averaging dilutes localised extremes: a heatwave that scorches one region of a large country doesn't show up at full strength in the country-wide average. The same effect is visible in this site's Frost Days figures, which run roughly 40-50% below station-based numbers for the same reason, while the year-to-year rank order still matches closely.
Is 2026 included?
Not yet on this page. The underlying dataset (World Bank CCKP) publishes WSDI as an annual-only index, refreshed roughly once or twice a year as a whole-history replacement - the current record runs through 2025. A genuinely current 2026 figure is planned as a later addition to this page.
What's the link between heatwave days and global warming?
Globally, heatwave days have tracked the warming trend closely since around 2010, consistent with the wider research literature - heatwave duration is increasing nonlinearly with warming, with the longest, rarest heatwaves lengthening fastest. But the strength of that link varies a lot by country, shaped by factors like ENSO phase, jet-stream blocking, soil moisture, aerosols, and (for coastal areas) nearby marine heatwaves.
What role does El Niño / La Niña play?
ENSO shifts the jet stream and Walker Circulation, changing the odds of a heatwave in a given region independent of the long-term warming trend - El Niño years, for example, have been shown to amplify trans-Pacific heatwave synchronisation. The shaded bands on the chart mark El Niño (red) and La Niña (blue) episodes across their full duration - darker shading means a stronger episode - so you can see how they line up with the bars for any region.
Which regions are covered?
The chart covers the global average, all seven continents, 160+ individual countries, all 50 US states, 9 US climate regions, and the UK nations and regions with heatwave data available. Use the search bar or the map to find any location.