Heatwave Days – Trends & Drivers
The number of heatwave days each year has climbed sharply since the 1970s, in step with the long-term warming trend. This page tracks the Warm Spell Duration Index (days per year in a 6+ day hot spell) since 1950 for the world, every continent, 160+ countries, US states and UK regions, with Temperature and ENSO (El Niño / La Niña) overlays showing what else is driving the swings.
Heatwave Days – Global
Warm Spell Duration Index (WSDI) - days per year in a 6+ day run above the local 90th-percentile temperature. Land-area-weighted mean across 169 countries.
Heatwave Days (WSDI)
Toggle Temperature (right axis, tightly scaled so the trend is visually comparable to the heatwave bars) and ENSO (its own stacked panel below, aligned to the same years) independently or together - turning ENSO on also marks each El Niño / La Niña episode’s peak on the main chart, so you can see how it lines up with the heatwave spikes directly.
Source: World Bank Climate Change Knowledge Portal (ERA5-derived) (heatwave days), refreshed roughly 1-2x/year, ~7-8 months behind present · NOAA RONI (Relative Niño 3.4 index, ERSSTv5) - the same ENSO series shown on the ENSO Tracker.
What Drives Heatwaves
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). But at the country level the picture is far less uniform. Research points to several factors layering on top of the warming trend:
- ENSO - El Niño and La Niña shift the jet stream and Walker Circulation, changing regional heatwave odds independent of the warming trend; El Niño years have been shown to amplify trans-Pacific heatwave synchronisation (IOPscience).
- Jet-stream blocking - persistent “omega block” patterns have directly driven several major recent heatwaves by interrupting the westerly flow that would otherwise break up a hot spell.
- Soil moisture - land-atmosphere coupling means dry soils amplify heatwaves, with a strong but regionally uneven effect (npj Climate & Atmospheric Science).
- Aerosols - cleaner air (falling aerosol pollution) is an under-modelled contributor to recent heatwave trends in Europe and China.
- Marine heatwaves - for coastal countries, a nearby marine heatwave can amplify an adjacent land heatwave by up to 3.5x (Nature Scientific Reports).
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 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.