How we spot heatwaves

What the dashboard shows, where the numbers come from, how current they are, and what they can’t tell you.

The short answer

A heatwave is heat that is unusual for the place and the time of year. 35 °C is an ordinary day in Riyadh in summer and an emergency in London. So the dashboard compares each of 48 major cities, on every continent, with its own climate: the daily highs it had at the same time of year in 1991–2020. When a city is hotter than 9 in 10 of those days for at least three days in a row, we call it a heatwave. We also count how many heatwave days each city has had this year, against an average year.

How we know

Daily highs. NOAA’s Climate Prediction Center collects daily reports from weather stations worldwide and turns them into a map of daily maximum temperature on a grid of half-degree squares, about 50 km wide. It adds each day within a day or two. We read the square of each city every night from NOAA’s public files.

The normal. From the same NOAA analysis for 1991–2020 we worked out, for each city and each day of the year, the average daily high and the 90th percentile: the temperature that only 1 in 10 days exceeded. We use the 15 days around each date, so the normal changes smoothly through the year.

The heatwave rule. At least three days in a row above the 90th percentile. It is one of the standard definitions used by climate scientists (Perkins and Alexander 2013). Because it is relative to the time of year, an unusually warm spell in winter counts too: it is a heatwave in the sense of the record books, even if it is not dangerous.

The bigger picture. The world’s yearly average temperature comes from NOAA’s National Centers for Environmental Information. The statements on heatwaves and climate change are from the Intergovernmental Panel on Climate Change (IPCC) assessment of 2021, and the death tolls from peer-reviewed studies.

How current each part is

Daily highs: to the latest day NOAA has analysed, usually yesterday or the day before. NOAA may revise recent days, so we re-read the last three weeks every night.
Global temperature: complete years only; the current year appears after December.
Studies and IPCC statements: as published, checked 26 October 2026.

What the dashboard can’t tell you

It is not a heat warning. For warnings and advice in the coming days, follow your national weather service and health authorities.

A grid square is an average over about 50 by 50 km, not a thermometer in the city centre. City centres are often a few degrees hotter, and at the coast or near mountains the square can differ from the city’s weather station. For Los Angeles and Athens the nearest square is mostly mountains or sea, so we use the neighbouring square that matches the city’s station. Some squares, such as those of Lagos and Bangkok, contain no reporting station on most days; NOAA fills them from the nearest stations, so their values are smoother and less certain.

Daily highs are only part of the danger. Humidity, and nights that stay hot, matter as much for health, and they are not included here.

The normal is 1991–2020, which was already warmer than the 20th century. A year with “normal” heatwave days still has more than the climate our grandparents knew.

Credits

Daily temperatures from NOAA’s Climate Prediction Center, global temperature from NOAA’s National Centers for Environmental Information, both public US government data. Heat-death estimates and IPCC statements as cited. We only download and present their public data.