Explainer · Updated October 22, 2026
What is a heat storm?
A heat storm is a heat wave that has grown enormous: abnormal heat covering an area the size of half of Canada or more, all at once. The term was introduced in October 2026 to describe the largest heat events on the planet, and they are happening far more often than they used to.
Key facts
- Heat wave: at least five days in a row of abnormally high temperatures for a place and time of year.
- Heat storm: a heat wave that, at its peak, covers at least 1% of Earth’s surface, about 5.1 million square kilometers (roughly 2 million square miles).
- Cascade: heat storms that follow one another or overlap, with no break in between.
- How common: about 5 heat storms a year in the 1960s, about 102 a year so far this decade, according to Reuters’ analysis.
Where the term comes from
“Heat storm” was introduced by Reuters on October 20, 2026, in an investigation into supersized heat waves. Working with two dozen climate researchers, Reuters analyzed temperatures since the 1960s for more than a million grid squares covering the whole planet. Its conclusion: the phrase “heat wave” no longer captures the scale of the biggest events, some of which now spread across continents and last for months.
Reuters names each heat storm, the way hurricanes are named, and tracks them on its Climate Monitor. HeatStorms.org is an independent site and is not affiliated with Reuters.
Heat wave vs. heat storm
The difference is size. In Reuters’ method, a spot on the map counts as abnormally hot when its daily high reaches at least the 90th percentile for that date, compared with the years 1961–1990. When that heat holds for about five days, it’s a heat wave. Neighboring hot areas are merged into a single event, and once that event covers at least 1% of Earth’s surface, it becomes a heat storm.
| Term | What it means | Typical scale |
|---|---|---|
| Hot day | One day hotter than normal for that place and date | A single place |
| Heat wave | Abnormal heat that lasts five days or more | A city up to a large region |
| Heat storm | A heat wave covering at least 1% of Earth’s surface at its peak | About 2 million sq mi or more, roughly half of Canada |
| Cascade | Heat storms running back to back or at the same time | Can last months or even years |
Because everything is measured against what’s normal for the date, a heat storm doesn’t have to happen in summer. An unusually warm stretch in February can qualify.
How big can heat storms get?
Very big. According to Reuters:
- The largest heat storm on record, which Reuters named Vera, lasted 367 days across 2023 and 2024 and swept over nearly 85% of the globe during its lifetime.
- The heat storm active when Reuters published, Raimundo, began on February 28, 2026, and has covered almost three-quarters of Earth’s surface. It fueled major wildfires in Spain and France this summer.
- A cascade that began on July 6, 2008, has never stopped. For more than 18 years, at least one heat storm has been active somewhere on the planet.
Why they’re becoming more common
Reuters found that heat storms went from about 5 a year in the 1960s to about 102 a year in the current decade. Cascades grew longer too, from roughly two months on average in the 1960s to about nine months in the early 2000s. Over the same period, carbon dioxide in the atmosphere rose by more than 30%, and the average share of the planet likely to be covered by heat waves and heat storms increased more than twelvefold.
The basic idea is simple: as the planet’s baseline temperature rises, it takes less of a push for any given day to land in the hottest 10% of what used to be normal, so abnormal heat covers more ground, more often.
What causes the heat in the first place?
Large heat events are usually driven by a heat dome: a big area of high pressure in the upper atmosphere. Air under the dome sinks and warms as it’s compressed, and the sinking air keeps clouds and rain from forming. When the winds high above are weak, the pattern can stall in place for days, baking the same area day after day. When patterns like this are large, slow-moving, or several are active at once on top of a warmer baseline, heat waves can grow to heat-storm size.
Why the size matters
Heat is the number one weather-related killer in the United States, according to the National Weather Service. Worldwide, heat is estimated to kill almost half a million people a year. When the hot area is the size of a continent, far more people are exposed at the same time, and the effects pile up: strained power grids, damaged crops, drought and wildfire risk, and emergency services stretched thin across many regions at once.
That’s also the argument for giving these events a name. As Anthony Leiserowitz of the Yale Program on Climate Change Communication told Reuters, naming events is “a powerful way for people to understand that something is happening.”
How to stay safe when the heat hits
- Slow down. Move hard work and exercise to the coolest part of the day.
- Drink plenty of water or other non-alcoholic fluids, even before you feel thirsty.
- Spend the hottest hours somewhere air-conditioned: home, a library, a mall, or a local cooling center.
- Wear light, light-colored clothing.
- Never leave a child or pet in a parked car, even with the windows down.
- Check on older neighbors and anyone who lives alone.
Know the warning signs
Heat exhaustion: heavy sweating, weakness, cool, pale and clammy skin, a weak pulse, dizziness, nausea or vomiting, and fainting. Move to a cooler place, loosen clothing, and sip water.
Heat stroke: confusion or an altered mental state, a very high body temperature (106°F or higher), a rapid pulse, and possible unconsciousness. Heat stroke is a medical emergency. Call 911.
Tracking heat on HeatStorms.org
Our homepage brings the picture together for North America: a live heat risk map from the National Weather Service with a seven-day outlook, every active heat alert in the US, and a local check that compares this week’s highs where you live with normal for the date. A named heat storm tracker for North America is in the works. See how our data works.
Frequently asked questions
Is a heat storm an actual storm?
Not in the usual sense. A heat storm doesn’t need wind, rain or lightning. It’s a very large, long-lasting heat wave. The word “storm” signals its scale and danger, the same way names help people take hurricanes seriously.
Who decides when a heat wave becomes a heat storm?
There’s no official government definition yet. The term and the 1% of Earth’s surface threshold come from Reuters’ analysis. Weather agencies such as the National Weather Service don’t use the term, and the World Meteorological Organization has been cautious about naming heat waves.
Does the National Weather Service issue heat storm warnings?
No. In the US, official heat alerts are Heat Advisories, Extreme Heat Watches and Extreme Heat Warnings. Those are what you should act on, and you can see every active one on our homepage.
How are heat storms named?
Reuters names the heat storms it tracks using a list of 2,380 names drawn from popular names around the world, alternating between genders and world regions. Vera and Raimundo are two of them.
Can a heat storm happen in winter?
Yes. Heat storms are measured against what’s normal for the time of year, so a winter warm spell that’s far above normal can count. Heat Storm Raimundo began in late February.
Sources
- Reuters, “Giant ‘heat storms’ are scorching the Earth” and “How (and why) we developed ‘The Age of Heat Storms’,” October 20, 2026.
- National Weather Service, Heat Safety.
- American Meteorological Society, Glossary of Meteorology: Heat dome.
- World Meteorological Organization, Considerations regarding the naming of heatwaves, 2022.