Temperature · 5 min read

What Counts as a Heatwave?

By Emma Clarke · August 10, 2026

Not every hot week counts as a heatwave. Meteorological agencies use specific thresholds to decide when ordinary warm weather crosses into official heatwave territory, and those thresholds are not the same everywhere.

There's no single global definition

The World Meteorological Organization broadly describes a heatwave as a period of abnormally hot weather, but leaves the exact numbers to national weather agencies, because 'abnormal' depends heavily on what's normal for that climate. A stretch of 30°C days that would be an extreme heatwave in a typically cool northern-European climate is simply an average summer week in a hot, dry region — so most countries set their own threshold relative to their own local averages, rather than using one fixed temperature worldwide. The UK Met Office, for example, currently defines a heatwave as at least three consecutive days at or above a location-specific threshold that ranges from around 25°C in cooler parts of the country to 28°C in the warmest, reflecting just how much 'abnormal' varies even within a single country. A city with a naturally hot, dry climate might not declare a heatwave until temperatures climb well past 40°C for several days running.

How weather agencies typically define one

Most definitions combine two things: a temperature threshold and a minimum duration, often three or more consecutive days above that threshold, sometimes with an added requirement that overnight lows also stay unusually high. That overnight condition matters because a body that never gets a chance to cool down overnight is under far more sustained heat stress than one that gets relief after sunset, even if the daytime peak is identical. Heatwaves are often driven by a large, slow-moving area of high pressure sometimes described as a 'heat dome', which traps warm air in place for days and suppresses the cloud and rain that would normally help temperatures ease off. Because that pressure system tends to move slowly, heatwaves are also somewhat more predictable several days in advance than more localized weather events like an isolated thunderstorm.

What to watch for during one

During a heatwave, stay hydrated, avoid strenuous activity during the hottest part of the day, and check on relatives or neighbours who may be more vulnerable to heat, including older people and young children. Seek shade or air conditioning where possible, since the risk from heat builds cumulatively over consecutive hot days rather than resetting each morning. Watch for early signs of heat exhaustion in yourself and others: heavy sweating, dizziness, nausea or an unusually fast pulse. If someone shows these symptoms, moving them to a cooler spot, giving them water and loosening tight clothing usually helps; symptoms that don't improve quickly, or any sign of confusion, are worth treating as a medical emergency.

Planning outdoor time around heat

Shift exercise or outdoor plans to the early morning or evening, when temperatures are meaningfully lower than around midday. Keep an eye on humidity as well as the raw temperature — in humid air, heat is harder for your body to shed by sweating, which is part of why the same temperature can feel far more draining in one region than another. Indoors, closing blinds or curtains during the hottest part of the day and opening windows in the cooler evening and early morning can noticeably reduce how much heat builds up inside a home, especially in buildings not designed with strong insulation against heat. Keep drinking water regularly rather than waiting until you feel thirsty, since thirst is a late signal that you're already somewhat dehydrated.

Frequently asked questions

Is 30°C already a heatwave?
It depends entirely on the region. In a climate where 30°C is unusual, a run of several such days could qualify; in a naturally hot climate, 30°C might be an ordinary summer day and well below the local heatwave threshold.

Where can I check the official threshold for my area?
Your national meteorological service is the most reliable source — most publish the specific temperature and duration criteria they use to declare a heatwave.

Are heatwaves becoming more common?
Most national meteorological agencies and climate scientists report that heatwaves have become more frequent and intense in many regions in recent decades. Checking your local forecast regularly during warm spells is a reasonable habit regardless of the long-term trend.

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