What Is a Heat Wave and How Do We Define One?
A heat wave is a spell of unusually hot weather that lasts several days, hot compared with what is normal for that place and time of year. There is no single global definition, which is why one country’s heat wave can be another’s ordinary week.
Why definitions differ
Heat is relative. A daily high of 35 C is routine in parts of India and a national emergency in parts of the United Kingdom. Most agencies define a heat wave with a threshold that fits the local climate, and a minimum duration. Common choices include:
- a run of consecutive days above a fixed temperature;
- a run of days above a local percentile, such as the 90th percentile of daily maximum for that time of year;
- a run in which both daytime highs and night-time lows stay high, since warm nights stop the body from recovering;
- an index that combines temperature with humidity.
Each is reasonable and each gives different answers, so comparisons across countries need care. Health warnings often use their own thresholds, tied to the local effect on people, which differ again.
Why nights matter
Minimum temperatures are as important as maximums in a long heat spell. In a cooler night the body and buildings recover. When the night stays hot, heat builds up over days. That is why the site tracks record high minimum temperatures as their own type of record, not only highest maximums.
How we detect events
We do not start from a definition based on one threshold. We start from records. An event is a cluster of record events, mainly record highs, that are close together in space and time. To keep noise out, a cluster needs a minimum number of stations, a minimum number of days and a minimum area before it counts as an event. Once it does, it gets its own page with its start, end and peak day, the countries it touches, the records by type, the strongest records and similar past events. The page keeps its address for life. An active event becomes closed when the records stop.
This approach has strengths and limits.
Strengths. It is the same rule everywhere, so events are comparable across countries. It is anchored to each station’s own history, so it adapts to local climate without a threshold list. And it is based on observations, not on a model.
Limits. It only sees where there are qualifying stations, so regions with few stations show gaps, not calm. It finds records, so a long, hot spell that stays just under the records does not show up. And it says nothing about human effects, such as health impacts, which need their own data.
Heat wave or heat dome
People often say heat dome when they mean heat wave. A heat dome is a cause: a large, persistent area of high pressure aloft that traps hot air. A heat wave is the result, the hot weather at the surface. Confirming a dome needs upper air data, such as reanalysis, and we do not yet use it. Our rule is to call events heat waves until such confirmation exists. Our blog post on heat domes covers the physics and our reasoning.
Cold, rain and snow events
The same method covers other extremes. Cold outbreaks cluster record lows, extreme rain events cluster record daily precipitation and snow events cluster record snowfall or snow depth. Each has its own cluster rules and page type. See the events pages for the current list.
Heat and health
Heat is a leading cause of weather related illness, and the people most at risk are older adults, young children, people with chronic conditions and people who work outdoors. Health agencies therefore issue their own heat warnings, based on the local effect of heat on people. Those warnings and our events answer different questions. A warning says that heat is likely to cause harm. An event on this site says that many stations set records in a connected area over several days. The two often overlap, but neither replaces the other, and nothing here is a forecast or a safety notice. For advice on staying safe, follow your national weather service or public health agency.
Records within a heat wave
A heat wave often brings a mix of daily records, some monthly records and occasionally all-time records. Ties appear too, which is covered in our explainer on ties. To see how the counts of records are combined, read what a weather record is.
Reading an event page
Start with the dates and the countries, then the count of records by type. Check the peak day and how many stations were involved. Look at the strongest records and their margins. Then check the note on data: the last 60 days are provisional, and an active event can still change. Longer trends show up in the ratio of record highs to lows, and the rules are set out on the methodology page. The observation source and its licence are on the sources page.