This is a series of articles examining heatwaves from every angle, exploring their causes, consequences, and the science behind them. It includes eight parts:
- Introduction
- Heatwave patterns
- Impacts on human health
- Impacts on urban areas
- Extreme heat in 2024
- Are heatwaves linked to climate change?
- Adapting to more frequent periods of extreme heat
- Temperature, apparent temperature, and heat indices
For almost the entire week starting on 16 June 2025, the weather in the United Kingdom was dominated by a large area of high pressure. It brought calm, sunny conditions with gradually rising temperatures. In the middle of the week, advection of warm air intensified. As a result, temperatures in southern England steadily climbed to levels at which a heatwave is officially declared. On Saturday June 21st, temperatures in the south frequently reached around 30 °C, and in Charlwood, Surrey, they even peaked at 33.2 °C. Such an event is relatively rare in June – on average, it is expected to occur once every 25 years.
Daily maximum temperatures for southern United Kingdom (19–21 June 2025). Areas in dark orange exceed the heatwave threshold of 28 °C.
In this respect, the United Kingdom was no different from most of the Northern Hemisphere. In June 2025, extreme temperatures and heatwaves affected many regions. Several episodes brought intense heat to large parts of North America (including Mexico and the southern and eastern United States), southwestern and western Europe, the Middle East (with a potential Asian record of 54.3 °C currently being verified in Kuwait), as well as South Asia (India and Pakistan) and East Asia (Siberia, China, and Japan).
What is a heatwave, anyway?
There is no internationally unified definition of a heatwave, and the criteria for declaring one vary depending on the climate, geography, and social conditions of each region or country. The definition may also differ depending on how heatwave information is being used. The World Meteorological Organization (WMO) defines a heatwave as “a period of unusually hot weather, statistically unusual, lasting several days and nights.” Various meteorological services define heatwaves according to their own specific criteria.
In the United Kingdom, a heatwave is defined as a period of at least three consecutive days during which the maximum daily temperatures in a given county reach or exceed a specified temperature threshold. This threshold varies across British counties and ranges between 25 °C and 28 °C. The values were determined based on climate data on maximum daily temperatures from the 1991–2020 period, specifically for the midpoint of the meteorological summer (15 July).
In Australia, a heatwave occurs when both maximum and minimum temperatures are unusually high for three consecutive days. These temperature extremes are assessed relative to the local climate and historical weather conditions.
In the United States, a heatwave is defined as a period of abnormally and uncomfortably hot, and usually humid, weather lasting at least two days. In addition to temperature and duration, humidity levels are also taken into account when assessing heatwaves. The National Weather Service has established criteria for issuing heat advisories, warnings, and alerts based on the Heatwave Index.
In India, a heatwave is declared when the maximum temperature reaches at least 40 °C in plains or 30 °C in hilly areas. It is assessed either by the deviation from the long-term average (at least 4.5 °C) or by the absolute temperature. If the temperature reaches 45 °C or higher, a heatwave is declared automatically. A warning is issued if these conditions are met at a minimum of two stations in the affected area for two consecutive days. In coastal regions, the thresholds are slightly lower: the temperature must exceed 37 °C, and the deviation from normal must be at least 4.5 °C.
In the absence of a universally accepted definition of a heatwave, comparative climate statistics often consider a temperature abnormally high if it exceeds a specified percentile of daily values for a given date within the reference period. For example, the report Climate Change and the Escalation of Global Extreme Heat: Assessing and Addressing the Risks uses the 90th percentile threshold. To illustrate the impacts of extreme temperatures, various heat indices are often used, such as the Thermal Climate Index or UTCI.
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