Risk of extreme heat varies according to local climate, environmental conditions and the vulnerability of exposed populations. As a result, extreme heat can be defined and measured in different ways, and no single metric or indicator captures all its dimensions.

Heatwaves are one manifestation of extreme heat. What constitutes a heatwave depends on local climate conditions and how people, infrastructure and ecosystems are adapted to them. For example, temperatures above 30°C contributed to the worst heatwave on record in parts of sub-Arctic Norway, Sweden and Finland in 2025, whereas daily temperatures of 31–33°C are common throughout the year in tropical cities such as Kuala Lumpur (Malaysia). For this reason, heatwaves are generally defined using local thresholds rather than a single global temperature value.

A heatwave is not simply a sequence of hot days. A defining characteristic is the build-up of excess heat over successive days and nights. During normal conditions, cooler nights allow people, buildings and the environment to release heat absorbed during the day. When nights remain unusually warm, this recovery process is disrupted and less of that heat is dissipated. As heat intensifies, pressure on energy and transport systems grows, vegetation dries out, wildfire risk rises and crops, plants and animal species face increasing stress.

Dangerous heat is not limited to heatwaves alone. In many tropical regions, people experience persistent heat and humidity that create dangerous conditions even during relatively weak heatwaves. Marine heatwaves and rising temperatures are also affecting coastal ecosystems and fisheries, with implications for communities and the economies that depend on them.

Thermal heat stress refers to the strain placed on the human body when humid or dry heat conditions overwhelm the body's natural ability to regulate temperature. It depends on a combination of factors including temperature, humidity, solar radiation and wind speed.

While heatwaves and thermal heat stress can occur together, they do not always coincide. Differentiating between these hazards therefore helps improve monitoring, forecasting, warning services and impact assessments. 

Monitoring and forecasting

Monitoring extreme heat relies on observations collected through global observing systems, including surface weather stations, satellites, ships and aircraft, which provide information on temperature and other environmental conditions around the globe.

National Meteorological and Hydrological Services (NMHSs) use these observations to track temperatures in real time and to understand how extreme heat events and climate patterns are changing over time. In some cases, observations are combined within numerical climate and weather models to produce climate datasets known as reanalyses, which help fill observational gaps and improve our understanding of heat conditions across different regions.

Temperature alone does not always reflect how hot conditions feel to the human body. Both high humidity and drier heat in windy conditions reduce the effectiveness of sweating, which is the body's primary cooling mechanism. For this reason, scientists use a range of thermal indices that combine temperature with factors such as humidity, solar radiation and wind speed to better assess heat stress and associated health risks.

Weather forecasts can provide advance warning of extreme heat events, while seasonal and sub-seasonal outlook scan identify trends where temperatures are more likely to be above average. These forecasts support preparedness and decision-making across sectors including health, agriculture, energy, water management and emergency services.