1297. Definition of Heat Waves

As global warming progresses, the impacts of extreme heat are expected to grow even more, making it urgent to build effective early warning systems. According to an editorial in The Lancet Planetary Health, in the first two decades of the 21st century, the excess deaths from heat stroke were about 500,000 per year, of which about 250,000 were in Asia.

According to the editorial, many countries have yet to implement heat stroke warning systems, and even in those that have, the impacts of heat waves are not clearly defined. A major challenge in mitigating heat waves is the lack of a universally accepted definition of what a heat wave is. Given the wide range of uses of the term in climate, human health, and meteorology, it is not surprising that definitions of heat waves are diverse, but it is essential to exercise great care in applying each indicator.

Heat waves are an example of an extreme heat event and are defined based on regional and annual climatology. Regional climatology takes into account human adaptation to heat, whereas annual climatology distinguishes between warm periods (i.e., unseasonably hot outside the hot season) and extreme heat.

The World Meteorological Organization (WMO) defines a heat wave as a regional cumulative excess of heat that occurs during a series of abnormally hot days and nights, with cumulative and sustained heat in addition to being regional and excessive. The temperature excess is integrated over the day and night against a regional threshold to parameterize the cumulative excess heat. This definition requires maximum and minimum temperatures to parameterize a heat wave. High minimum temperatures provide little relief or recovery to those exposed to heat, and typically lead to a faster rise in temperature the next day, further accumulating heat. Dangerous heatwaves are usually identified by exceeding a maximum temperature threshold over several days, but there are still limited services that include minimum temperature thresholds or additional threshold tests to identify more levels of hazardous events.

The editorial stated that a proper definition of heatwaves based on intensity is needed to properly address heatwaves, and suggested that the definition of heatwaves be properly addressed in multi-hazard early warning systems based on the definition of "localized accumulated residual heat during a period of abnormally hot days and nights" established by the World Meteorological Organization (WMO) in 2025.

(Reference)
John Nairn, Simon J Mason, (2025) Extreme heat and heatwaves: hazard awareness and impact mitigation. The Lancet Planetary Health. https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(25)0016…

Contributor: IIYAMA Miyuki, Information Program