A heat wave is a prolonged period of abnormally high temperatures that is sufficiently intense to pose risks to human health, ecosystems or infrastructure. Meteorologists typically define a heat wave relative to local climate conditions — for example, when daily maximum temperatures exceed the long-term historical average of a region for at least two or more consecutive days. What qualifies as a heat wave depends on place and context: a temperature extreme in one location may be routine in another, so thresholds are set region by region.
In practice, agencies may refine the definition further: In the United States, the National Weather Service’s regions and districts set heat wave advisory thresholds around heat index values that pose a threat to their populations, depending on climate norms. Internationally, the World Meteorological Organization (WMO) defines a heat wave as a period of five or more consecutive days with daily maximum temperature exceeding the average by at least 5°C (9°F), relative to the 1961–1990 baseline.