A heat wave or heatwave, sometimes described as extreme heat, is a period of abnormally hot weather ^[1]: 2911 and natural disaster that lasts for multiple days. A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season.^[2] The main difficulties with this broad definition emerge when one must quantify what the 'normal' temperature state is, and what the spatial extent of the event may or must be. Temperatures that humans from a hotter climate consider normal can be regarded as a heat wave in a cooler area. This would be the case if the warm temperatures are outside the normal climate pattern for that area. Heat waves have become more frequent, and more intense over land, across almost every area on Earth since the 1950s, the increase in frequency and duration being caused by climate change. According to the World Meteorological Organization, heat waves continued to intensify in 2024, with record-breaking temperatures reported in Europe, North America, and China. Many regions experienced consecutive days above 45°C, highlighting the increasing frequency and severity of extreme heat events worldwide.^[3].

Heat waves form when a high-pressure area in the upper atmosphere strengthens and remains over a region for several days up to several weeks.^[4] This traps heat near the earth's surface. It is usually possible to forecast heat waves, thus allowing the authorities to issue a warning in advance.

Heat waves have an impact on the economy. They can reduce labour productivity, disrupt agricultural and industrial processes and damage infrastructure.^[5][6] Severe heat waves have caused catastrophic crop failures and thousands of deaths from hyperthermia. They have increased the risk of wildfires in areas with drought. They can lead to widespread electricity outages because more air conditioning is used. A heat wave counts as extreme weather. It poses a danger to human health, because heat and sunlight overwhelm the thermoregulation in humans.