Heat wave

A heat wave^[1] (or heatwave^[2]), sometimes described as extreme heat, is a period of abnormally hot weather.^[3]: 2911 High humidity often accompanies heat waves. This is especially the case in oceanic climate countries. Definitions vary but are similar.^[4] A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season.^[3]: 2911 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.^[5] Heat waves have become more frequent, and more intense over land, across almost every area on Earth since the 1950s. This is due to climate change.^[6][7]

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.^[8] This traps heat near the earth's surface. It is usually possible to detect heat waves by using forecasting instruments. This allows the authorities to issue a warning.

Heat waves often have complex effects on human economic activities. They reduce labour productivity, disrupt agricultural and industrial processes and damage infrastructure not suitable for extreme heat. These events end up affecting the local hydrological patterns, which can end up leading to soil erosion, floodings, and depletion of local water resources, all this not only leading to problems on a small scale, but also affecting societies on a large scale. ^[9][10] 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 danger to human health, because heat and sunlight overwhelm the thermoregulation in humans.