Heat wave

A heat wave or heatwave, sometimes described as extreme heat, is a period of abnormally hot weather.^[1]: 2911 A time period of five consecutive days is often used to define a heat wave but the exact definition of a heat waves can vary for different countries. A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season.^[1]: 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.^[2] High humidity often occurs during heat waves as well. This is especially the case in oceanic climate countries. 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.^{[3]: 8–10 [4]}

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.^[5] 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.^[6][7] 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.