Heat wave

A heat wave (or heatwave), sometimes known as extreme heat, is a period of abnormally hot weather.^: 2911 High humidity often accompanies heat waves. This is especially the case in oceanic climate countries. Definitions vary but are similar. A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season.^: 2911 Temperatures that people from a hotter climate consider normal can be called 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. This is due to climate change.

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. This traps heat near the ground.

Heat waves often have complex effects on human economies. They reduce labour productivity, disrupt agricultural and industrial processes and damage infrastructure not suitable for extreme heat. 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 power outages because people use more air conditioning. A heat wave counts as extreme weather. It poses danger to human health because heat and sunlight overwhelm the human body's cooling system. It is usually possible to detect heat waves by using forecasting instruments. This allows the authorities to issue a warning.