Heat wave

 A heat wave, or heatwave,[1] is a period of excessively hot weather, which may be accompanied by high humidity, especially in oceanic climate countries. While definitions vary,[2] a heat wave is usually measured relative to the usual weather in the area and relative to normal temperatures for the season. Temperatures that people from a hotter climate consider normal can be called a heat wave in a cooler area if they are outside the normal climate pattern for that area.[3]

The term is applied both to hot weather variations and to extraordinary spells of hot which may occur only once a century. Severe heat waves have caused catastrophic crop failures, thousands of deaths from hyperthermia, and widespread power outages due to increased use of air conditioning. A heat wave is considered extreme weather that can be a natural disaster, and a danger because heat and sunlight may overheat the human body. Heat waves can usually be detected using forecasting instruments so that a warning call can be issued 

Heat waves form when high pressure aloft (from 10,000–25,000 feet (3,000–7,600 metres)) strengthens and remains over a region for several days up to several weeks.[16] This is common in summer (in both Northern and Southern Hemispheres) as the jet stream 'follows the sun'. On the equator side of the jet stream, in the upper layers of the atmosphere, is the high pressure area.

Summertime weather patterns are generally slower to change than in winter. As a result, this upper level high pressure also moves slowly. Under high pressure, the air subsides (sinks) toward the surface, warming and drying adiabatically, inhibiting convection and preventing the formation of clouds. Reduction of clouds increases shortwave radiation reaching the surface. A low pressure at the surface leads to surface wind from lower latitudes that brings warm air, enhancing the warming. Alternatively, the surface winds could blow from the hot continental interior towards the coastal zone, leading to heat waves there, or from a high elevation towards low elevation, enhancing the subsidence and therefore the adiabatic warming.[17][18]

In the Eastern United States a heat wave can occur when a high pressure system originating in the Gulf of Mexico becomes stationary just off the Atlantic Seaboard (typically known as a Bermuda High). Hot humid air masses form over the Gulf of Mexico and the Caribbean Sea while hot dry air masses form over the desert Southwest and northern Mexico. The SW winds on the back side of the High continue to pump hot, humid Gulf air northeastward resulting in a spell of hot and humid weather for much of the Eastern States.[19]

In the Western Cape Province of South Africa, a heat wave can occur when a low pressure offshore and high pressure inland air combine to form a Bergwind. The air warms as it descends from the Karoo interior, and the temperature will rise about 10 °C from the interior to the coast. Humidities are usually very low, and the temperatures can be over 40 °C in summer. The highest official temperatures recorded in South Africa (51.5 °C) was recorded one summer during a bergwind occurring along the Eastern Cape coastlin 

June 2019 was the hottest month on record worldwide, the effects of this were especially prominent in Europe.[53] The effects of climate change have been projected to make heat waves in places such as Europe up to five times more likely to occur. Among other effects, increased wildfires in places such as Spain can also be attributed to heat waves.[54]

In July 2019, over 50 million people in the United States were present in a jurisdiction with any type of heat advisory. Scientists predicted that in the days following the issuance of these warnings, many records for highest low temperatures will be broken: i.e. the lowest temperature in a 24-hour period will be higher than any low temperature measured before.[55]

In addition to posing a threat to human health, heat waves significantly threaten agricultural production. In 2019, heat waves in the Mulanje region of Malawi experienced temperatures as high as 40 °C (104 °F). Heat waves and a late rain season resulted in significant leaf scorching of tea leaves in Malawi, leading to reduced yields.[56]

The 2021 Western North America heat wave resulted in some of the highest temperatures ever recorded in the region, including 49.6 °C (121.3 °F), the highest temperature ever measured in Canada