A heat wave[1] or heatwave,[2] sometimes described as extreme heat, is a period of abnormally hot weather. Definitions vary but are similar.[3] A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season. 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.[4] 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.[5] [6]
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 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.[7] [8] 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.
There are several definitions of heat waves:
Denmark defines a national heat wave (hedebølge) as a period of at least 3 consecutive days in which the average maximum temperature across more than half the country exceeds 28°C. The Danish Meteorological Institute also has a definition for a "warmth wave" (varmebølge). It defines this using the same criteria for a 25°C temperature.[14] Sweden defines a heat wave as at least five days in a row with a daily high exceeding 25°C.[15]
In Greece, the Hellenic National Meteorological Service defines a heat wave as occurring over three consecutive days with temperatures at 39 °C (102 °F) or higher. In the same period the minimum temperature is 26°C or more. During this period, there are either no winds or only weak winds. These conditions occur in a broad area.
The Netherlands defines a heat wave as a period of at least five consecutive days in which the maximum temperature in De Bilt exceeds 25°C. During this period the maximum temperature in De Bilt must exceed 30°C for at least three days. Belgium also uses this definition of a heat wave with Ukkel as a reference point. So does Luxembourg.
In the United Kingdom, the Met Office operates a Heat Health Watch system. This places each Local Authority region into one of four levels. Heat wave conditions occur when the maximum daytime temperature and minimum nighttime temperature rise above the threshold for a particular region. The length of time above that threshold determines the level. Level 1 represents normal summer conditions. Level 2 occurs when there is a 60% or higher risk that the temperature will be above the threshold levels for two days and the intervening night. Level 3 arises when the temperature has been above the threshold for the preceding day and night, and there is a 90% or higher chance that it will stay above the threshold in the following day. Level 4 is triggered if conditions are more severe than those of the preceding three levels. Each of the first three levels gives rise to a particular state of readiness and response by the social and health services. Level 4 involves a more widespread response.[16] The threshold for a heat wave occurs when there are at least three days above 25°C across much of the country. Greater London has a threshold of 28°C.[17]
In the United States, definitions also vary by region. They usually involve a period of at least two or more days of excessively hot weather.[18] In the Northeast, a heat wave is typically when the temperature reaches or exceeds 90°F for three consecutive days. This is not always the case. This is because the high temperature ties in with humidity levels to determine a heat index threshold.[19] The same does not apply to drier climates. A heat storm is a Californian term for an extended heat wave. Heat storms occur when the temperature reaches 100°F for three or more consecutive days over a wide area (tens of thousands of square miles).[20] The National Weather Service issues heat advisories and excessive heat warnings when it expects unusual periods of hot weather.
In Adelaide, South Australia, a heat wave is five consecutive days at or above 35°C, or three consecutive days at or over 40°C.[21] The Australian Bureau of Meteorology defines a heat wave as three or more days of unusual maximum and minimum temperatures.[22] Before this new Pilot Heatwave Forecast there was no national definition for heat waves or measures of heat wave severity.
See main article: List of heat waves. It is possible to compare heat waves in different regions of the world with different climates thanks to a general indicator that appeared in 2015.[23] With these indicators, experts estimated heat waves at the global scale from 1901 to 2010. They found a substantial and sharp increase in the number of affected areas in the last two decades.[24]
One study in 2021 investigated 13,115 cities. It found that extreme heat exposure of a wet bulb globe temperature above 30 Celsius tripled between 1983 and 2016, and if the effect of population growth (increasing the urban heat island effect) during those years is excluded, the exposure increased a further 50%. The researchers compiled a comprehensive list of past urban extreme heat events.[25] [26]
Heat waves form when a high pressure area at an altitude of 10000- strengthens and remains over a region for several days and up to several weeks.[27] This is common in summer in both the Northern and Southern Hemispheres. This is because the jet stream 'follows the sun'. The high pressure area is on the equator side of the jet stream in the upper layers of the atmosphere.
Weather patterns are generally slower to change in summer than in winter. So, this upper level high pressure also moves slowly. Under high pressure, the air sinks toward the surface. It warms and dries adiabatically. This inhibits convection and prevents the formation of clouds. A reduction of clouds increases the shortwave radiation reaching the surface. A low pressure area at the surface leads to surface wind from lower latitudes that brings warm air, enhancing the warming. The surface winds could also blow from the hot continental interior towards the coastal zone. This would lead to heat waves on the coast. They could also blow from high towards low elevations. This enhances the subsidence or sinking of the air and therefore the adiabatic warming.[28] [29]
In the eastern regions of the 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. Hot humid air masses form over the Gulf of Mexico and the Caribbean Sea. At the same time hot dry air masses form over the desert Southwest and northern Mexico. The southwest winds on the back side of the high continue to pump hot, humid Gulf air northeastwards. This results in a spell of hot and humid weather for much of the eastern United States and into southeastern Canada.[30]
In the Western Cape Province of South Africa, a heat wave can occur when the low-pressure area offshore and the high-pressure area inland combine to form a bergwind. The air warms as it descends from the Karoo interior. The temperature will rise about 10 Celsius from the interior to the coast. Humidity is usually very low. The temperature can be over 40 Celsius in summer. The highest temperature recorded in South Africa (51.5 Celsius) occurred one summer during a berg wind along the Eastern Cape coastline.[31] [32]
The level of soil moisture can intensify heat waves in Europe.[33] [34] Low soil moisture leads to a number of complex feedback mechanisms. These in turn can result in increased surface temperatures. One of the main mechanisms is reduced evaporative cooling of the atmosphere.[33] When water evaporates, it consumes energy. So, it will lower the surrounding temperature. If the soil is very dry, then incoming radiation from the sun will warm the air. But there will be little or no cooling effect from moisture evaporating from the soil.
The number of heat fatalities is probably highly underreported. This is due to a lack of reports and to misreporting.[35] When considering heat-related illnesses as well, actual death tolls from extreme heat may be six times higher than official figures. This is based on studies of California[36] and Japan.[37]
Part of the mortality during a heat wave may be due to short-term forward mortality displacement. In some heat waves there is a decrease in overall mortality in the weeks after a heat wave. These compensatory reductions in mortality suggest that heat affects people who would have died anyway, and brings their deaths forward.[38]
Social institutions and structures influence the effects of risks. This factor can also help explain the underreporting of heat waves as a health risk. The deadly French heat wave in 2003 showed that heat wave dangers result from a combination of natural and social factors.[39] Social invisibility is one such factor. Heat-related deaths can occur indoors, for instance among elderly people living alone. In these cases it can be challenging to assign heat as a contributing factor.[40]
The heat index in the table above is a measure of how hot it feels when relative humidity is factored with the actual air temperature.
Excessive heat causes psychological stress as well as physical stress. This can affect performance. It may also lead to an increase in violent crime.[41] High temperatures are associated with increased conflict between individuals and at the social level. In every society, crime rates go up when temperatures go up. This is particularly the case with violent crimes such as assault, murder and rape. In politically unstable countries, high temperatures can exacerbate factors that lead to civil war.[42]
High temperatures also have a significant effect on income. A study of countries in the United States found that the economic productivity of individual days declines by about 1.7 percent for each degree Celsius above 15°C.[43]
High temperatures also make the effects of ozone pollution in urban areas worse. This raises heat-related mortality during heat waves.[44] During heat waves in urban areas, ground level ozone pollution can be 20 percent higher than usual.[45]
One study looked at fine particle concentrations and ozone concentrations from 1860 to 2000. It found that the global population-weighted fine particle concentrations increased by 5 percent due to climate change. Near-surface ozone concentrations rose by 2 percent.[46]
An investigation to assess the joint mortality effects of ozone and heat during the European heat waves in 2003 concluded that these appear to be reinforce each other and increase mortality when combined.[47]
Calculations from 2022 suggest that heat waves will shrink the global economy by about 1 percent decrease by the middle of the 21st century.[48] [49] [50]
Heat waves often have complex effects on economies. They reduce labour productivity, disrupt agricultural and industrial processes and damage infrastructure that is not suitable for extreme heat. In 2016, a marine heatwave in Chile and its subsequent harmful algal bloom caused $800 million (USD) in export losses for the aquaculture industry as salmon and shellfish died off.[51]
See main article: Effects of climate change on agriculture.
Heat waves are a big threat to agricultural production. In 2019 heat waves in the Mulanje region of Malawi involved temperatures as high as . This and a late rain season scorched tea leaves and reduced yields.[52]
Heat waves cause roads and highways to buckle and melt,[53] water lines to burst, and power transformers to detonate, causing fires. A heat wave can also damage railways, by buckling and kinking rails. This can slow down or delay traffic. It can even lead to cancellations of service when rails are too dangerous to traverse by trains.
Heat waves often lead to spikes in electricity demand because there is more use of air conditioning. This can create power outages, making the problem worse. During the 2006 North American heat wave, thousands of homes and businesses went without power, especially in California. In Los Angeles, electrical transformers failed, leaving thousands without power for as long as five days.[54] The early 2009 southeastern Australia heat wave caused major power disruptions in the city of Melbourne. They left over half a million people without power as the heat wave blew transformers and overloaded a power grid.
A heat wave occurring during a drought can contribute to bushfires and wildfires. This is because a drought dries out vegetation, so it is more likely to catch fire. During the disastrous heat wave that struck Europe in 2003, fires raged through Portugal. They destroyed over 3010km2 of forest and 440km2 of agricultural land. They caused about €1 billion worth of damage.[55] High end farmlands have irrigation systems to back up crops.
Heat waves can also contribute to flooding. Because hot air is able to carry more moisture, heatwaves may be followed by extreme rainfall especially in mid-latitude regions.[56] For example, the record-breaking heat wave that afflicted Pakistan beginning in May 2022 led to glacier melt and moisture flow. These were factors in the devastating floods that began in June and claimed over 1,100 lives.[57]
Researchers have predicted that roughly 10-40% of all land vertebrate species will be affected by heat waves by 2099, depending on the amount of future greenhouse gas emissions.[58] Heatwaves present an additional form of stress and evolutionary pressure for species that already deal with habitat loss and climate change.
Species have a thermal range of tolerance that describes the temperatures where they perform best. Temperature conditions that are outside of this range may experience decreased fitness and the inability to reproduce.[59] The species with sufficient genetic variation will be able to ensure some individuals can survive frequent days of high temperatures in the future.[60]
Marine heatwaves may cause mass mortality in fish populations, especially for species that are better adapted to cooler temperatures.[61] Species that have adapted to warmer temperatures may expand their range during a heatwave. These invasive species may outcompete the native species that experience higher mortality during a heatwave, which disrupts ecosystem functioning. Marine heatwaves have also been correlated with negative impacts on foundation species such as coral and kelp.[62]
A possible public health measure during heat waves is to set up air-conditioned public cooling centres. Adding air conditioning in schools provides a cooler work place.[63] But it can result in additional greenhouse gas emissions unless solar energy is used.
Policymakers, funders and researchers have created the Extreme Heat Resilience Alliance coalition under the Atlantic Council. This advocates for naming heat waves, measuring them, and ranking them to build better awareness of their impacts.[64] [65]
See main article: List of heat waves.
In July 2019, there were over 50 million people in the United States in jurisdictions with heat advisories. Scientists predicted that many records for highest low temperatures would be broken in the days following these warnings. This means the lowest temperature in a 24-hour period will be higher than any low temperature measured before.[66]
According to a 2022 study, 107 million people in the US will experience extremely dangerous heat in the year 2053.[67]
Heat waves are the most lethal type of weather phenomenon in the United States. Between 1992 and 2001, deaths from excessive heat in the United States numbered 2,190, compared with 880 deaths from floods and 150 from tropical cyclones.[68] About 400 deaths a year on average are directly due to heat in the United States. The 1995 Chicago heat wave, one of the worst in US history, led to approximately 739 heat-related deaths over 5 days.[69] In the United States, the loss of human life in hot spells in summer exceeds that caused by all other weather events. These include lightning, rain, floods, hurricanes, and tornadoes.[70] [71]
About 6,200 Americans need hospital treatment each summer, according to data from 2008. This is due to excessive heat, and those at highest risk are poor, uninsured or elderly.[72]
The relationship between extreme temperature and mortality in the United States varies by location. Heat is more likely to increase the risk of death in cities in the northern part of the country than in southern regions. As a whole, people in the United States appear to be adapting to hotter temperatures further north each decade. This might be due to better infrastructure, more modern building design and better public awareness.[73]