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A heat wave, or heatwave, is a period of excessively hot weather, which may be accompanied by high humidity, especially in oceanic climate countries. While definitions vary, a heat wave is usually measured relative to the usual climate 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.
The term is applied both to hot weather variations and to extraordinary spells of hot weather which may occur only once a century. Severe heat waves have caused catastrophic crop failures, thousands of deaths from hyperthermia, increased risk of wildfires in areas with drought, and widespread power outages due to increased use of air conditioning. A heat wave is considered extreme weather, and poses danger to human health because heat and sunlight overwhelm the human body's cooling system. Heat waves can usually be detected using forecasting instruments so that a warning can be issued.
According to the IPCC, heatwaves have become more frequent, and over land more intense, almost everywhere since the 1950s, due to climate change.
Heatwaves often have complex effects on human economies, due to less productivity of workers, disruption of agricultural and industrial processes and damage to infrastructure not adapted for extreme heat. Recent projections suggest heatwaves alone will cause ~1% decrease of GDP to economies by mid 21st century.
A definition based on Frich et al.'s Heat Wave Duration Index is that a heat wave occurs when the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 °C (9 °F), the normal period being 1961–1990.
A formal, peer-reviewed definition from the Glossary of Meteorology is:
A period of abnormally and uncomfortably hot and usually humid weather.
- To be a heat wave such a period should last at least one day, but conventionally it lasts from several days to several weeks. In 1900, A. T. Burrows more rigidly defined a "hot wave" as a spell of three or more days on each of which the maximum shade temperature reaches or exceeds 90 °F (32.2 °C). More realistically, the comfort criteria for any one region are dependent upon the normal conditions of that area. [Robert DeCourcy Ward]
The World Meteorological Organization, defines a heat wave as five or more consecutive days of prolonged heat in which the daily maximum temperature is higher than the average maximum temperature by 5 °C (9 °F) or more.
In the Netherlands, a heat wave is defined as a period of at least five consecutive days in which the maximum temperature in De Bilt exceeds 25 °C (77 °F), provided that on at least three days in this period the maximum temperature in De Bilt exceeds 30 °C (86 °F). This definition of a heat wave is also used in Belgium (with Ukkel as reference point) and Luxembourg.
In Denmark, a national heat wave (hedebølge) is defined as a period of at least 3 consecutive days of which period the average maximum temperature across more than fifty percent of the country exceeds 28 °C (82.4 °F) – the Danish Meteorological Institute further defines a "warmth wave" (varmebølge) when the same criteria are met for a 25 °C (77.0 °F) temperature, while in Sweden, a heat wave is defined as at least five days in a row with a daily high exceeding 25 °C (77.0 °F).
In the United States, definitions also vary by region, usually meaning a period of at least two or more days of excessively hot weather. In the Northeast, a heat wave is typically defined as three consecutive days where the temperature reaches or exceeds 90 °F (32.2 °C), but not always as this ties in with humidity levels to determine a heat index threshold. 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 (37.8 °C) for three or more consecutive days over a wide area (tens of thousands of square miles). The National Weather Service issues heat advisories and excessive heat warnings when unusual periods of hot weather are expected.
In Adelaide, South Australia, a heat wave is defined as five consecutive days at or above 35 °C (95 °F), or three consecutive days at or over 40 °C (104 °F). The Australian Bureau of Meteorology defines a heat wave as "three days or more of maximum and minimum temperatures that are unusual for the location". Until the introduction of this new Pilot Heatwave Forecast there was no national definition that described heatwave or measures of heatwave severity.
In Greece, according to the Hellenic National Metereological Service, a heat wave is defined as three consecutive days at or above 39 °C (102 °F) and a minimum temperature in the same period at or over 26 °C (79 °F), with no winds or with weak winds, and the above conditions being observed in a broad area.
In the United Kingdom, the Met Office operates a Heat Health Watch system which places each Local Authority region into one of four levels. Heatwave conditions are defined by the maximum daytime temperature and minimum nighttime temperature rising above the threshold for a particular region. The length of time spent above that threshold determines the particular level. Level 1 is normal summer conditions. Level 2 is reached 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 is triggered 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 is associated with a particular state of readiness and response by the social and health services, and Level 4 is associated with more widespread response.
A more general indicator that allows comparing heat waves in different regions of the World, characterized by different climates, was published in 2015. This was used to estimate heat waves occurrence at the global scale from 1901 to 2010, finding a substantial and sharp increase in the number of affected areas in the last two decades.
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. 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. 
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.
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 coastline.
The role of soil moisture can also contribute to the intensification of heat waves in Europe. Low soil moisture leads to a number of complex feedback mechanisms, which can in turn result in increased surface temperatures. One of the main mechanisms is reduced evaporative cooling of the atmosphere. When water evaporates, it consumes energy and thus will lower the surrounding temperature. If the soil is very dry, then incoming radiation from the sun will warm the air with little or no cooling effect from moisture evaporating from the soil.
Main article: Effects of climate change § Heat waves and temperature extremes
Heatwaves over land have become more frequent and more intense since the 1950s due to climate change in almost all world regions. Furthermore, heat waves are more likely to occur simultaneously with droughts. Marine heatwaves have also increased in frequency, with a doubling since 1980. The intensity of individual heat waves can often be attributed to global warming. Some extreme events would have been nearly impossible without human influence on the climate system. A heatwave that would occur once every ten years before global warming started, now occurs 2.8 times as often. Under further warming, heatwaves are set to become more frequent. An event that would occur each ten year, would occur every other year if global warming reaches 2 °C.
Heat waves and droughts as a result, minimize ecosystem carbon uptake. Carbon uptake is also known as carbon sequestration. Extreme heat wave events are predicted to happen with increased global warming, which puts stress on ecosystems. Stress on ecosystems due to future intensified heat waves will reduce biological productivity. This will cause changes in the ecosystem's carbon cycle feedback because there will be less vegetation to hold the carbon from the atmosphere, which will only contribute more to atmospheric warming.
Policy makers, funders and researchers responding to the increasing heatwaves created the Extreme Heat Resilience Alliance coalition under the Atlantic Council to advocate for naming heatwaves, measuring them, and ranking them to build better awareness of their impacts.
Main article: List of heat waves
June 2019 was the hottest month on record worldwide, the effects of this were especially prominent in Europe. Increased wildfires in places such as Spain can also be attributed to heat waves.
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.
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.
A study that investigated 13,115 cities found that extreme heat exposure of a wet bulb globe temperature above 30 °C tripled between 1983 and 2016. It increased by ~50% when the population growth in these cities is not taken into account. Urban areas and living spaces are often significantly warmer than surrounding rural areas, partly due to the urban heat island effect. The researchers compiled a comprehensive inventory of past urban extreme heat events.
According to estimates of a 2022 study by the climate research group First Street Foundation, 107 million people in the US will experience extremely dangerous heat in the year 2053.
|80 °F (27 °C)||82 °F (28 °C)||84 °F (29 °C)||86 °F (30 °C)||88 °F (31 °C)||90 °F (32 °C)||92 °F (33 °C)||94 °F (34 °C)||96 °F (36 °C)||98 °F (37 °C)||100 °F (38 °C)||102 °F (39 °C)||104 °F (40 °C)||106 °F (41 °C)||108 °F (42 °C)||110 °F (43 °C)|
|40%||80 °F (27 °C)||81 °F (27 °C)||83 °F (28 °C)||85 °F (29 °C)||88 °F (31 °C)||91 °F (33 °C)||94 °F (34 °C)||97 °F (36 °C)||101 °F (38 °C)||105 °F (41 °C)||109 °F (43 °C)||114 °F (46 °C)||119 °F (48 °C)||124 °F (51 °C)||130 °F (54 °C)||136 °F (58 °C)|
|45%||80 °F (27 °C)||82 °F (28 °C)||84 °F (29 °C)||87 °F (31 °C)||89 °F (32 °C)||93 °F (34 °C)||96 °F (36 °C)||100 °F (38 °C)||104 °F (40 °C)||109 °F (43 °C)||114 °F (46 °C)||119 °F (48 °C)||124 °F (51 °C)||130 °F (54 °C)||137 °F (58 °C)|
|50%||81 °F (27 °C)||83 °F (28 °C)||85 °F (29 °C)||88 °F (31 °C)||91 °F (33 °C)||95 °F (35 °C)||99 °F (37 °C)||103 °F (39 °C)||108 °F (42 °C)||113 °F (45 °C)||118 °F (48 °C)||124 °F (51 °C)||131 °F (55 °C)||137 °F (58 °C)|
|55%||81 °F (27 °C)||84 °F (29 °C)||86 °F (30 °C)||89 °F (32 °C)||93 °F (34 °C)||97 °F (36 °C)||101 °F (38 °C)||106 °F (41 °C)||112 °F (44 °C)||117 °F (47 °C)||124 °F (51 °C)||130 °F (54 °C)||137 °F (58 °C)|
|60%||82 °F (28 °C)||84 °F (29 °C)||88 °F (31 °C)||91 °F (33 °C)||95 °F (35 °C)||100 °F (38 °C)||105 °F (41 °C)||110 °F (43 °C)||116 °F (47 °C)||123 °F (51 °C)||129 °F (54 °C)||137 °F (58 °C)|
|65%||82 °F (28 °C)||85 °F (29 °C)||89 °F (32 °C)||93 °F (34 °C)||98 °F (37 °C)||103 °F (39 °C)||108 °F (42 °C)||114 °F (46 °C)||121 °F (49 °C)||128 °F (53 °C)||136 °F (58 °C)|
|70%||83 °F (28 °C)||86 °F (30 °C)||90 °F (32 °C)||95 °F (35 °C)||100 °F (38 °C)||105 °F (41 °C)||112 °F (44 °C)||119 °F (48 °C)||126 °F (52 °C)||134 °F (57 °C)|
|75%||84 °F (29 °C)||88 °F (31 °C)||92 °F (33 °C)||97 °F (36 °C)||103 °F (39 °C)||109 °F (43 °C)||116 °F (47 °C)||124 °F (51 °C)||132 °F (56 °C)|
|80%||84 °F (29 °C)||89 °F (32 °C)||94 °F (34 °C)||100 °F (38 °C)||106 °F (41 °C)||113 °F (45 °C)||121 °F (49 °C)||129 °F (54 °C)|
|85%||85 °F (29 °C)||90 °F (32 °C)||96 °F (36 °C)||102 °F (39 °C)||110 °F (43 °C)||117 °F (47 °C)||126 °F (52 °C)||135 °F (57 °C)|
|90%||86 °F (30 °C)||91 °F (33 °C)||98 °F (37 °C)||105 °F (41 °C)||113 °F (45 °C)||122 °F (50 °C)||131 °F (55 °C)|
|95%||86 °F (30 °C)||93 °F (34 °C)||100 °F (38 °C)||108 °F (42 °C)||117 °F (47 °C)||127 °F (53 °C)|
|100%||87 °F (31 °C)||95 °F (35 °C)||103 °F (39 °C)||112 °F (44 °C)||121 °F (49 °C)||132 °F (56 °C)|
The heat index (as shown in the table above) is a measure of how hot it feels when relative humidity is factored with the actual air temperature. Hyperthermia, also known as heat stroke, becomes commonplace during periods of sustained high temperature and humidity. Older adults, very young children, and those who are sick or overweight are at a higher risk for heat-related illness. The chronically ill and elderly are often taking prescription medications (e.g., diuretics, anticholinergics, antipsychotics, and antihypertensives) that interfere with the body's ability to dissipate heat.
Heat edema presents as a transient swelling of the hands, feet, and ankles and is generally secondary to increased aldosterone secretion, which enhances water retention. When combined with peripheral vasodilation and venous stasis, the excess fluid accumulates in the dependent areas of the extremities. The heat edema usually resolves within several days after the patient becomes acclimated to the warmer environment. No treatment is required, although wearing support stockings and elevating the affected legs will help minimize the edema.
Heat rash, also known as prickly heat, is a maculopapular rash accompanied by acute inflammation and blocked sweat ducts. The sweat ducts may become dilated and may eventually rupture, producing small pruritic vesicles on an erythematous base. Heat rash affects areas of the body covered by tight clothing. If this continues for a duration of time it can lead to the development of chronic dermatitis or a secondary bacterial infection. Prevention is the best therapy. It is also advised to wear loose-fitting clothing in the heat. Once heat rash has developed, the initial treatment involves the application of chlorhexidine lotion to remove any desquamated skin. The associated itching may be treated with topical or systemic antihistamines. If infection occurs a regimen of antibiotics is required.
Heat cramps are painful, often severe, involuntary spasms of the large muscle groups used in strenuous exercise. Heat cramps tend to occur after intense exertion. They usually develop in people performing heavy exercise while sweating profusely and replenishing fluid loss with non-electrolyte containing water. This is believed to lead to hyponatremia that induces cramping in stressed muscles. Rehydration with salt-containing fluids provides rapid relief. Patients with mild cramps can be given oral .2% salt solutions, while those with severe cramps require IV isotonic fluids. The many sport drinks on the market are a good source of electrolytes and are readily accessible.
Heat syncope is related to heat exposure that produces orthostatic hypotension. This hypotension can precipitate a near-syncopal episode. Heat syncope is believed to result from intense sweating, which leads to dehydration, followed by peripheral vasodilation and reduced venous blood return in the face of decreased vasomotor control. Management of heat syncope consists of cooling and rehydration of the patient using oral rehydration therapy (sport drinks) or isotonic IV fluids. People who experience heat syncope should avoid standing in the heat for long periods of time. They should move to a cooler environment and lie down if they recognize the initial symptoms. Wearing support stockings and engaging in deep knee-bending movements can help promote venous blood return.
Heat exhaustion is considered by experts to be the forerunner of heat stroke (hyperthermia). It may even resemble heat stroke, with the difference being that the neurologic function remains intact. Heat exhaustion is marked by excessive dehydration and electrolyte depletion. Symptoms may include diarrhea, headache, nausea and vomiting, dizziness, tachycardia, malaise, and myalgia. Definitive therapy includes removing patients from the heat and replenishing their fluids. Most patients will require fluid replacement with IV isotonic fluids at first. The salt content is adjusted as necessary once the electrolyte levels are known. After discharge from the hospital, patients are instructed to rest, drink plenty of fluids for 2–3 hours, and avoid the heat for several days. If this advice is not followed it may then lead to heat stroke.
One public health measure taken during heat waves is the setting-up of air-conditioned public cooling centers.
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 hurricanes. The average annual number of fatalities directly attributed to heat in the United States is about 400. Federal data from 2020 estimates more than 1300 people die in the United States every year as a result of extreme heat. The 1995 Chicago heat wave, one of the worst in US history, led to approximately 739 heat-related deaths over a period of 5 days. Eric Klinenberg has noted that in the United States, the loss of human life in hot spells in summer exceeds that caused by all other weather events combined, including lightning, rain, floods, hurricanes, and tornadoes. Despite the dangers, Scott Sheridan, professor of geography at Kent State University, found that less than half of people 65 and older abide by heat-emergency recommendations such as drinking plenty of water. In his study of heat-wave behavior, focusing particularly on seniors in Philadelphia, Phoenix, Toronto, and Dayton, Ohio, he found that people over 65 "don't consider themselves seniors". One of his older respondents said: "Heat doesn't bother me much, but I worry about my neighbors."
According to the Agency for Health care Research and Quality, about 6,200 Americans are hospitalized each summer due to excessive heat, and those at highest risk are poor, uninsured or elderly. More than 70,000 Europeans died as a result of the 2003 European heat wave. Also more than 2,000 people died in Karachi, Pakistan in June 2015 due to a severe heat wave with temperatures as high as 49 °C (120 °F).
The concern now is focusing on predicting the future likelihood of heat waves and their severity. Additionally, due to the fact that in most of the world, most of those suffering the impacts of a heat wave will be inside a building. This will modify the temperatures they are exposed to, and as a result, there is the need to link climate models to building models. This means producing example time series of future weather. Other work has shown that future mortality due to heat waves could be reduced if buildings were better designed to modify the internal climate, or if the occupants were better educated about the issues, so they can take action in time.
The number of heat fatalities is likely highly underreported due to a lack of reports and misreports. When factoring in heat-related illnesses, actual death tolls linked to extreme heat may be six times as high as official figures, as suggested for California and Japan.
Part of the mortality observed during a heat wave can be attributed to short-term forward mortality displacement. It has been observed that for some heat waves, there is a compensatory decrease in overall mortality during the subsequent weeks after a heat wave. Such compensatory reductions in mortality suggest that heat affects especially those so ill that they "would have died in the short term anyway".
Another explanation for underreporting is the social attenuation in most contexts of heat waves as a health risk. As shown by the deadly French heat wave in 2003, heat wave dangers result from the intricate association of natural and social factors. Social invisibility is one such factor. In places where heat-related deaths often occur indoors, among elderly people living alone, it can be challenging to assign heat as a contributing factor.
In addition to physical stress, excessive heat causes psychological stress, to a degree which affects performance, and is also associated with an increase in violent crime. High temperatures are associated with increased conflict both at the interpersonal level and at the societal level. In every society, crime rates go up when temperatures go up, particularly violent crimes such as assault, murder, and rape. Furthermore, in politically unstable countries, high temperatures are an aggravating factor that lead toward civil wars.
Additionally, high temperatures have a significant effect on income. A study of counties in the United States found that economic productivity of individual days declines by about 1.7% for each degree Celsius above 15 °C (59 °F).
Further information: Ozone § Ozone air pollution, and Ozone § Low level ozone in urban areas
Ozone pollution in urban areas is especially concerning with increasing temperatures, raising heat-related mortality during heat waves. During heat waves in urban areas, ground level ozone pollution can be 20% higher than usual.
One study concluded that from 1860 to 2000, the global population-weighted fine particle concentrations increased by 5% and near-surface ozone concentrations by 2% due to climate change.
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 additive.
Main article: Effects of climate change on agriculture
Heat waves significantly threaten agricultural production. In 2019, heat waves in the Mulanje region of Malawi involved temperatures as high as 40 °C (104 °F). This and a late rain season resulted in significant tea leaf scorching and reduced yields.
If a heat wave occurs during a drought, which dries out vegetation, it can contribute to bushfires and wildfires. During the disastrous heat wave that struck Europe in 2003, fires raged through Portugal, destroying over 3,010 square kilometres (1,160 sq mi) or 301,000 hectares (740,000 acres) of forest and 440 square kilometres (170 sq mi) or 44,000 hectares (110,000 acres) of agricultural land and causing an estimated €1 billion worth of damage. High end farmlands have irrigation systems to back up crops with. Heat waves cause wildfires.
Heat waves can also contribute to severe flooding. The record-breaking heat wave that afflicted Pakistan beginning in May 2022 led to glacier melt and moisture flow, which were factors in the devastating floods that began in June and claimed over 1,100 lives.
Heat waves can and do cause roads and highways to buckle and melt, water lines to burst, and power transformers to detonate, causing fires. Heat waves can also damage rail roads, such as buckling and kinking rails, which can lead to slower traffic, delays, and even cancellations of service when rails are too dangerous to traverse by trains.
Heat waves often lead to electricity spikes due to increased air conditioning use, which can create power outages, exacerbating the problem. 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. The 2009 South Eastern Australia Heat Wave caused the city of Melbourne, Australia to experience some major power disruptions which left over half a million people without power as the heat wave blew transformers and overloaded a power grid.