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A heat wave, or heatwave, is a period of excessively hot weather, which may be accompanied by high
humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...
, especially in
oceanic climate An oceanic climate, also known as a marine climate, is the humid temperate climate sub-type in Köppen classification ''Cfb'', typical of west coasts in higher middle latitudes of continents, generally featuring cool summers and mild winters ( ...
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 Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...
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 Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat than it dissipates. When extreme ...
, increased risk of
wildfires A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identif ...
in areas with
drought A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D.  Jiang, A.  Khan, W.  Pokam Mba, D.  Rosenfeld, J. Tierney, an ...
, and widespread power outages due to increased use of air conditioning. A heat wave is considered
extreme weather Extreme weather or extreme climate events includes unexpected, unusual, severe, or unseasonal weather; weather at the extremes of the historical distribution—the range that has been seen in the past. Often, extreme events are based on a locat ...
, 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 Forecasting is the process of making predictions based on past and present data. Later these can be compared (resolved) against what happens. For example, a company might estimate their revenue in the next year, then compare it against the actual ...
instruments so that a warning can be issued. According to the
IPCC The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to advance scientific knowledge about climate change caused by human activities. The World Meteorological Organization (WMO) a ...
, heatwaves have become more frequent, and over land more intense, almost everywhere since the 1950s, due to
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
. 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.


Definitions

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 , the normal period being 1961–1990. A formal,
peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work (peers). It functions as a form of self-regulation by qualified members of a profession within the relevant field. Peer review ...
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 . More realistically, the comfort criteria for any one region are dependent upon the normal conditions of that area. /nowiki>Robert_DeCourcy_Ward.html" ;"title="Robert_DeCourcy_Ward.html" ;"title="/nowiki>Robert DeCourcy Ward">/nowiki>Robert DeCourcy Ward">Robert_DeCourcy_Ward.html" ;"title="/nowiki>Robert DeCourcy Ward">/nowiki>Robert DeCourcy Ward/nowiki>
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 De Bilt () is a municipality and town in the province of Utrecht, Netherlands. It had a population of in . De Bilt houses the headquarters of the Royal Netherlands Meteorological Institute (KNMI). It is the ancestral home and namesake for the pro ...
exceeds , provided that on at least three days in this period the maximum temperature in De Bilt exceeds . This definition of a heat wave is also used in Belgium and
Luxembourg Luxembourg ( ; lb, Lëtzebuerg ; french: link=no, Luxembourg; german: link=no, Luxemburg), officially the Grand Duchy of Luxembourg, ; french: link=no, Grand-Duché de Luxembourg ; german: link=no, Großherzogtum Luxemburg is a small lan ...
. 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 – the
Danish Meteorological Institute The Danish Meteorological Institute (DMI; da, Danmarks Meteorologiske Institut) is the official Danish meteorological institute, administrated by the Ministry of Energy, Utilities and Climate. The institute makes weather forecasts and observati ...
further defines a "warmth wave" (''varmebølge'') when the same criteria are met for a temperature, while in Sweden, a heat wave is defined as at least five days in a row with a daily high exceeding . 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 The points of the compass are a set of horizontal, radially arrayed compass directions (or azimuths) used in navigation and cartography. A compass rose is primarily composed of four cardinal directions—north, east, south, and west—each se ...
, a heat wave is typically defined as three consecutive days where the temperature reaches or exceeds , 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 for three or more consecutive days over a wide area (tens of thousands of square miles). The
National Weather Service The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...
issues heat advisories and
excessive heat warning An excessive heat warning is a notice issued by the National Weather Service of the United States within 12 hours of the heat index reaching one of two criteria levels. In most areas, a warning will be issued if there is a heat index of at least f ...
s when unusual periods of hot weather are expected. In
Adelaide Adelaide ( ) is the capital city of South Australia, the state's largest city and the fifth-most populous city in Australia. "Adelaide" may refer to either Greater Adelaide (including the Adelaide Hills) or the Adelaide city centre. The dem ...
,
South Australia South Australia (commonly abbreviated as SA) is a state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Australia's states and territories ...
, a heat wave is defined as five consecutive days at or above , or three consecutive days at or over . 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 th
Hellenic National Metereological Service
a heat wave is defined as three consecutive days at or above and a minimum temperature in the same period at or over , with no winds or with weak winds, and the above conditions being observed in a broad area. In the United Kingdom, the
Met Office The Meteorological Office, abbreviated as the Met Office, is the United Kingdom's national weather service. It is an executive agency and trading fund of the Department for Business, Energy and Industrial Strategy and is led by CEO Penelope E ...
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.


Formation

Heat waves form when high pressure aloft (from ) 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 Jet streams are fast flowing, narrow, meandering thermal wind, air currents in the Atmosphere of Earth, atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are west ...
'follows the sun'. On the equator side of the jet stream, in the upper layers of the atmosphere, is the
high pressure area A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interpl ...
. 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 Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
and preventing the formation of clouds. Reduction of clouds increases
shortwave radiation Shortwave radiation (SW) is radiant energy with wavelengths in the visible (VIS), near-ultraviolet (UV), and near-infrared (NIR) spectra. There is no standard cut-off for the near-infrared range; therefore, the shortwave radiation range is also v ...
reaching the surface. A
low pressure In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather (such as cloudy, windy, with possible ...
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 A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interpl ...
originating in the
Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an oceanic basin, ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of ...
becomes stationary just off the Atlantic Seaboard (typically known as a
Bermuda High The Azores High also known as North Atlantic (Subtropical) High/Anticyclone or the Bermuda-Azores High, is a large subtropical semi-permanent centre of high atmospheric pressure typically found south of the Azores in the Atlantic Ocean, at the Ho ...
). 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 The Western Cape is a province of South Africa, situated on the south-western coast of the country. It is the fourth largest of the nine provinces with an area of , and the third most populous, with an estimated 7 million inhabitants in 2020 ...
Province of
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
, a heat wave can occur when a
low pressure In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather (such as cloudy, windy, with possible ...
offshore and
high pressure In science and engineering the study of high pressure examines its effects on materials and the design and construction of devices, such as a diamond anvil cell, which can create high pressure. By ''high pressure'' is usually meant pressures of th ...
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.


Climate change

Heatwaves over land have become more frequent and more intense since the 1950s due to
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
in almost all world regions. Furthermore, heat waves are more likely to occur simultaneously with droughts.
Marine heatwaves A marine heatwave (MHW) is a period of abnormally high temperatures relative to the average seasonal temperature in a particular region of a sea or ocean. Marine heatwaves are caused by a variety of factors, including shorter term weather phenomena ...
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 Earth's climate system is a complex system having five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things). '' ...
. 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
drought A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D.  Jiang, A.  Khan, W.  Pokam Mba, D.  Rosenfeld, J. Tierney, an ...
s as a result, minimize ecosystem carbon uptake. Carbon uptake is also known as
carbon sequestration Carbon sequestration is the process of storing carbon in a carbon pool. Carbon dioxide () is naturally captured from the atmosphere through biological, chemical, and physical processes. These changes can be accelerated through changes in land ...
. Extreme heat wave events are predicted to happen with increased
global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
, 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 The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
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 The Atlantic Council is an American think tank in the field of international affairs, favoring Atlanticism, founded in 1961. It manages sixteen regional centers and functional programs related to international security and global economic prosp ...
to advocate for naming heatwaves, measuring them, and ranking them to build better awareness of their impacts.


Examples

June 2019 was the hottest month on record worldwide, the effects of this were especially prominent in Europe. Increased
wildfires A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identif ...
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 , 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 The wet-bulb globe temperature (WBGT) is a type of apparent temperature used to estimate the effect of temperature, humidity, wind speed (wind chill), and visible and infrared radiation (usually sunlight) on humans. It is used by industrial h ...
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 An urban heat island (UHI) is an urban or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference is usually larger at night than during the day, and is most apparen ...
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.


Health effects

The
heat index The heat index (HI) is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The result is als ...
(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 Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat than it dissipates. When extreme ...
, 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.,
diuretic A diuretic () is any substance that promotes diuresis, the increased production of urine. This includes forced diuresis. A diuretic tablet is sometimes colloquially called a water tablet. There are several categories of diuretics. All diuretics in ...
s,
anticholinergic Anticholinergics (anticholinergic agents) are substances that block the action of the neurotransmitter called acetylcholine (ACh) at synapses in the central and peripheral nervous system. These agents inhibit the parasympathetic nervous system ...
s,
antipsychotic Antipsychotics, also known as neuroleptics, are a class of Psychiatric medication, psychotropic medication primarily used to manage psychosis (including delusions, hallucinations, paranoia or disordered thought), principally in schizophrenia but ...
s, and
antihypertensive Antihypertensives are a class of drugs that are used to treat hypertension (high blood pressure). Antihypertensive therapy seeks to prevent the complications of high blood pressure, such as stroke and myocardial infarction. Evidence suggests tha ...
s) that interfere with the body's ability to dissipate heat. Heat
edema Edema, also spelled oedema, and also known as fluid retention, dropsy, hydropsy and swelling, is the build-up of fluid in the body's Tissue (biology), tissue. Most commonly, the legs or arms are affected. Symptoms may include skin which feels t ...
presents as a transient swelling of the hands, feet, and ankles and is generally secondary to increased
aldosterone Aldosterone is the main mineralocorticoid steroid hormone produced by the zona glomerulosa of the adrenal cortex in the adrenal gland. It is essential for sodium conservation in the kidney, salivary glands, sweat glands, and colon. It plays a c ...
secretion 440px Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical ...
, which enhances water retention. When combined with peripheral vasodilation and
venous stasis Venostasis, or venous stasis, is a condition of slow blood flow in the veins, usually of the legs. Presentation Complications Potential complications of venous stasis are: * Venous ulcers * Blood clot formation in veins (venous thrombosis), tha ...
, 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 Miliaria, also called heat rash, sweat rash, or prickly heat, is a skin disease marked by small, itchy rashes due to sweat trapped under the skin by clogged sweat-gland ducts. Miliaria is a common ailment in hot and humid conditions, such as in ...
, also known as prickly heat, is a
maculopapular rash A maculopapular rash is a type of rash characterized by a flat, red area on the skin that is covered with small confluent bumps. It may only appear red in lighter-skinned people. The term "maculopapular" is a compound: ''macules'' are small, flat ...
accompanied by acute inflammation and blocked sweat ducts. The sweat ducts may become dilated and may eventually rupture, producing small pruritic vesicles on an
erythema Erythema (from the Greek , meaning red) is redness of the skin or mucous membranes, caused by hyperemia (increased blood flow) in superficial capillaries. It occurs with any skin injury, infection, or inflammation. Examples of erythema not assoc ...
tous 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 Dermatitis is inflammation of the skin, typically characterized by itchiness, redness and a rash. In cases of short duration, there may be small blisters, while in long-term cases the skin may become thickened. The area of skin involved can v ...
or a secondary
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
l 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 Chlorhexidine (CHX) (commonly known by the salt forms chlorhexidine gluconate and chlorhexidine digluconate (CHG) or chlorhexidine acetate) is a disinfectant and antiseptic that is used for skin disinfection before surgery and to sterilize surg ...
lotion to remove any desquamated skin. The associated itching may be treated with topical or systemic antihistamines. If infection occurs a regimen of
antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...
s is required.
Heat cramps Heat cramps, a type of heat illness, are muscle spasms that result from loss of large amount of salt and water through exercise. Heat cramps are associated with cramping in the abdomen, arms and calves. This can be caused by inadequate consumption ...
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 Hyponatremia or hyponatraemia is a low concentration of sodium in the blood. It is generally defined as a sodium concentration of less than 135 mmol/L (135 mEq/L), with severe hyponatremia being below 120 mEq/L. Symptoms can be abs ...
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
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
s and are readily accessible.
Heat syncope Heat syncope is fainting or dizziness as a result of overheating (''syncope'' is the medical term for fainting). It is a type of heat illness. The basic symptom of heat syncope is fainting, with or without mental confusion. Heat syncope is caus ...
is related to heat exposure that produces
orthostatic hypotension Orthostatic hypotension, also known as postural hypotension, is a medical condition wherein a person's blood pressure drops when standing up or sitting down. Primary orthostatic hypertension is also often referred to as neurogenic orthostatic hyp ...
. This hypotension can precipitate a near-syncopal episode. Heat syncope is believed to result from intense sweating, which leads to
dehydration In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...
, 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 Oral rehydration therapy (ORT) is a type of fluid replacement used to prevent and treat dehydration, especially due to diarrhea. It involves drinking water with modest amounts of sugar and salts, specifically sodium and potassium. Oral rehydrati ...
(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 Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat than it dissipates. When extreme ...
). 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 Diarrhea, also spelled diarrhoea, is the condition of having at least three loose, liquid, or watery bowel movements each day. It often lasts for a few days and can result in dehydration due to fluid loss. Signs of dehydration often begin wi ...
,
headache Headache is the symptom of pain in the face, head, or neck. It can occur as a migraine, tension-type headache, or cluster headache. There is an increased risk of depression in those with severe headaches. Headaches can occur as a result ...
,
nausea Nausea is a diffuse sensation of unease and discomfort, sometimes perceived as an urge to vomit. While not painful, it can be a debilitating symptom if prolonged and has been described as placing discomfort on the chest, abdomen, or back of the ...
and
vomiting Vomiting (also known as emesis and throwing up) is the involuntary, forceful expulsion of the contents of one's stomach through the mouth and sometimes the Human nose, nose. Vomiting can be the result of ailments like Food-poisoning, foo ...
,
dizziness Dizziness is an imprecise term that can refer to a sense of disorientation in space, vertigo, or lightheadedness. It can also refer to disequilibrium or a non-specific feeling, such as giddiness or foolishness. Dizziness is a common medical c ...
,
tachycardia Tachycardia, also called tachyarrhythmia, is a heart rate that exceeds the normal resting rate. In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults. Heart rates above the resting rate may be normal (su ...
,
malaise As a medical term, malaise is a feeling of general discomfort, uneasiness or lack of wellbeing and often the first sign of an infection or other disease. The word has existed in French since at least the 12th century. The term is often used ...
, and
myalgia Myalgia (also called muscle pain and muscle ache in layman's terms) is the medical term for muscle pain. Myalgia is a symptom of many diseases. The most common cause of acute myalgia is the overuse of a muscle or group of muscles; another likel ...
. 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.


Mortality

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 A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
. 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 The July 1995 Chicago heat wave led to 739 heat-related deaths in Chicago over a period of five days. Most of the victims of the heat wave were elderly poor residents of the city, who could not afford air conditioning and did not open windows or ...
, 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 Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...
,
rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
,
flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
s,
hurricane A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
s, and
tornado A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
es.
Dead Heat: Why don't Americans sweat over heat-wave deaths?
' By Eric Klinenberg. Slate.com. Posted Tuesday, 30 July 2002
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."

' By Amanda Ripley. Time. 28 August 2006.
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 The 2003 European heat wave saw the hottest summer recorded in Europe since at least 1540. France was hit especially hard. The heat wave led to health crises in several countries and combined with drought to create a crop shortfall in parts of S ...
. Also more than 2,000 people died in
Karachi Karachi (; ur, ; ; ) is the most populous city in Pakistan and 12th most populous city in the world, with a population of over 20 million. It is situated at the southern tip of the country along the Arabian Sea coast. It is the former cap ...
, Pakistan in June 2015 due to a severe
heat wave 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 ...
with temperatures as high as . 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.


Underreporting and displacement

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 In epidemiology, mortality displacement is the occurrence of deaths at an earlier time than they would have otherwise occurred, meaning the deaths are ''displaced'' from the future into the present. The displacement may be described as the resul ...
. 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.


Psychological and sociological effects

In addition to physical
stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
, 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 In ordinary language, a crime is an unlawful act punishable by a State (polity), state or other authority. The term ''crime'' does not, in modern criminal law, have any simple and universally accepted definition,Farmer, Lindsay: "Crime, definit ...
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 In the United States, a county is an administrative or political subdivision of a state that consists of a geographic region with specific boundaries and usually some level of governmental authority. The term "county" is used in 48 states, wh ...
found that economic
productivity Productivity is the efficiency of production of goods or services expressed by some measure. Measurements of productivity are often expressed as a ratio of an aggregate output to a single input or an aggregate input used in a production proces ...
of individual days declines by about 1.7% for each degree Celsius above .


Surface ozone (air pollution)

Ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
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 Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
and heat during the European heat waves in 2003, concluded that these appear to be additive.


Leaf scorching

Heat waves significantly threaten agricultural production. In 2019, heat waves in the
Mulanje Mulanje, formerly called Mlanje, is a town in the Southern Region of Malawi, close to the border with Mozambique, to the east. It is near the Mulanje Massif. Location The town of Mulanje lies along the M-2 highway from Thyolo to the west to t ...
region of
Malawi Malawi (; or aláwi Tumbuka: ''Malaŵi''), officially the Republic of Malawi, is a landlocked country in Southeastern Africa that was formerly known as Nyasaland. It is bordered by Zambia to the west, Tanzania to the north and northeast ...
involved temperatures as high as . This and a late rain season resulted in significant tea leaf scorching and reduced yields.


Wildfires

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 or of forest and or of agricultural land and causing an estimated
The euro sign () is the currency sign used for the euro, the official currency of the eurozone and unilaterally adopted by Kosovo and Montenegro. The design was presented to the public by the European Commission on 12 December 1996. It consists ...
1 billion worth of damage. High end
farmland Agricultural land is typically land ''devoted to'' agriculture, the systematic and controlled use of other forms of lifeparticularly the rearing of livestock and production of cropsto produce food for humans. It is generally synonymous with bot ...
s have irrigation systems to back up
crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydroponic ...
s with. Heat waves cause wildfires.


Floods

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.


Infrastructural damage

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.


Power outages

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
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s failed, leaving thousands without power for as long as five days. The 2009 South Eastern Australia Heat Wave caused the city of
Melbourne, Australia Melbourne ( ; Boonwurrung/Woiwurrung: ''Narrm'' or ''Naarm'') is the capital and most populous city of the Australian state of Victoria, and the second-most populous city in both Australia and Oceania. Its name generally refers to a metropol ...
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.


See also

* Cold wave * List of heat waves * List of severe weather phenomena


References


External links


Social & Economic Costs of Temperature Extremes
from NOAA Socioeconomics website initiative, National Centers for Environmental Information {{DEFAULTSORT:Heat Wave Heat waves, Meteorological phenomena Natural disasters Torridness Weather events Weather hazards