Heat wave

{{Weather}}

There are several definitions of heat waves:

• The IPCC defines heatwave as "a period of abnormally hot weather, often defined with reference to a relative temperature threshold, lasting from two days to months."{{Cite web |title=Heat Waves Explained |date=20 May 2023 |url=https://compass.rauias.com/disaster-management/heat-waves-heat-dome/}}IPCC, 2022: Annex II: [https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_Annex-II.pdf Glossary] [Möller, V., R. van Diemen, J.B.R. Matthews, C. Méndez, S. Semenov, J.S. Fuglestvedt, A. Reisinger (eds.)]. In: [https://www.ipcc.ch/report/ar6/wg2/ Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [H.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.)]. Cambridge University Press, Cambridge, UK and New York, pp. 2897–2930, {{doi|10.1017/9781009325844.029}}.{{rp|2911}}
• A definition based on the 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 {{convert|5|C-change|0|lk=on}}, the normal period being 1961–1990.{{cite journal |last=Frich |first= A. |author2=L.V. Alexander |author3=P. Della-Marta |author4=B. Gleason |author5=M. Haylock |author6=A.M.G. Klein Tank |author7=T. Peterson |date=January 2002 |title=Observed coherent changes in climatic extremes during the second half of the twentieth century |journal=Climate Research |volume= 19 |pages=193–212 |url=http://www.vsamp.com/resume/publications/Frich_et_al.pdf |doi=10.3354/cr019193|bibcode=2002ClRes..19..193F |doi-access=free }} The same definition is used by the World Meteorological Organization.{{Cite web |title=Heat wave meteorology |url=https://www.britannica.com/science/heat-wave-meteorology |access-date=2019-04-01 |publisher=Encyclopedia Britannica }}
• A definition from the Glossary of Meteorology is:{{cite book |last=Glickman |first=Todd S. |title=Glossary of Meteorology |publisher=American Meteorological Society |date= 2000 |isbn=9781878220493 }} "A period of abnormally and uncomfortably hot and usually humid weather."
• Marine heatwaves are generally described as prolonged discrete periods of unusually warm sea surface temperatures in a specific region. At this time the most commonly accepted definition is that proposed by Hobday et. al.{{cite journal|last=Hobday |first=A.J. |display-authors=etal|date=2018|title=Categorizing and naming marine heatwaves. |journal=Oceanography |volume=31 |issue=2 |pages=162–173}} which refers to an algorithm that uses percentile values for temperatures, and defines a threshold set as the 90th percentile for a given day of the year, above which one can say a marine heatwave is occurring. This definition can be used with temperature data acquired anywhere in the world, allowing for comparisons across different observations and latitudes.

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 {{convert|28|°C|°F|1}}. The Danish Meteorological Institute also has a definition for a "warmth wave" (varmebølge). It defines this using the same criteria for a {{convert|25|°C|°F|1}} temperature.{{cite web|url=http://www.dmi.dk/dmi/danmark_faar_varme-_og_hedeboelge |title=Danmark får varme- og hedebølge |language=da |publisher=Danish Meteorological Institute |date=22 July 2008 |access-date=18 July 2013 |url-status=dead |archive-url=https://web.archive.org/web/20080723170544/http://www.dmi.dk/dmi/danmark_faar_varme-_og_hedeboelge |archive-date=23 July 2008 }} Sweden defines a heat wave as at least five days in a row with a daily high exceeding {{convert|25|°C|°F|1}}.{{cite web|url=http://www.smhi.se/kunskapsbanken/klimat/varmebolja-1.22372 |title=Värmebölja Klimat: Kunskapsbanken SMHI |language=sv |publisher=Smhi.se |access-date=17 July 2013}}

In Greece, the [http://www.hnms.gr/emy/el/ 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 {{convert|26|°C|°F}} 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 {{convert|25|°C|°F}}. During this period the maximum temperature in De Bilt must exceed {{convert|30|°C|°F}} 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.{{cite web |date=31 August 2011 |title=Heat-health watch |url=http://www.metoffice.gov.uk/public/weather/heat-health/ |access-date=17 July 2013 |publisher=Met Office}} The threshold for a heat wave occurs when there are at least three days above {{convert|25|°C|°F}} across much of the country. Greater London has a threshold of {{convert|28|°C|°F}}.{{cite web |date=26 May 2023 |title=What is a heatwave?|url=https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/temperature/heatwave |access-date=26 May 2023 |publisher=Met Office}}

In Ireland, a heat wave is defined as temperatures exceeding {{convert|25|°C|°F}} for five or more consecutive days.{{cite web |date=2 Jun 2023 |title=Summer Centre|url=https://www.met.ie/climate/summer-centre |access-date=1 Feb 2025 |publisher=Met Éireann}}

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.{{cite web |url=http://www.weather.gov/glossary/index.php?word=heat+wave |title=Glossary |publisher=NOAA's National Weather Service |date=25 June 2009 |access-date=17 July 2013}} In the Northeast, a heat wave is typically when the temperature reaches or exceeds {{convert|90|°F|°C|1}} for three or more 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.{{cite web |last=Singer |first=Stephen |title=Half the country wilts under unrelenting heat |url=http://old.news.yahoo.com/s/ap/us_hot_weather |publisher=Yahoo! News |url-status=dead |archive-url=https://archive.today/20120716024044/http://old.news.yahoo.com/s/ap/us_hot_weather |archive-date=16 July 2012 |df=dmy-all}} 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 {{convert|100|°F|°C|1}} for three or more consecutive days over a wide area (tens of thousands of square miles).{{cite web|title=Staying Cool and Safe|publisher=Pacific Gas and Electric Company|location=Oakland, California|date=24 March 2017|accessdate=26 June 2023|url=https://www.pge.com/includes/docs/pdfs/myhome/edusafety/seasonal/coolingcenters/staying_cooling_and_safe_english.pdf|archive-date=27 June 2023|archive-url=https://web.archive.org/web/20230627064913/https://www.pge.com/includes/docs/pdfs/myhome/edusafety/seasonal/coolingcenters/staying_cooling_and_safe_english.pdf|url-status=dead}} The National Weather Service issues heat advisories and excessive heat warnings when it expects unusual periods of hot weather.

In Canada, heat waves are defined using the daily maximum and minimum temperatures, and in most of the country, the humidex as well, exceeding a regional threshold for two or more days. The threshold in which daily maximum temperatures must exceed ranges between {{convert|28|°C|°F}} in Newfoundland and {{convert|35|°C|°F}} in interior British Columbia, though this threshold is much lower in Nunavut, ranging between {{convert|22|°C|°F}} and {{convert|26|°C|°F}}.{{cite web |date=31 Jul 2024|title=Criteria for public weather alerts|url=https://www.canada.ca/en/environment-climate-change/services/types-weather-forecasts-use/public/criteria-alerts.html#heat|access-date=1 Feb 2025 |publisher=Environment and Climate Change Canada}}

In Adelaide, South Australia, a heat wave is five consecutive days at or above {{convert|35|°C|°F}}, or three consecutive days at or over {{convert|40|°C|°F}}.{{cite web |url=http://www.bom.gov.au/announcements/media_releases/sa/20100115_First_Heatwave_SA_Jan.shtml |title=Extreme Heat Services for South Australia |publisher=Bureau of Meteorology |date=15 January 2010 |access-date=17 July 2013}} The Australian Bureau of Meteorology defines a heat wave as three or more days of unusual maximum and minimum temperatures.{{Cite web|title = Australia Weather and Warnings|url = http://www.bom.gov.au/weather-services/about/heatwave-forecast.shtml|publisher = Bureau of Meteorology|access-date = 2016-01-17|language = en|archive-date = 16 October 2015|archive-url = https://web.archive.org/web/20151016140923/http://www.bom.gov.au/weather-services/about/heatwave-forecast.shtml|url-status = dead}} Before this new Pilot Heatwave Forecast there was no national definition for heat waves or measures of heat wave severity.

In New Zealand, heat waves thresholds depend on local climatology, with the temperature threshold ranging between {{convert|27|°C|°F}} in Greymouth and {{convert|32|°C|°F}} in Gisborne.{{cite web |date=2024|title=Heat|url=https://getready.govt.nz/emergency/heat|access-date=1 Feb 2025 |publisher=Get Ready - National Emergency Management Agency}}

Marine heatwaves have become a prominent subject of research in recent years, reflecting the fact that since the turn of this century many ocean areas have experienced peaks of temperatures, along with more frequent, more intense, more prolonged warming events than ever met on record.Marine heatwaves in the Mediterranean Sea and beyond - an overview. 2024. pp. 5-24 in CIESM Monograph 51’’ (F. Briand, Ed.) ISSN 1726-5886 [https://www.researchgate.net/publication/384727584] The genesis of marine heatwaves is mainly driven by a combination of oceanic and atmospheric factors, often triggered by high pressure systems that will reduce cloud cover and increase solar absorption by the sea surface. Human-induced climate change appears bound to play a growing role in the development of marine heatwaves, with increasing impacts on marine ecosystems, such as mass mortality in benthic communities, coral bleaching events, disruptions in fishery catches, and shifts in species distributions.

{{Main|List of heat waves}}

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| image1= 1951+ Percent of global area at temperature records - Seasonal comparison - NOAA.svg |caption1= New high temperature records have outpaced new low temperature records on a growing portion of Earth's surface.{{cite web |title=Mean Monthly Temperature Records Across the Globe / Timeseries of Global Land and Ocean Areas at Record Levels for October from 1951-2023 |url=https://www.ncei.noaa.gov/access/monitoring/monthly-report/global/202310/supplemental/page-3 |publisher=National Centers for Environmental Information (NCEI) NCEI.NOAA.gov of the National Oceanic and Atmospheric Administration (NOAA)|archive-url=https://web.archive.org/web/20231116185412/https://www.ncei.noaa.gov/access/monitoring/monthly-report/global/202310/supplemental/page-3 |archive-date=16 November 2023 |date=November 2023 |url-status=live}} (change "202310" in URL to see years other than 2023, and months other than 10=October)

| image2= 20211109 Frequency of extreme weather for different degrees of global warming - bar chart IPCC AR6 WG1 SPM.svg |caption2= Large increases in both the frequency and intensity of extreme weather events (for increasing degrees of global warming) are expected.IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf Summary for Policymakers]. In: [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 3−32, {{doi|10.1017/9781009157896.001}}{{rp|18}}

}}[[File:Map of increasing heatwave trends over the midlatitudes and Europe.webp|thumb|upright=1.35|Map of increasing heat wave trends (frequency and cumulative intensity) over the midlatitudes and Europe, July–August 1979–2020{{cite journal |last1=Rousi |first1=Efi |last2=Kornhuber |first2=Kai |last3=Beobide-Arsuaga |first3=Goratz |last4=Luo |first4=Fei |last5=Coumou |first5=Dim |date=4 July 2022 |title=Accelerated western European heatwave trends linked to more-persistent double jets over Eurasia |journal=Nature Communications |volume=13 |issue=1 |pages=3851 |bibcode=2022NatCo..13.3851R |doi=10.1038/s41467-022-31432-y |pmc=9253148 |pmid=35788585 |doi-access=free}}

• News report: {{cite news |last1=Fountain |first1=Henry |date=18 July 2022 |title=Why Europe Is Becoming a Heat Wave Hot Spot |work=The New York Times |url=https://www.nytimes.com/2022/07/19/climate/europe-heat-wave-science.html |access-date=21 August 2022}}]]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.{{cite journal |last1=Russo |first1=Simone |last2=Sillmann |first2=Jana |last3=Fischer |first3=Erich M |year=2015 |title=Top ten European heatwaves since 1950 and their occurrence in the coming decades |url=https://www.research-collection.ethz.ch/bitstream/20.500.11850/109725/2/Russo_109725.pdf |journal=Environmental Research Letters |volume=10 |issue=12 |pages=124003 |bibcode=2015ERL....10l4003R |doi=10.1088/1748-9326/10/12/124003 |doi-access=free}} 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.{{cite journal |last1=Zampieri |first1=Matteo |last2=Russo |first2=Simone |last3=Di Sabatino |first3=Silvana |last4=Michetti |first4=Melania |last5=Scoccimarro |first5=Enrico |last6=Gualdi |first6=Silvio |year=2016 |title=Global assessment of heatwave magnitudes from 1901 to 2010 and implications for the river discharge of the Alps |journal=Science of the Total Environment |volume=571 |pages=1330–9 |bibcode=2016ScTEn.571.1330Z |doi=10.1016/j.scitotenv.2016.07.008 |pmid=27418520}}

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.{{cite news |last1=Henson |first1=Bob |title=Exposure to extreme urban heat has tripled worldwide since the 1980s, study finds |newspaper=Washington Post |url=https://www.washingtonpost.com/weather/2021/11/09/extreme-heat-exposure-urban-climate/ |access-date=15 November 2021}}{{cite journal |last1=Tuholske |first1=Cascade |last2=Caylor |first2=Kelly |last3=Funk |first3=Chris |last4=Verdin |first4=Andrew |last5=Sweeney |first5=Stuart |last6=Grace |first6=Kathryn |last7=Peterson |first7=Pete |last8=Evans |first8=Tom |date=12 October 2021 |title=Global urban population exposure to extreme heat |journal=Proceedings of the National Academy of Sciences |language=en |volume=118 |issue=41 |pages=e2024792118 |bibcode=2021PNAS..11824792T |doi=10.1073/pnas.2024792118 |issn=0027-8424 |pmc=8521713 |pmid=34607944 |doi-access=free}}

Heat waves form when a high pressure area at an altitude of {{convert|3000|-|7600|m|abbr=off}} strengthens and remains over a region for several days and up to several weeks.{{Cite web |url=https://www.weather.gov/jetstream/hi |title=NWS JetStream - Heat Index |publisher=US Department of Commerce NOAA weather.gov |access-date=2019-02-09}} 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.{{cite journal |journal=Journal of Climate |title=A Model Study of Heat Waves over North America: Meteorological Aspects and Projections for the Twenty-First Century |volume=25 |pages=4761–4784 |year=2012 |last1=Lau |first1=N |last2=Nath |first2=Mary Jo|issue=14 |doi=10.1175/JCLI-D-11-00575.1 |bibcode=2012JCli...25.4761L |doi-access=free }}{{cite web |title=Heat Index |publisher=US National Weather Service |url=http://www.srh.noaa.gov/jetstream/global/hi.htm}}

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.{{cite web|title=Heat Index |publisher=Pasquotank County, NC, U. S. Website |url=http://www.co.pasquotank.nc.us/departments/911/webpage/heatwaves.htm |url-status=dead |archive-url=https://web.archive.org/web/20120318154040/http://www.co.pasquotank.nc.us/departments/911/webpage/heatwaves.htm |archive-date=18 March 2012 }}

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.{{cite web |title=Bergwind Info |publisher=1stweather.com |url=http://www.1stweather.com/regional/education/bergwind.shtml |url-status=dead |archive-url=https://web.archive.org/web/20120415080019/http://www.1stweather.com/regional/education/bergwind.shtml |archive-date=15 April 2012 |df=dmy-all }}{{cite web|title=Natural Hazards - Heat Wave|publisher=City of Cape Town, South Africa Website|url=http://www.capetown.gov.za/en/DRM/Pages/HeatWave.aspx|url-status=dead|archive-url=https://web.archive.org/web/20120608111607/http://www.capetown.gov.za/en/DRM/Pages/HeatWave.aspx|archive-date=8 June 2012|df=dmy-all}}

The level of soil moisture can intensify heat waves in Europe.{{Cite journal |last1=Miralles |first1=D. G. |last2=van den Berg |first2=M. J. |last3=Teuling |first3=A. J. |last4=de Jeu |first4=R. A. M. |date=November 2012 |title=Soil moisture-temperature coupling: A multiscale observational analysis |url=https://doi.org/10.1029/2012GL053703 |journal=Geophysical Research Letters |volume=39 |issue=21 |pages=n/a |doi=10.1029/2012gl053703 |bibcode=2012GeoRL..3921707M |s2cid=53668167 |issn=0094-8276}}{{Cite journal |last1=Seneviratne |first1=Sonia I. |last2=Corti |first2=Thierry |last3=Davin |first3=Edouard L. |last4=Hirschi |first4=Martin |last5=Jaeger |first5=Eric B. |last6=Lehner |first6=Irene |last7=Orlowsky |first7=Boris |last8=Teuling |first8=Adriaan J. |date=2010-05-01 |title=Investigating soil moisture–climate interactions in a changing climate: A review |url=https://www.sciencedirect.com/science/article/pii/S0012825210000139 |journal=Earth-Science Reviews |language=en |volume=99 |issue=3 |pages=125–161 |doi=10.1016/j.earscirev.2010.02.004 |bibcode=2010ESRv...99..125S |issn=0012-8252|url-access=subscription }} 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. 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.

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The number of heat fatalities is probably highly underreported. This is due to a lack of reports and to misreporting.{{cite journal |last=Basu |first=Rupa |author2=Jonathan M. Samet |year=2002 |title=Relation between Elevated Ambient Temperature and Mortality: A Review of the Epidemiologic Evidence |journal=Epidemiologic Reviews |volume=24 |issue=2 |pages=190–202 |doi=10.1093/epirev/mxf007 |pmid=12762092 |doi-access=free}} 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{{Cite web |title=Heat waves are far deadlier than we think. How California neglects this climate threat |url=https://www.latimes.com/projects/california-extreme-heat-deaths-show-climate-change-risks/ |access-date=2022-09-04 |website=Los Angeles Times |date=7 October 2021 |language=en}} and Japan.{{Cite journal |last1=Fujibe |first1=Fumiaki |last2=Matsumoto |first2=Jun |date=2021 |title=Estimation of Excess Deaths during Hot Summers in Japan |url=https://www.jstage.jst.go.jp/article/sola/17/0/17_2021-038/_article/-char/ja |journal=Scientific Online Letters on the Atmosphere |volume=17 |pages=220–223 |bibcode=2021SOLA...17..220F |doi=10.2151/sola.2021-038 |s2cid=241577645 |doi-access=free}}

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.{{cite journal |last1=Huynen |first1=Maud M. T. E |last2=Martens |first2=Pim |last3=Schram |first3=Dieneke |last4=Weijenberg |first4=Matty P |last5=Kunst |first5=Anton E |year=2001 |title=The Impact of Heat Waves and Cold Spells on Mortality Rates in the Dutch Population |journal=Environmental Health Perspectives |volume=109 |issue=5 |pages=463–70 |doi=10.2307/3454704 |jstor=3454704 |pmc=1240305 |pmid=11401757}}

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.{{cite journal |last1=Poumadère |first1=M. |last2=Mays |first2=C. |last3=Le Mer |first3=S. |last4=Blong |first4=R. |year=2005 |title=The 2003 Heat Wave in France: Dangerous Climate Change Here and Now |url=http://www.coe.int/t/dg4/majorhazards/activites/murcia_26-27oct2009/heatwave2003_poumadere2005.pdf |journal=Risk Analysis |volume=25 |issue=6 |pages=1483–1494 |citeseerx=10.1.1.577.825 |doi=10.1111/j.1539-6924.2005.00694.x |pmid=16506977 |bibcode=2005RiskA..25.1483P |s2cid=25784074}} 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.{{Cite journal |last=Ro |first=Christine |date=2022-09-01 |title=Can Japan really reach "zero deaths" from heat stroke? |url=https://www.bmj.com/content/378/bmj.o2107 |journal=BMJ |language=en |volume=378 |pages=o2107 |doi=10.1136/bmj.o2107 |issn=1756-1833 |s2cid=251954370 |doi-access=free|url-access=subscription }}

{{HeatTable}}

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.{{cite journal |last=Simister |first=John |author2=Cary Cooper |title=Thermal stress in the U.S.A.: effects on violence and on employee behaviour |journal=Stress and Health |volume=21 |issue=1 |pages=3–15 |date=October 2004 |doi=10.1002/smi.1029 |doi-access=free }} 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.{{cite journal |last1=Hsiang |first1=Solomon |last2=Burke |first2=Marshall |last3=Miguel |first3=Edward |title=Climate and Conflict |journal=Annual Review of Economics |volume=7 |issue=1 |pages=577–617 |year=2015 |doi=10.1146/annurev-economics-080614-115430 |s2cid=17657019 }}

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 {{convert|15|°C|°F}}.{{cite journal |last1=Solomon |first1=Hsiang |last2=Tatyana |first2=Deryugina |title=Does the Environment Still Matter? Daily Temperature and Income in the United States |journal=NBER Working Paper No. 20750 |date=December 2014 |doi=10.3386/w20750 |doi-access=free }}

{{Further|Ozone#Ozone air pollution|Ozone#Low level ozone in urban areas}}High temperatures also make the effects of ozone pollution in urban areas worse. This raises heat-related mortality during heat waves.{{Cite journal |last1=Diem |first1=Jeremy E. |last2=Stauber |first2=Christine E. |last3=Rothenberg |first3=Richard |date=2017-05-16 |editor-last=Añel |editor-first=Juan A. |title=Heat in the southeastern United States: Characteristics, trends, and potential health impact |journal=PLOS ONE |volume=12 |issue=5 |pages=e0177937 |bibcode=2017PLoSO..1277937D |doi=10.1371/journal.pone.0177937 |issn=1932-6203 |pmc=5433771 |pmid=28520817 |doi-access=free}} During heat waves in urban areas, ground level ozone pollution can be 20 percent higher than usual.{{Cite journal |last1=Hou |first1=Pei |last2=Wu |first2=Shiliang |date=July 2016 |title=Long-term Changes in Extreme Air Pollution Meteorology and the Implications for Air Quality |journal=Scientific Reports |volume=6 |issue=1 |pages=23792 |bibcode=2016NatSR...623792H |doi=10.1038/srep23792 |issn=2045-2322 |pmc=4815017 |pmid=27029386}}

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.{{Cite journal |last1=Orru |first1=H. |last2=Ebi |first2=K. L. |last3=Forsberg |first3=B. |date=2017 |title=The Interplay of Climate Change and Air Pollution on Health |journal=Current Environmental Health Reports |volume=4 |issue=4 |pages=504–513 |doi=10.1007/s40572-017-0168-6 |issn=2196-5412 |pmc=5676805 |pmid=29080073|bibcode=2017CEHR....4..504O }}

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.{{cite journal |author=Kosatsky T. |date=July 2005 |title=The 2003 European heat waves |url=http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=552 |journal=Eurosurveillance |volume=10 |issue=7 |pages=3–4 |doi=10.2807/esm.10.07.00552-en |pmid=29208081 |access-date=14 January 2014 |doi-access=free}}

File:09 Aus heatwave map.PNG

Calculations from 2022 suggest that heat waves will shrink the global economy by about 1 percent decrease by the middle of the 21st century.{{Cite web |last=Benedek |first=Réfi |date=2022-07-12 |title=The cost of heatwaves |url=https://hypeandhyper.com/en/the-cost-of-heatwaves/ |access-date=2022-07-15 |publisher=HYPEANDHYPER }}{{Cite magazine |title=Rising Heat is Making it Harder to Work in the U.S. — the Costs for the Economy Will Soar with Climate Change |url=https://time.com/6093845/how-heat-hurts-the-economy/ |magazine=Time |access-date=2022-07-15}}

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.{{Cite journal |last1=Trainer |first1=Vera L. |last2=Moore |first2=Stephanie K. |last3=Hallegraeff |first3=Gustaaf |last4=Kudela |first4=Raphael M. |last5=Clement |first5=Alejandro |last6=Mardones |first6=Jorge I. |last7=Cochlan |first7=William P. |date=2020-01-01 |title=Pelagic harmful algal blooms and climate change: Lessons from nature's experiments with extremes |journal=Harmful Algae |series=Climate change and harmful algal blooms |volume=91 |pages=101591 |doi=10.1016/j.hal.2019.03.009 |issn=1568-9883|doi-access=free |pmid=32057339 |bibcode=2020HAlga..9101591T }}

{{Main|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 {{cvt|40|°C}}. This and a late rain season scorched tea leaves and reduced yields.{{Cite web |title=Malawi heatwaves threaten tea yields and livelihoods |publisher=Future Climate Africa |url=https://futureclimateafrica.org/news/malawi-heatwaves-threaten-tea-yields-and-livelihoods/ |access-date=2020-09-24 }}

{{excerpt|Effects of climate change on livestock#Health impacts of heat stress}}

Heat waves cause roads and highways to buckle and melt,{{Cite news |date=15 July 2013 |title=When does tarmac melt? |url=https://www.bbc.com/news/magazine-23315384 |publisher=BBC News}} 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.{{cite news |last=Doan |first=Lynn |author2=Covarrubias, Amanda |date=27 July 2006 |title=Heat Eases, but Thousands of Southern Californians Still Lack Power |url=http://www.latimes.com/news/local/la-me-heat27jul27,1,4111447.story |access-date=16 June 2014 |work=Los Angeles Times}}

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 {{convert|3010|km2|sqmi}} of forest and {{convert|440|km2|sqmi}} of agricultural land. They caused about €1 billion worth of damage.{{cite news |last=Bell |first=M. |author2=A. Giannini |author3=E. Grover |author4=M. Hopp |author5=B. Lyon |author6=A. Seth |date=September 2003 |title=Climate Impacts |work=IRI Climate Digest |publisher=The Earth Institute |url=http://iri.columbia.edu/climate/cid/Sep2003/impacts.html |access-date=28 July 2006}} 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.{{Cite journal |last1=Sauter |first1=Christoph |last2=Fowler |first2=Hayley J. |last3=Westra |first3=Seth |last4=Ali |first4=Haider |last5=Peleg |first5=Nadav |last6=White |first6=Christopher J. |date=2023-06-01 |title=Compound extreme hourly rainfall preconditioned by heatwaves most likely in the mid-latitudes |url=https://www.sciencedirect.com/science/article/pii/S2212094723000166 |journal=Weather and Climate Extremes |volume=40 |pages=100563 |doi=10.1016/j.wace.2023.100563 |bibcode=2023WCE....4000563S |issn=2212-0947}} 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.{{Cite web |last1=Clarke |first1=Ben |last2=Otto |first2=Friederike |last3=Harrington |first3=Luke |title=Pakistan floods: what role did climate change play? |url=http://theconversation.com/pakistan-floods-what-role-did-climate-change-play-189833 |access-date=2022-09-04 |publisher=The Conversation |date=2 September 2022 }}

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.{{Cite journal |last1=Murali |first1=Gopal |last2=Iwamura |first2=Takuya |last3=Meiri |first3=Shai |last4=Roll |first4=Uri |date=March 2023 |title=Future temperature extremes threaten land vertebrates |url=https://www.nature.com/articles/s41586-022-05606-z |journal=Nature |language=en |volume=615 |issue=7952 |pages=461–467 |doi=10.1038/s41586-022-05606-z |pmid=36653454 |bibcode=2023Natur.615..461M |issn=1476-4687|url-access=subscription }} 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.{{Cite web |title=Physiological Optima and Critical Limits |work= Scitable |url=https://www.nature.com/scitable/knowledge/library/physiological-optima-and-critical-limits-45749376/ |access-date=2024-04-19 |publisher=Nature |last1=Miller|first1=N. A. |last2=Stillman|first2=J. H. |language=en}}{{Citation |last=Cullum |first=A. J. |title=Tolerance Range |date=2008-01-01 |encyclopedia=Encyclopedia of Ecology (Second Edition) |pages=640–646 |editor-last=Fath |editor-first=Brian |url=https://www.sciencedirect.com/science/article/pii/B9780444637680005564 |access-date=2024-04-26 |place=Oxford |publisher=Elsevier |isbn=978-0-444-64130-4}} The species with sufficient genetic variation will be able to ensure some individuals can survive frequent days of high temperatures in the future.{{Cite web |date=2024-04-19 |title=Extreme heat triggers mass die-offs and stress for wildlife in the West |url=https://www.nationalgeographic.com/animals/article/extreme-heat-triggers-mass-die-offs-and-stress-for-wildlife-in-the-west |access-date=2024-04-19 |website=Animals |language=en}}

Marine heatwaves may cause mass mortality in fish populations, especially for species that are better adapted to cooler temperatures.{{Cite journal |last1=Smith |first1=Kathryn E. |last2=Burrows |first2=Michael T. |last3=Hobday |first3=Alistair J. |last4=King |first4=Nathan G. |last5=Moore |first5=Pippa J. |last6=Sen Gupta |first6=Alex |last7=Thomsen |first7=Mads S. |last8=Wernberg |first8=Thomas |last9=Smale |first9=Dan A. |date=2023-01-16 |title=Biological Impacts of Marine Heatwaves |url=https://www.annualreviews.org/doi/10.1146/annurev-marine-032122-121437 |journal=Annual Review of Marine Science |language=en |volume=15 |issue=1 |pages=119–145 |doi=10.1146/annurev-marine-032122-121437 |bibcode=2023ARMS...15..119S |issn=1941-1405|hdl=11250/3095845 |hdl-access=free }} 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.{{Cite journal |last1=Smale |first1=Dan A. |last2=Wernberg |first2=Thomas |last3=Oliver |first3=Eric C. J. |last4=Thomsen |first4=Mads |last5=Harvey |first5=Ben P. |last6=Straub |first6=Sandra C. |last7=Burrows |first7=Michael T. |last8=Alexander |first8=Lisa V. |last9=Benthuysen |first9=Jessica A. |last10=Donat |first10=Markus G. |last11=Feng |first11=Ming |last12=Hobday |first12=Alistair J. |last13=Holbrook |first13=Neil J. |last14=Perkins-Kirkpatrick |first14=Sarah E. |last15=Scannell |first15=Hillary A. |date=April 2019 |title=Marine heatwaves threaten global biodiversity and the provision of ecosystem services |url=https://www.nature.com/articles/s41558-019-0412-1 |journal=Nature Climate Change |language=en |volume=9 |issue=4 |pages=306–312 |doi=10.1038/s41558-019-0412-1 |bibcode=2019NatCC...9..306S |issn=1758-6798|hdl=2160/3a9b534b-03ab-4619-9637-2ab06054fe70 |hdl-access=free }}

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.{{Cite news |last=Kaufman |first=Leslie |date=2011-05-23 |title=A City Prepares for a Warm Long-Term Forecast |url=https://www.nytimes.com/2011/05/23/science/earth/23adaptation.html |access-date=2023-02-08 |work=The New York Times |issn=0362-4331}} 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.{{Cite web |title=Extreme Heat Resilience Alliance: Reducing Extreme Heat Risk for Vulnerable People |url=https://wcr.ethz.ch/news-and-events/EHRA.html |url-status=dead |archive-url=https://web.archive.org/web/20200821212642/https://wcr.ethz.ch/news-and-events/EHRA.html |archive-date=21 August 2020 |access-date=2020-09-02 |publisher=wcr.ethz.ch}}{{Cite web |title=The world's getting hotter. Can naming heat waves raise awareness of the risks? |date=19 August 2020 |url=https://www.pri.org/stories/2020-08-19/world-s-getting-hotter-can-naming-heat-waves-raise-awareness-risks |access-date=2020-09-02 |publisher=The World from PRX}}

{{Main|List of heat waves}}

{{excerpt|List of heat waves#2024}}

{{excerpt|2024 Indian heat wave}}

{{excerpt|2024 Southeast Asia heat wave}}

A study found the average resident in China was exposed to 16 days of heat waves in 2023, with more than 37,000 heat wave-related deaths. Besides, the number of work hours lost due to heat stress in China was 36.9 billion in 2023, and China’s citizens experienced a 60% surge in lost safe outdoor activity hours, with each person losing 2.2 hours on average each day. The study predicted that by the 2060s, annual heat wave-related mortality is expected to reach 29,000 to 38,000 in China, with a 28% to 37% increase in work hours lost.{{cite web|author=Ding Rui|work=Sixth Tone|url=https://www.sixthtone.com/news/1016149|title=Heat Wave-Related Deaths Soared in China in 2023, Report Finds|date=November 7, 2024}}

{{ multiple image | total_width=450

| image1 = 1960- Heat wave indicators - US.svg |caption1= US heat waves have increased in frequency, average duration, and intensity.

Also, heat wave seasons have grown in length.{{cite web |title=Climate Change Indicators: Heat Waves |url=https://www.epa.gov/climate-indicators/climate-change-indicators-heat-waves |publisher=U.S. Environmental Protection Agency (EPA) |archive-url=https://web.archive.org/web/20241007114317/https://www.epa.gov/climate-indicators/climate-change-indicators-heat-waves |archive-date=7 October 2024 |date=June 2024 |url-status=live }} EPA cites data source: NOAA, 2024.

| image2 = 1960- Annual average number of days spent in heat waves - US.svg |caption2= Over decades, the average number of days spent in heat waves in the U.S. annually has increased, based on increases in both the average annual number of heat waves and on their average durations.

}}

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.{{cite news |last1=Rosane |first1=Olivia |title=50 Million Americans Are Currently Living Under Some Type of Heat Warning |publisher=Ecowatch |url=https://www.ecowatch.com/heat-wave-central-eastern-us-2639217633.html |access-date=19 July 2019}}

According to a 2022 study, 107 million people in the US will experience extremely dangerous heat in the year 2053.{{Cite web |last1=Miller |first1=Brandon |last2=Waldrop |first2=Theresa |date=2022-08-16 |title=An 'extreme heat belt' will impact over 100 million Americans in the next 30 years, study finds |url=https://www.cnn.com/2022/08/15/weather/extreme-heat-belt-us-impact-study-climate/index.html |access-date=2022-08-22 |publisher=CNN}}

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.{{cite web |title=Hot Weather Tips and the Chicago Heat Plan |url=http://chicago.about.com/library/blank/bl_hot_weather_tips.htm |url-status=dead |archive-url=https://web.archive.org/web/20060621110942/http://chicago.about.com/library/blank/bl_hot_weather_tips.htm |archive-date=21 June 2006 |access-date=27 July 2006 |work=About.com}} 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.[http://www.annals.org/cgi/content/abstract/129/3/173 Near-Fatal Heat Stroke during the 1995 Heat Wave in Chicago]. Annals of Internal Medicine Vol. 129 Issue 3 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.{{cite book |last=Klinenberg |first=Eric |title=Heat Wave: A Social Autopsy of Disaster in Chicago |publisher=Chicago University Press |year=2002 |isbn=9780226443218}}[http://slate.com/id/2068612/ Dead Heat: Why don't Americans sweat over heat-wave deaths?] By Eric Klinenberg. Slate.com. Posted Tuesday, 30 July 2002

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.[http://newswise.com/articles/view/542519/ Most People Struck Down by Summer Heat Are Poor] Newswise, Retrieved on 9 July 2008.

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.{{cite journal |author1=Robert E. Davis |author2=Paul C. Knappenberger |author3=Patrick J. Michaels |author4=Wendy M. Novicoff |date=November 2003 |title=Changing heat-related mortality in the United States |journal=Environmental Health Perspectives |volume=111 |issue=14 |pages=1712–1718 |doi=10.1289/ehp.6336 |pmc=1241712 |pmid=14594620|bibcode=2003EnvHP.111.1712D }}

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• List of heat waves
• List of severe weather phenomena
• Urban heat island

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