View source for Earth:Heat wave

{{Short description|Prolonged period of excessively hot weather}}
[[File:Heat Wave.jpg|thumb|upright=1.5|A high pressure system in the upper atmosphere traps heat near the ground, forming a heatwave (for North America as an example)]]

A '''heat wave'''<ref>{{cite web|url=https://www.merriam-webster.com/dictionary/heat%20wave|title=Heatwave – noun – Definition|website=Merriam-Webster }}</ref> (or '''heatwave'''<ref>{{cite web|url=https://www.oxfordlearnersdictionaries.com/definition/english/heatwave|title=Heatwave – noun – Definition|website=gcunoxfohoarnersdictionaries.com}}</ref>), sometimes known as '''extreme heat''', is a period of abnormally hot weather.<ref name=":42" />{{rp|2911}} High [[Physics:Humidity|humidity]] often accompanies heat waves. This is especially the case in [[Earth:Oceanic climate|oceanic climate]] countries.  Definitions vary but are similar.<ref name="pmid15310900">{{cite journal |doi=10.1126/science.1098704 |pmid=15310900 |title=More Intense, More Frequent, and Longer Lasting Heat Waves in the 21st Century |journal=Science |volume=305 |issue=5686 |pages=994–997 |year=2004 |last1=Meehl |first1=G. A |bibcode=2004Sci...305..994M |doi-access=free }}</ref> We usually measure a heat wave relative to the usual climate in the area and to normal temperatures for the season.<ref name=":42" />{{rp|2911}} Temperatures that people from a hotter climate consider normal can be called a heat wave in a cooler area. This would be the case if the warm temperatures are outside the normal [[Earth:Climate|climate]] pattern for that area.<ref>{{cite journal |doi=10.1175/1520-0450(2001)040<0762:OTDOAH>2.0.CO;2 |year=2001 |volume=40 |issue=4 |pages=762–775 |title=On the Definition of a Heat Wave |journal=Journal of Applied Meteorology |last1=Robinson |first1=Peter J |bibcode=2001JApMe..40..762R |doi-access=free }}</ref> Heat waves have become more frequent, and more intense over land, almost everywhere since the 1950s. This is due to [[Earth:Climate change|climate change]].<ref name="IPCC6AR_ExtremeEvents">{{cite book |title=Climate Change 2021: The Physical Science Basis |publisher=Intergovernmental Panel on Climate Change |year=2021 |pages=8–10 |chapter=Summary for Policymakers |chapter-url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM_final.pdf |archive-url=https://web.archive.org/web/20211104175351/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM_final.pdf |archive-date=4 November 2021 |url-status=live}}</ref><ref>Thompson, Andrea, [https://www.scientificamerican.com/article/this-summers-record-breaking-heat-waves-would-not-have-happened-without-climate-change/ "This Summer’s Record-Breaking Heat Waves Would Not Have Happened without Climate Change"], ''Scientific American'', July 25, 2023.</ref>

Heat waves form when a [[Earth:High-pressure area|high pressure area]] in the upper atmosphere strengthens and remains over a region for several days up to several weeks.<ref name=":1" /> This traps heat near the ground.

Heat waves often have complex effects on human economies. They reduce labour productivity, disrupt agricultural and industrial processes and damage infrastructure not suitable for extreme heat.<ref name=":3">{{Cite web |last=Bottollier-Depois |first=Amélie |title=Deadly heatwaves threaten economies too |url=https://phys.org/news/2022-06-deadly-heatwaves-threaten-economies.html |access-date=2022-07-15 |website=phys.org |language=en}}</ref><ref name=":4">{{Cite journal |last1=García-León |first1=David |last2=Casanueva |first2=Ana |last3=Standardi |first3=Gabriele |last4=Burgstall |first4=Annkatrin |last5=Flouris |first5=Andreas D. |last6=Nybo |first6=Lars |date=2021-10-04 |title=Current and projected regional economic impacts of heatwaves in Europe |journal=Nature Communications |language=en |volume=12 |issue=1 |pages=5807 |bibcode=2021NatCo..12.5807G |doi=10.1038/s41467-021-26050-z |issn=2041-1723 |pmc=8490455 |pmid=34608159}}</ref> Severe heat waves have caused catastrophic crop failures and thousands of deaths from [[Medicine:Hyperthermia|hyperthermia]]. They have increased the risk of [[Biology:Wildfire|wildfires]] in areas with [[Earth:Drought|drought]]. They can lead to widespread power outages because people use more air conditioning. A heat wave counts as [[Earth:Extreme weather|extreme weather]]. It poses danger to human health because heat and sunlight overwhelm the human body's [[Physics:Thermoregulation in humans|cooling system]].  It is usually possible to detect heat waves by using forecasting instruments. This allows the authorities to issue a warning.

== Definitions ==
{{Weather}}
There are several definitions of heat waves:
* The [[Organization:Intergovernmental Panel on Climate Change|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."<ref name=":42">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}}.</ref>{{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.<ref>{{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 }}</ref> The same definition is used by the [[Organization:World Meteorological Organization|World Meteorological Organization]].<ref name=":2">{{Cite web |title=Heat wave {{!}} meteorology |url=https://www.britannica.com/science/heat-wave-meteorology |access-date=2019-04-01 |website=Encyclopedia Britannica |language=en}}</ref>
* A definition from the ''Glossary of Meteorology'' is:<ref name="Glossary of Meteorology">{{cite book |last=Glickman |first=Todd S. |title=Glossary of Meteorology |publisher=American Meteorological Society |date= 2000 |location=Boston |url=https://glossary.ametsoc.org/ |isbn=978-1-878220-49-3 }}</ref> "A period of abnormally and uncomfortably hot and usually humid weather."
We can use the term in two cases. One is variations in hot weather. The other is extraordinary spells of hot weather which may occur only once a century. 

=== Definitions by country ===
==== Europe ====
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 as the same criteria for a {{convert|25|°C|°F|1}} temperature.<ref>{{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 }}</ref> Sweden defines a heat wave as at least five days in a row with a daily high exceeding {{convert|25|°C|°F|1}}.<ref>{{cite web|url=http://www.smhi.se/kunskapsbanken/klimat/varmebolja-1.22372 |title=Värmebölja &#124; Klimat &#124; Kunskapsbanken &#124; SMHI |language=sv |publisher=Smhi.se |access-date=17 July 2013}}</ref>

In Greece, the [http://www.hnms.gr/emy/el/ Hellenic National Meteorological Service] defines a heat wave as three consecutive days at {{convert|39|°C|°F}} or more. In the same period the minimum temperature is {{convert|26|°C|°F}} or more. There are 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 [[Company:Met Office|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 is 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.<ref>{{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}}</ref> 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}}.<ref>{{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}}</ref>

==== Other regions ====
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.<ref>{{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}}</ref> In the Northeast, a heat wave typically when the temperature reaches or exceeds {{convert|90|°F|°C|1}} for three consecutive days. This is not always the case. This is because the high temperature ties in with humidity levels to determine a heat index threshold.<ref>{{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}}</ref> 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).<ref>{{cite web|title=Staying Cool and Safe|publisher=[[Company:Pacific Gas and Electric Company|Pacific Gas and Electric Company]]|location=Oakland, California|date=March 24, 2017|accessdate=June 26, 2023|url=https://www.pge.com/includes/docs/pdfs/myhome/edusafety/seasonal/coolingcenters/staying_cooling_and_safe_english.pdf}}</ref> The [[Earth:National Weather Service|National Weather Service]] issues heat advisories and excessive heat warnings when it expects unusual periods of hot weather.

In Adelaide, South Australia, a heat wave is five consecutive days at or above {{convert|35|°C|°F}}. It can also be three consecutive days at or over {{convert|40|°C|°F}}.<ref>{{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}}</ref> The Australian Bureau of Meteorology defines a heat wave as three or more days of unusual maximum and minimum temperatures.<ref name=":0">{{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}}</ref> Before this new Pilot Heatwave Forecast there was no national definition for heat waves or measures of heat wave severity.<ref name=":0" />

== Observations ==
{{multiple image | total_width=500
| 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.<ref name=NOAA_October>{{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 |website=NCEI.NOAA.gov |publisher=National Centers for Environmental Information (NCEI) 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)</ref>
| 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 [[Earth:Extreme weather|extreme weather]] events (for increasing degrees of global warming) are expected.<ref>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}}</ref>{{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<ref>{{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}}</ref>]]It is possible to compare heat waves in different regions of the World with different climates thanks to a general indicator. This appeared in 2015.<ref>{{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}}</ref> 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.<ref>{{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}}</ref>

In July 2023 the world hit a new record high temperature.<ref>{{Cite web |date=2023-07-10 |title=Preliminary data shows hottest week on record. Unprecedented sea surface temperatures and Antarctic sea ice loss |url=https://public-old.wmo.int/en/media/news/preliminary-data-shows-hottest-week-record-unprecedented-sea-surface-temperatures-and |archive-url=https://web.archive.org/web/20231218183142/https://public-old.wmo.int/en/media/news/preliminary-data-shows-hottest-week-record-unprecedented-sea-surface-temperatures-and |url-status=dead |archive-date=18 December 2023 |access-date=2023-07-24 |website=public.wmo.int |language=en}}</ref> Increased wildfires in places such as Spain and Greece can also be attributed to heat waves.<ref>{{cite news |last1=Duncan |first1=Conrad |date=3 July 2019 |title=June was hottest ever recorded on Earth, European satellite agency announces |agency=The Independent |url=https://www.independent.co.uk/news/world/june-hottest-month-ever-earth-2019-weather-heatwave-hot-a8984691.html |url-status=live |url-access=subscription |access-date=4 July 2019 |archive-url=https://ghostarchive.org/archive/20220509/https://www.independent.co.uk/news/world/june-hottest-month-ever-earth-2019-weather-heatwave-hot-a8984691.html |archive-date=9 May 2022}}</ref>

The 2021 Western North America heat wave resulted in some of the highest temperatures ever recorded in the region. These included a record {{cvt|49.6|°C}} for Canada.<ref>{{Cite web |last=Berardelli |first=Jeff |date=2021-06-29 |title=Pacific Northwest bakes under once-in-a-millennium heat dome |url=https://www.cbsnews.com/news/heat-wave-dome-2021-seattle-portland-weather/ |access-date=2021-06-30 |website=www.cbsnews.com |language=en-US}}</ref>

One study in 2021 investigated 13,115 cities. It found that extreme heat exposure of a wet bulb globe temperature above 30&nbsp;°C tripled between 1983 and 2016. It increased by about 50% if you exclude the effect of population growth in these cities. Urban areas and living spaces are often significantly warmer than surrounding rural areas. This is partly due to the [[Earth:Urban heat island|urban heat island]] effect. The researchers compiled a comprehensive list of past urban extreme heat events.<ref>{{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}}</ref><ref>{{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}}</ref>

== Causes ==
Heat waves form when a [[Earth:High-pressure area|high pressure area]] at an altitude of {{convert|10000|-|25000|ft|m|abbr=off}} strengthens and remains over a region for several days and up to several weeks.<ref name=":1">{{Cite web|url=https://www.weather.gov/jetstream/hi|title=NWS JetStream - Heat Index|last=US Department of Commerce|first=NOAA|website=www.weather.gov|language=EN-US|access-date=2019-02-09}}</ref> This is common in summer in both the Northern and Southern Hemispheres. This is because the [[Earth:Jet stream|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 [[Physics:Adiabatic process#Adiabatic heating and cooling|adiabatically]]. This inhibits [[Earth:Atmospheric convection|convection]] and prevents the formation of clouds. A reduction of clouds increases the [[Shortwave radiation|shortwave radiation]] reaching the surface. A [[Physics:Low-pressure area|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.<ref>{{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 }}</ref><ref>{{cite web |title=Heat Index |publisher=US National Weather Service |url=http://www.srh.noaa.gov/jetstream/global/hi.htm}}</ref>

In the eastern regions of the United States a heat wave can occur when a [[Earth:High-pressure area|high pressure system]] originating in the Gulf of Mexico becomes stationary just off the Atlantic Seaboard. We usually call this a Bermuda High. 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.<ref>{{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 }}</ref>

In the Western Cape Province of South Africa, a heat wave can occur when [[Physics:Low-pressure area|low pressure]] offshore and [[Earth:High-pressure area|high pressure]] inland air combine to form a berg wind. The air warms as it descends from the Karoo interior. The temperature will rise about 10&nbsp;°C from the interior to the coast. Humidity is usually very low. The temperature can be over 40&nbsp;°C in summer. The highest temperature recorded in South Africa (51.5&nbsp;°C) occurred one summer during a berg wind along the Eastern Cape coastline.<ref>{{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 }}</ref><ref>{{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}}</ref>

The level of [[Chemistry:Soil moisture|soil moisture]] can intensify heat waves in Europe.<ref name="Miralles n/a–n/a">{{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}}</ref><ref>{{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}}</ref> 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.<ref name="Miralles n/a–n/a"/> 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.

=== Climate change ===
{{excerpt|Effects of climate change#Heat waves and temperature extremes|paragraphs=1-2|file=no}}

== Impacts on human health ==
[[File:Heat stroke treatment, Baton Rouge, 2016 Louisiana floods.jpg|thumb|[[Medicine:Heat stroke|Heat stroke]] treatment at Baton Rouge during the 2016 Louisiana floods]]
=== Heat-related health effects for vulnerable people ===
{{excerpt|heat illness|paragraphs=1-2|file=no}}

{{excerpt|Effects of climate change on human health#Heat-related health effects for vulnerable people|paragraphs=1-2|file=no}}

=== Mortality ===
[[File:NWS HeatRisk Categories.jpg|thumb|The National Weather Service risk categories for ''NWS HeatRisk''.]]
{{excerpt|Effects of climate change on human health#Heat-related mortality|paragraphs=1-2|file=no}}

==== Underreporting of fatalities ====
The number of heat fatalities is probably highly underreported. This is due to a lack of reports and to misreporting.<ref name="heat epidemiology">{{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}}</ref> If we factor in heat-related illnesses, actual death tolls linked to extreme heat may be six times as high as official figures. This is based on studies of California<ref>{{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}}</ref> and Japan.<ref>{{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}}</ref> 

Part of the mortality during a heat wave may be due to short-term forward [[Medicine:Mortality displacement|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.<ref name="pmid11401757">{{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}}</ref>

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.<ref>{{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}}</ref> 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.<ref>{{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}}</ref>

===Heat index for temperature and relative humidity===
The [[Earth:Heat index|heat index]] in the table above is a measure of how hot it feels when relative humidity is factored with the actual air temperature.

=== Psychological and sociological effects ===
Excessive heat causes psychological stress as well as physical [[Medicine:Stress|stress]]. This can affect performance. It may also lead to an increase in violent crime.<ref name="heat and stress">{{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 }}</ref> High temperatures are associated with increased conflict between individuals and at the social level. In every society, [[Philosophy:Crime|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.<ref>{{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 }}</ref>

High temperatures also have a significant effect on income. A study of counties in the United States found that the economic productivity of individual days declines by about 1.7% for each degree Celsius above {{convert|15|°C|°F}}.<ref>{{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 }}</ref>

=== Surface ozone (air pollution) ===

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% due to climate change. Near-surface ozone concentrations rose by 2%.<ref>{{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}}</ref>

An investigation to assess the joint mortality effects of ozone and heat during the European heat waves in 2003 concluded that these appear to be reinforce each other and increase mortality when combined.<ref>{{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=January 14, 2014 |doi-access=free}}</ref>

== Other impacts ==

=== Reduced GDP ===
Calculations from 2022 suggest heat waves will shrink the global economy by about 1% decrease by the middle of the 21st century.<ref>{{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 |website=HYPEANDHYPER |language=en-GB}}</ref><ref>{{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 |language=en |access-date=2022-07-15}}</ref><ref>{{Cite journal |last1=García-León |first1=David |last2=Casanueva |first2=Ana |last3=Standardi |first3=Gabriele |last4=Burgstall |first4=Annkatrin |last5=Flouris |first5=Andreas D. |last6=Nybo |first6=Lars |date=2021-10-04 |title=Current and projected regional economic impacts of heatwaves in Europe |journal=Nature Communications |language=en |volume=12 |issue=1 |pages=5807 |bibcode=2021NatCo..12.5807G |doi=10.1038/s41467-021-26050-z |issn=2041-1723 |pmc=8490455 |pmid=34608159}}</ref>

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.<ref name=":3" /><ref name=":4" /> 

=== Reduced agricultural yields ===
{{Main|Biology: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.<ref>{{Cite web |title=Malawi heatwaves threaten tea yields and livelihoods – Future Climate Africa |url=https://futureclimateafrica.org/news/malawi-heatwaves-threaten-tea-yields-and-livelihoods/ |access-date=2020-09-24 |language=en-US}}</ref>

=== Wildfires ===
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&nbsp;billion worth of damage.<ref>{{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=[[Organization:The Earth Institute|The Earth Institute]] |url=http://iri.columbia.edu/climate/cid/Sep2003/impacts.html |access-date=28 July 2006}}</ref> High end farmlands have irrigation systems to back up crops.

=== Floods ===
Heat waves can also contribute to flooding. 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.<ref>{{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 |website=The Conversation |date=2 September 2022 |language=en}}</ref>

=== Infrastructural damage ===
Heat waves cause roads and highways to buckle and melt,<ref>{{Cite news |date=15 July 2013 |title=When does tarmac melt? |work=BBC News |url=https://www.bbc.com/news/magazine-23315384}}</ref> 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.

=== Power outages ===
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.<ref>{{cite news |last=Doan |first=Lynn |author2=Covarrubias, Amanda |date=27 July 2006 |title=Heat Eases, but Thousands of Southern Californians Still Lack Power |work={{wipe|Los Angeles Times}} |url=http://www.latimes.com/news/local/la-me-heat27jul27,1,4111447.story |access-date=16 June 2014}}</ref>
The 2009 South Eastern 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.

== Options for reducing impacts of heat waves on people ==

A possible public health measure during heat waves is to set up air-conditioned public cooling centres. There are novel designs for cooling systems that are relatively low-cost. They do not use electrical components, are [[Physics:Off-the-grid#Electrical power|off-grid]] and store solar energy chemically for use on demand.<ref>{{cite news |date=20 September 2021 |title=Sunlight and salt water join forces in electricity-free cooling system |work=New Atlas |url=https://newatlas.com/good-thinking/sunlight-salt-water-electricity-free-cooling-system/ |access-date=20 October 2021}}</ref><ref>{{cite journal |last1=Wang |first1=Wenbin |last2=Shi |first2=Yusuf |last3=Zhang |first3=Chenlin |last4=Li |first4=Renyuan |last5=Wu |first5=Mengchun |last6=Zhuo |first6=Sifei |last7=Aleid |first7=Sara |last8=Wang |first8=Peng |date=1 September 2021 |title=Conversion and storage of solar energy for cooling |journal=Energy & Environmental Science |language=en |volume=15 |pages=136–145 |doi=10.1039/D1EE01688A |issn=1754-5706 |s2cid=239698764 |doi-access=free|hdl=10754/670903 |hdl-access=free }}</ref>

Adding air conditioning in schools<ref name="chicago">{{Cite news |last=Kaufman |first=Leslie |date=2011-05-23 |title=A City Prepares for a Warm Long-Term Forecast |language=en-US |work=The New York Times |url=https://www.nytimes.com/2011/05/23/science/earth/23adaptation.html |access-date=2023-02-08 |issn=0362-4331}}</ref> provides a cooler work place. But it can result in additional [[Earth:Greenhouse gas emissions|greenhouse gas emissions]] unless [[Physics:Solar energy|solar energy]] is used.

== Examples by country ==

=== United States ===
[[Image:Summer 1936 US Temperature.gif|thumb|right|The [[1936 North American heat wave]].
[[File:202307 Survey - climate change "major factor" in extreme weather - Washington Post.svg |thumb |Perceptions differ along political lines, on whether climate change was a "major factor" contributing to various extreme weather events.<ref name=WashPost_20230823>{{cite news |last1=Ajasa |first1=Amudalat |last2=Clement |first2=Scott |last3=Guskin |first3=Emily |title=Partisans remain split on climate change contributing to more disasters, and on their weather becoming more extreme |url=https://www.washingtonpost.com/climate-environment/2023/08/23/extreme-weather-climate-change-poll/ |newspaper=The Washington Post |date=23 August 2023 |archive-url=https://archive.today/20230823125945/https://www.washingtonpost.com/climate-environment/2023/08/23/extreme-weather-climate-change-poll/ |archive-date=23 August 2023 |url-status=live }}</ref>]]
Record temperatures were based on 112-year records]]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.<ref>{{cite news |last1=Rosane |first1=Olivia |title=50 Million Americans Are Currently Living Under Some Type of Heat Warning |agency=Ecowatch |url=https://www.ecowatch.com/heat-wave-central-eastern-us-2639217633.html |access-date=19 July 2019}}</ref>

According to a 2022 study, 107 million people in the US will experience extremely dangerous heat in the year 2053.<ref>{{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 |website=CNN}}</ref>

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 [[Physics:Tropical cyclone|hurricanes]].<ref>{{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}}</ref> About 400 deaths a year on average are directly due to heat in the United States.<ref name="heat epidemiology" /> The 1995 Chicago heat wave, one of the worst in US history, led to approximately 739 heat-related deaths over 5 days.<ref>''[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</ref> In the United States, the loss of human life in hot spells in summer exceeds that caused by all other weather events. These include [[Astronomy:Lightning|lightning]], rain, [[Earth:Flood|flood]]s, hurricanes, and [[Earth:Tornado|tornado]]es.<ref name="Klinenberg">{{cite book |last=Klinenberg |first=Eric |url=https://archive.org/details/heatwave00eric |title=Heat Wave: A Social Autopsy of Disaster in Chicago |publisher=Chicago University Press |year=2002 |isbn=978-0-226-44321-8 |location=Chicago, IL}}</ref><ref name="slate">''[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</ref> 

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

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. Unusually hot summertime temperatures in Chicago, Denver, or New York City lead to predictions of higher levels of illness and death. Parts of the country that are mild to hot all year have a lower public health risk from excessive heat. Residents of southern cities such as Miami, Tampa, Los Angeles, and Phoenix tend to be acclimatized to hot weather conditions. They are therefore less vulnerable to heat-related deaths. 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.<ref>{{cite journal |author1=Robert E. Davis |author2=Paul C. Knappenberger |author3=Patrick J. Michaels |author4=Wendy M. Novicoff |name-list-style=amp |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}}</ref>

== Society and culture ==
Policymakers, funders and researchers have created the Extreme Heat Resilience Alliance coalition under the [[Organization:Atlantic Council|Atlantic Council]]. This advocates for naming heat waves, measuring them, and ranking them to build better awareness of their impacts.<ref>{{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 |website=wcr.ethz.ch |language=en}}</ref><ref>{{Cite web |title=The world's getting hotter. Can naming heat waves raise awareness of the risks? |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 |website=The World from PRX |language=en}}</ref>

== See also ==
* [[Physics:Cold wave|Cold wave]]
* List of heat waves
* [[Earth:List of severe weather phenomena|List of severe weather phenomena]]
* [[Earth:Urban heat island|Urban heat island]]

== References ==
{{Reflist}}

== External links ==

* [http://www.ncdc.noaa.gov/oa/esb/?goal=climate&file=events/temp/ Social & Economic Costs of Temperature Extremes] from NOAA Socioeconomics website initiative, [[Organization:National Centers for Environmental Information|National Centers for Environmental Information]]

* [https://earth.nullschool.net/#current/wind/surface/level/overlay=misery_index/winkel3 Current global map of extreme temperatures]
* [https://www.science.org/doi/10.1126/sciadv.aaw1838 The emergence of heat and humidity too severe for human tolerance], American Association for the Advancement of Science research paper
* [http://www.statsguy.co.uk/wet-bulb-temperatures-part-1/#comment-472390 Statistical analysis] of [[Earth:Met Office Hadley Centre|Met Office Hadley Centre]] HadISD datasets
* [https://www.metoffice.gov.uk/hadobs/hadisd/HadISD Global Sub-daily Station HadISD Dataset]



{{DEFAULTSORT:Heat Wave}}
[[Category:Meteorological phenomena]]
[[Category:Weather hazards]]

{{Sourceattribution|Heat wave}}