Editing Heat wave (section)

{{Short description|Prolonged period of excessively hot weather}}
{{Distinguish|heat burst}}
{{For|the disease that is produced by heat|Heat stroke}}
{{Redirect|Heat waves|the Glass Animals song|Heat Waves|other uses|Heat wave (disambiguation)}}
{{Use British English|date=August 2023}}
{{Use dmy dates|date=June 2021}}
[[File:Heat Wave.jpg|thumb|upright=1.5|A [[High-pressure area|high pressure system]] in the [[upper atmosphere]] traps heat near the ground, forming a heat wave (for North America in this example)]]
[[File:Heatwave in London (9217).jpg|thumb|Scorched grass in Greenwich Park, London, England, during a heatwave in August 2022.]]
A '''heat wave''' or '''heatwave''', sometimes described as '''extreme heat''', is a period of abnormally hot weather <ref name=":42" />{{rp|2911}} generally considered to be at least ''five consecutive days''. A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season.<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> The main difficulties with this broad definition emerge when one must quantify what the 'normal' temperature state is, and what the spatial extent of the event may or must be. Temperatures that humans from a hotter climate consider normal can be regarded as a heat wave in a cooler area. This would be the case if the warm temperatures are outside the normal [[climate]] pattern for that area. Heat waves have become more frequent, and more intense over land, across almost every area on Earth since the 1950s, the increase in frequency and duration being caused by [[climate change]].<ref name="SPM IPCC 2021" />{{rp|8–10}}<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]]'' 25 July 2023</ref>

Heat waves form when a [[high-pressure area]] in the upper atmosphere strengthens and remains over a region for several days up to several weeks.<ref name=":1" /> This traps heat near the earth's surface. It is usually possible to forecast heat waves, thus allowing the authorities to issue a warning in advance.

Heat waves have an impact on the economy. They can reduce labour productivity, disrupt agricultural and industrial processes and damage infrastructure.<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 |publisher=phys.org }}</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 |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 [[hyperthermia]]. They have increased the risk of [[wildfire]]s in areas with [[drought]]. They can lead to widespread electricity outages because more air conditioning is used. A heat wave counts as [[extreme weather]]. It poses danger to human health, because heat and sunlight overwhelm the [[thermoregulation in humans]].