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Heat wave
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== Causes == Heat waves form when a [[High-pressure area|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.<ref name=":1">{{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}}</ref> 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 [[Adiabatic process#Adiabatic heating and cooling|adiabatically]]. This inhibits [[Atmospheric convection|convection]] and prevents the formation of clouds. A reduction of clouds increases the [[shortwave radiation]] reaching the surface. A [[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 [[High-pressure area|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.<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 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.<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 [[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|url-access=subscription }}</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}}
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