Editing Urban heat island (section)

== Measurements and predictions ==

=== Urban Heat Island Index (UHII) ===
One method to quantify the UHI effect within urban areas is the UHI Index created by the [[California Environmental Protection Agency|Californian EPA]] in 2015. It compares the temperature of a surveyed area and rural reference points upwind from the surveyed area, at a height of two meters above ground level. The difference in temperature in degrees [[Celsius]] is taken hourly and differences with an increased urban temperature compared to the reference points are summed up, creating an amount of degree-Celsius-hours, which is the UHI Index of the surveyed area. The measure of Celsius-hours might be averaged over many days, but is specified as Celsius-hours per averaged day.<ref>{{Cite web |date=October 2016 |title=Executive Summary: Creating and Mapping an Urban Heat Island Index for California |url=https://calepa.ca.gov/wp-content/uploads/sites/6/2016/10/UrbanHeat-Report-ExSummary.pdf |url-status=live |archive-url=https://web.archive.org/web/20190731003514/https://calepa.ca.gov/wp-content/uploads/sites/6/2016/10/UrbanHeat-Report-ExSummary.pdf |archive-date=2019-07-31 |access-date=2024-07-24 |website=CalEPA {{!}} California Environmental Protection Agency}}</ref><ref name=":6">{{Cite web |title=Understanding the Urban Heat Island Index {{!}} CalEPA |url=https://calepa.ca.gov/climate/urban-heat-island-index-for-california/understanding-the-urban-heat-island-index/ |url-status=live |archive-url=https://web.archive.org/web/20171219054828/https://calepa.ca.gov/climate/urban-heat-island-index-for-california/understanding-the-urban-heat-island-index/ |archive-date=2017-12-19 |access-date=2024-07-24 |website=CalEPA {{!}} California Environmental Protection Agency}}</ref><ref name=":7">{{Cite web |date=October 2016 |title=Creating and Mapping an Urban Heat Island Index for California |url=https://calepa.ca.gov/wp-content/uploads/sites/6/2016/10/UrbanHeat-Report-Report.pdf |url-status=live |archive-url=https://web.archive.org/web/20190731003434/https://calepa.ca.gov/wp-content/uploads/sites/6/2016/10/UrbanHeat-Report-Report.pdf |archive-date=2019-07-31 |access-date=2024-07-24 |website=CalEPA {{!}} California Environmental Protection Agency}}</ref>

The index was created to estimate the expected use of air conditioning and resulting [[greenhouse gas emissions]] in California.<ref name=":6" /> The index does not consider values of or differences in wind-speed, humidity, or [[Solar irradiance|solar influx]], which might influence [[Apparent temperature|perceived temperature]] or the operation of air conditioners.<ref name=":7" />

=== Models and simulations ===
If a city or town has a good system of taking weather observations the UHI can be measured directly.<ref>{{cite journal |last1=Steeneveld |first1=G. J. |year=2011 |title=Quantifying urban heat island effects and human comfort for cities of variable size and urban morphology in the Netherlands |journal=Journal of Geophysical Research |volume=116 |issue=D20 |pages=D20129 |bibcode=2011JGRD..11620129S |doi=10.1029/2011JD015988 |doi-access=free}}</ref> An alternative is to use a complex simulation of the location to calculate the UHI, or to use an approximate empirical method.<ref>{{cite journal |last1=Kershaw |first1=T. J. |last2=Sanderson |first2=M. |last3=Coley |first3=D. |last4=Eames |first4=M. |year=2010 |title=Estimation of the urban heat island for UK climate change projections |journal=Building Services Engineering Research and Technology |volume=31 |issue=3 |pages=251–263 |doi=10.1177/0143624410365033 |doi-access=free |hdl-access=free |hdl=10871/13934}}</ref><ref>{{cite journal |last1=Theeuwes |first1=N. E. |last2=Steeneveld |first2=G. J. |last3=Ronda |first3=R. J. |last4=Holtslag |first4=A. A. M. |year=2017 |title=A diagnostic equation for the daily maximum urban heat island effect for cities in northwestern Europe |journal=International Journal of Climatology |volume=37 |issue=1 |pages=443–454 |bibcode=2017IJCli..37..443T |doi=10.1002/joc.4717 |s2cid=131437962}}</ref> Such models allow the UHI to be included in estimates of future temperatures rises within cities due to climate change.

Leonard O. Myrup published the first comprehensive numerical treatment to predict the effects of the urban heat island (UHI) in 1969.<ref name=":3">{{cite journal |last1=Myrup |first1=Leonard O. |year=1969 |title=A Numerical Model of the Urban Heat Island |journal=Journal of Applied Meteorology |volume=8 |issue=6 |pages=908–918 |bibcode=1969JApMe...8..908M |doi=10.1175/1520-0450(1969)008<0908:ANMOTU>2.0.CO;2 |doi-access=free}}</ref> The heat island effect was found to be the net result of several competing physical processes. In general, reduced evaporation in the city center and the thermal properties of the city building and paving materials are the dominant parameters.<ref name=":3" /> Modern simulation environments include [[ENVI-met]], which simulates all interactions between building and ground surfaces, plants and ambient air.<ref>{{Cite web |last=n.n. |title=ENVI-met-Alternativen für Mac — Altapps.net |url=https://de.altapps.net/soft/envi-met?platform=mac |access-date=2022-06-01 |website=de.altapps.net |language=de}}</ref>