{{Short description|30-year average of a weather variable for a given time of year}} thumb|right|Annual U.S. temperature compared to the 20th-century average for each U.S. Climate Normals period from 1901–1930 to 1991–2020. '''Climatological normal''' or '''climate normal''' ('''CN''') is a 30-year average of a weather variable for a given time of year.<ref name=var>[http://www.miseagrant.umich.edu/downloads/climate/11-703-Climate-Variability-and-Climate-Change.pdf Climate Variability and Climate Change] {{Webarchive|url=https://web.archive.org/web/20140517124329/http://www.miseagrant.umich.edu/downloads/climate/11-703-Climate-Variability-and-Climate-Change.pdf |date=2014-05-17 }}; WHAT IS THE DIFFERENCE? Michigan Sea Grant</ref> Most commonly, a CN refers to a particular month of year, but it may also refer to a broader scale, such as a specific meteorological season.<ref>{{cite web |url=https://library.wmo.int/index.php?lvl=notice_display&id=20130#.Xznu2ChKg2x |title=WMO Guidelines on the Calculation of Climate Normals |author=<!--Not stated--> |date=2017 |website=World Meteorological Organization |access-date=3 July 2021 }}</ref> More recently, CN have been reported for narrower scales, such as day of year and even hourly scale.<ref>{{cite journal |title=NOAA's 1981–2010 U.S. Climate Normals: An Overview |author=<!--Not stated--> |date=1 November 2012 |journal=Bulletin of the American Meteorological Society |volume=93 |issue=11 |pages=1687–1697 |publisher=American Meteorological Society |doi=10.1175/BAMS-D-11-00197.1 |bibcode=2012BAMS...93.1687A |doi-access=free }}</ref>
Climatological normals are used as an average or baseline to evaluate climate events and provide context for year-to-year variability. Normals can be calculated for a variety of weather variables including temperature and precipitation and rely on data from weather stations. Variability from the 30-year averages is typical and climate variability looks at the magnitude of extremes.<ref name=var/> '''Climatological standard normals''' are overlapping periods updated every decade: 1971–2000, 1981–2010, 1991–2020, etc.
The term "normal" first appeared in the literature by Heinrich Wilhelm Dove in 1840 and the concept was formalized by the International Meteorological Committee in 1872.<ref name="Guttman">{{cite journal |issn=1520-0477 |year=1989 |volume=70 |page=602 |doi=10.1175/1520-0477(1989)070<0602:SDOC>2.0.CO;2 |title=Statistical Descriptors of Climate |last1=Guttman |first1=Nathaniel B. |journal=Bulletin of the American Meteorological Society |issue=6 |bibcode=1989BAMS...70..602G|doi-access=free }}</ref> The use of the 30-year period of normals began in 1935 with the 1901-30 period.<ref>{{cite book |last=Trewin |first=Blair C. |author-link= |date=2007 |title=The Role of Climatological Normals in a Changing Climate |url=https://library.wmo.int/doc_num.php?explnum_id=4546 |series= World Climate Data and Monitoring Programme |location= |publisher= World Meteorological Organization |page=7 |isbn=}}</ref> The continued use of 30 year normals has increasingly been called into question due to substantial evidence that the stationarity of climate statistics can no longer be taken for granted due to climate change.<ref name="Guttman" /><ref>{{cite journal |title=The Definition of the Standard WMO Climate Normal: The Key to Deriving Alternative Climate Normals |last1=Arguez |first1=Anthony |last2=Vose |first2=Russell S. |date=1 June 2011 |journal=Bulletin of the American Meteorological Society |volume=92 |issue=6 |pages=699–704 |publisher=American Meteorological Society |doi=10.1175/2010BAMS2955.1 |bibcode=2011BAMS...92..699A |doi-access=free }}</ref> This has led to alternative definitions such as "Optimal Climate Normal" and the "Hinge Fit" approach to supplement the standard 30 year normals which are still commonly used.<ref>{{cite web |url=https://www.ncdc.noaa.gov/news/defining-climate-normals-new-ways |title=Defining Climate Normals in New Ways |author=<!--Not stated--> |date= |website=National Centers for Environmental Information |publisher=NOAA |access-date=3 July 2021 |quote= |archive-date=14 April 2022 |archive-url=https://web.archive.org/web/20220414113707/https://www.ncdc.noaa.gov/news/defining-climate-normals-new-ways |url-status=dead }}</ref> More recently, modern smoothing techniques such as spline-based methods have also been proposed and used operationally to estimate the expected (mean) state of climate variables for the coming year or years .<ref>{{Cite journal|last=Rigal|first=Alix|last2=Azaïs|first2=Jean-Marc|last3=Ribes|first3=Aurélien|title=Estimating daily climatological normals in a changing climate |journal=Climate Dynamics |volume=53 |issue=1 |pages=275–286 |date=2019-07-01 |doi=10.1007/s00382-018-4584-6 |url=https://link.springer.com/article/10.1007/s00382-018-4584-6 }}</ref>
==See also== *Climate anomaly *Climatology *Seasonality
==References== {{reflist}}
Category:Climate and weather statistics