Climatological normal or climate normal (CN) is a 30-year average of a weather variable for a given time of year.Climate Variability and Climate Change
; WHAT IS THE DIFFERENCE? Michigan Sea Grant Most commonly, a CN refers to a particular

month of year An ordinal date is a calendar date typically consisting of a ''year'' and a day of the year or ordinal day number (or simply ordinal day or day number), an ordinal number ranging between 1 and 366 (starting on January 1), though year may sometime ...

, but it may also refer to a broader scale, such as a specific

meteorological season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and pol ...

. More recently, CN have been reported for narrower scales, such as

day of year An ordinal date is a calendar date typically consisting of a ''year'' and a day of the year or ordinal day number (or simply ordinal day or day number), an ordinal number ranging between 1 and 366 (starting on January 1), though year may sometime ...

and even hourly scale. 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 station A weather station is a facility, either on land or sea, with instruments and equipment for measuring atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include tempera ...

s. Variability from the 30-year averages is typical and

climate variability Climate variability includes all the variations in the climate that last longer than individual weather events, whereas the term climate change only refers to those variations that persist for a longer period of time, typically decades or more ...

looks at the magnitude of extremes. 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 Heinrich Wilhelm Dove (6 October 1803 – 4 April 1879) was a Prussian physicist and meteorologist. Early years Dove was born in Liegnitz in the Kingdom of Prussia. Dove studied history, philosophy, and the natural sciences at the University of B ...

in 1840 and the concept was formalized by the

International Meteorological Committee The International Meteorological Organization (IMO; 1873–1951) was the first organization formed with the purpose of exchanging weather information among the countries of the world. It came into existence from the realization that weather systems ...

in 1872. The use of the 30 year period of normals began in 1935 with the 1901-30 period. 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 In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...

. 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.

References

{{reflist Climate and weather statistics

See also

References