What are Climate Normals?
A "normal" is the 30-year average of a particular variable’s measurements, calculated for a uniform time period. Climate normals are derived from weather and climate observations captured by weather stations. The monthly minimum temperature normal in January for a given station is computed by averaging the 30 January values of monthly averaged minimum temperatures from 1991–2020 after they have been adjusted for any changes in the observing station or observing practices, and had missing months estimated from nearby more complete stations. Temperature daily normals are calculated using a procedure that ensures the daily normals for a month agree with the monthly normals when averaged. Climate Normals also include many statistics other than averages.
NOAA produces Climate Normals in accordance with the World Meteorological Organization (WMO), of which the United States is a member. The WMO requires each member nation to compute 30-year meteorological quantity averages at least every 30 years (1931–1960, 1961–1990, 1991–2020, etc.), and recommends an update each decade, in part to incorporate newer weather stations. NCEI is also responsible for fulfilling the congressional mandate "... to establish and record the climatic conditions of the United States." This stems from a provision of the Organic Act of October 1, 1890, which established the Weather Bureau as a civilian agency (15 U.S.C. 311).
In addition to basic averages for temperature, precipitation, and snowfall, more than 500 separate types of weather and climate statistics are available as part of the U.S. Normals dataset. Some are well known metrics like heating and cooling degree days, growing season length for various temperature thresholds, numbers of days with precipitation or with temperatures below freezing, etc. Other statistics are less well known, but are needed by various user communities, including percentiles and probabilities of key variables. All available variables serve a purpose, and are described in more detailed documentation available through the Data Access tool.
Normals Calculations
The science and methodologies used to generate official climate normals for the United States were well established during the creation of the 1981–2010 U.S. Climate Normals. A team of NCEI researchers spent considerable time and effort improving and automating these processes, particularly for daily and hourly normals. These methods are documented in a series of five peer-reviewed publications (Applequist et al. 2012; Arguez et al. 2012; Arguez and Applequist 2013; Durre et al. 2013; Durre and Squires 2015).
The 1991–2020 Normals calculation software was updated to incorporate feedback and recommendations from WMO Guidelines on the Calculation of Climate Normals (No. 1203) published in 2017, as well as users requests for calculation changes and new normals variables. These changes are summarized in the Normals Calculation Methodology 2020 document, along with additional technical documentation released with 1981–2010 Normals.
2023 Update
Version 1.0.1 reflects changes in the daily, monthly, seasonal, and annual normals at a total of 23 stations. Hourly and agricultural normals remain unchanged.
During the year following the initial release of the 1991-2020 station-based U.S. Climate Normals in May 2021, National Weather Service Forecast Offices submitted inquiries regarding the Normals at a total of 60 stations. Staff at NCEI carefully adjudicated each inquiry in collaboration with its NWS partners. At 21 stations, NWS staff was able to provide data additions and corrections, and NCEI staff recalculated or added the affected normals parameters based on the updated data records. In most cases, changes the monthly normals were <0.5°C for temperature, <0.1 in for precipitation, and <0.5 in for snowfall. At two additional sites, certain normals parameters were removed because they were judged to be inaccurate, and no correction to the underlying data was feasible. The following stations were affected by changes in one or more of their Normals parameters.
References
- Applequist, S., A. Arguez, I. Durre, M. F. Squires, R. S. Vose, and X. Yin, 2012: 1981–2010 U.S. Hourly Normals. Bulletin of the American Meteorological Society, 93, 1637-1640. doi:10.1175/BAMS-D-11-00173.1.
- Arguez, A., I. Durre, S. Applequist, R. S. Vose, M. F. Squires, X. Yin, R. R. Heim, Jr., and T. W. Owen, 2012: NOAA's 1981–2010 U.S. Climate Normals: An Overview. Bulletin of the American Meteorological Society, 93, 1687-169. doi:10.1175/BAMS-D-11-00197.1.
- Arguez, A., and S. Applequist, 2013: A Harmonic Approach for Calculating Daily Temperature Normals Constrained by hom*ogenized Monthly Temperature Normals. Journal of Atmospheric and Oceanic Technology, 30, 1259–1265. doi:10.1175/JTECH-D-12-00195.1.
- Durre, I., M. F. Squires, R. S. Vose, X. Yin, A. Arguez, and S. Applequist, 2012: NOAA's 1981–2010 U.S. Climate Normals: Monthly Precipitation, Snowfall, and Snow Depth. Journal of Applied Meteorology and Climatology, 52, 2377- 2395. doi:10.1175/JAMC-D-13-051.1.
- Durre, I., and M. F. Squires, 2015: White Christmas? An Application of NOAA's 1981–2010 Daily Normals. Bulletin of the American Meteorological Society, 96, 1853-1858. doi:10.1175/BAMS-D-15-00038.1.
Frequently Asked Questions
Why were the Alaska temperature normals updated in July 2021?
During the temperature data hom*ogenization process, abrupt but real changes in sea ice on the west and northern coasts of Alaska could be mistaken for artificial air temperature changes at stations close to the coast (north of 70°N or west of 160°W). To address this issue, we replaced the hom*ogenized temperature records with original observations that were quality controlled but not hom*ogenized, and reran the normals process.
Normals at 18 stations within this zone were slightly modified. In addition, normals for 7 stations near the zone were under the influence of the coastal stations for filling missing data or estimating normals, and also had changed values.
New temperature normals for impacted stations include annual/seasonal, monthly, and daily temperature normals and temperature-related normals (seasonal variables, exceedance counts, etc.) for both 1991-2020 and 2006-2020. The quick access tool will have the changes available for temperature averages, while the full access tool can extract all temperature variables. New versions of files containing Alaska temperature normals have been uploaded to our Web Accessible Folders that contain the entire dataset, including both by-variable and by-station bulk files.
AK Modified Stations List
Were the 1991–2020 Climate Normals computed in the same way as 1981–2010 version?
Yes. Station normals calculations for 1991– 2020follow the approaches pioneered in the last cycle. The Normals Calculation Methodology 2020 documentation includes references to all the detailed normals methodology publications. Some technical changes to the way normals values are rounded, percentiles calculation, and data formatting have been adopted to align with World Meteorological Organization (WMO) Guidelines on the Calculation of Climate Normals (No. 1203).
What criteria determines which stations are included in U.S. Climate Normals products?
Climate Normals are computed for as many NOAA and partner weather stations with sufficient data for 1991–2020 as reasonably possible, including those in the National Weather Services (NWS) Cooperative Observer Program (COOP) Network and the Automated Surface Observing System (ASOS) Network found at many airports. Normals are also calculated for some stations with a Weather Bureau–Army–Navy (WBAN) station identification number, including stations from the U.S. Climate Reference Network (USCRN). Finally, precipitation normals are being calculated for selected U.S. Department of Agriculture Snow Telemetry (SNOTEL) Network stations and citizen science Community Collaborative Rain, Hail, and Snow (CoCoRaHS) Network for the first time.
How many stations are included in the1991–2020 U.S. Climate Normals?
The 1991–2020 Climate Normals includes data from more than 15,000 stations reporting precipitation and more than 7,300 stations reporting temperature. Of the stations reporting precipitation, more than 5,700 have adequate observations to report snowfall and snow depth normals. About 467 stations report hourly normals.
The 2006–2020 Climate Normals includes data from more than 13,000 stations reporting precipitation and more than 5,500 stations reporting temperature. Of the stations reporting precipitation, more than 1,600 have adequate observations to report snowfall and snow depth normals. About 1,150 stations report hourly normals.
What do climate normals tell us about global warming or climate change?
Climate Normals were not developed to track or document climate change, but instead to characterize the current climate. However, the differences between each installment do provide some evidence of climate change. This is especially apparent if current normals are compared to a long term average, such as the 1901-2000 20th Century Average often used by the NCEI Monitoring Section to compare to long time series of climate variables.
What are heating and cooling degree days? What are growing degree days?
Heating and cooling degree days are energy demand metrics associated with the variation of mean temperature across space and time. Growing degree days are climate metrics that measure the temperature inputs into agricultural systems, also as a function of mean temperature.
Degree day computations use certain threshold temperatures (e.g., 65°F for heating and cooling degree days, 50°F for growing degree days ) as base temperatures, and accumulate degree day metrics daily as departures from these baselines. Different baselines are useful for different assessments, so the Climate Normals product provides access to numerous degree day normals types with different baselines.
Does the 1991–2020 installment include Supplemental Temperature Normals?
Supplemental Temperature Normals were not developed for 1991–2020. Instead, an entire suite of 15-year normals for 2006-2020 have been calculated for all variables using the same methods used for the 30-year normals. The 15 year normals are optimized for use cases that require more recent climate information, such as predicting energy system loads and other economic decisions.
However, NCEI partners at the High Plains Regional Climate Center have developed a custom climatology tool that allows users to calculate monthly temperature normals for any time frame within the current period for the same set of stations as the conventional normals.
Do other agencies and individuals compute their own Climate Normals?
Yes. Many agencies, including the NOAA Climate Prediction Center, develop their own averages and change base periods for specific applications and/or internal use. Similarly, numerous individuals calculate their own Climate Normals for a variety of reasons. NCEI is the official source for the conventional 30-year U.S. Climate Normals.
