Frequently Asked Questions

Are the 1991-2020 Normals calculations different from 1981-2010 Normals? Just a little

Station normals calculations have followed the approaches pioneered in the last cycle. Some technical changes to the way normals values are rounded, percentiles are calculated, and data formatting have been adopted to align with the World Meteorological Organization (WMO) Guidelines on the Calculation of Climate Normals (No. 1203). You can also find all the details about the methodology within the Normals Calculation Methodology 2020 publication.

Why are the Normals calculated for a 30-year period? History, tradition, and WMO requirements.

Normals serve two purposes: a reference period for monitoring current weather and climate, and a good description of the expected climate at a location over the seasons. They provide a basis for determining whether today’s weather is warmer or colder, wetter or drier. They also can be used to plan for conditions beyond the time span of reliable weather forecasts.

A 30-year time period was chosen by the governing body of international meteorology in the 1930s, so the first normals were for 1901-1930, the longest period for which most countries had reliable climate records. International normals were called for in 1931-1960 and 1961-1990, but many countries updated normals more frequently, every 10 years, so as to keep them up to date. In 2015 this was made the WMO standard, so all countries will be creating normals for 1991-2020.

What are Climate Normals used for?

Meteorologists and climatologists regularly use Climate Normals for placing recent climate conditions into a historical context. NOAA's Climate Normals are commonly seen on local weather news segments for comparisons with the day's weather conditions. In addition to weather and climate comparisons, Climate Normals are utilized in seemingly countless applications across a variety of sectors. These include regulation of power companies, energy load forecasting, crop selection and planting times, construction planning, building design, and many others.

Why are my Averages Different from NCEI's Normals?

Short Answer:

There are three main reasons why your temperature averages may differ from NCEI’s temperature normals: differences in the data used, different approaches to handling data gaps, and homogeneity adjustments applied by NCEI. These methodological differences can result in differences as large as if not larger than 1 deg. F between a "raw" temperature average and the corresponding temperature normal from NCEI, although most are much smaller. For the "Long Answer" visit Why are my Averages Different from NCEI's Normals?

Are there normals for periods other than 30 years? Yes

There is a set of alternative normals provided by NOAA NCEI for the 15-year period from 2006-2020. However, instead of being available for temperature only, there is a full set of normals for all variables for those users needing a representation of climate based on more recent conditions.

The High Plains Regional Climate Center (HPRCC) at the University of Nebraska, Lincoln, has also developed a Custom Climatology Tool that allows users to generate a normal-style average for any recent period of time from a chosen start date to a chosen end date from the special underlying data set that is used to generate official normals, including corrections for station discontinuities and time of observation changes.

Will there be visual maps of changes in Normals from 1981-2010 to 1991-2020? Yes

New sets of gridded normals are under development for monthly and daily temperature and precipitation based on the NOAA NCEI nClimGrid product. Maps of the changes in normals will be available, along with monthly temperature and precipitation gridded normals. By spring 2022, the 1991-2020 daily gridded normals will be released to the public, providing a new source of normals for spatial applications. While station normals are still the gold standard for specific locations, these gridded normals will be representative of all places in the conterminous U.S.

What qualifies or disqualifies a station to be included in Climate Normals products?

Climate Normals are computed for as many stations operated by the National Weather Service (NWS) as reasonably possible. Some stations do not have sufficient data over the 1981–2010 period to be included in the Climate Normals, and this is the primary reason a station may not be included. Climate Normals are computed for stations that are part of the NWS Cooperative Observer Program (COOP) Network. Some additional stations are included that have a Weather Bureau–Army–Navy (WBAN) station identification number including the U.S. Climate Reference Network (USCRN). Climate Normals are only computed for stations in the United States (including Alaska and Hawaii) as well as U.S. territories, commonwealths, compact of free association nations, and one Canadian USCRN station.

How many stations are included in the 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 designed to be metrics of climate change. In fact, when the widespread practice of computing Climate Normals commenced in the 1930s, the generally accepted notion of the climate was that underlying long-term averages of climate time series were constant. Changes from one installment of Climate Normals to the next do, nonetheless, provide some evidence of climate change impacts. However, care must be taken when interpreting changes between one Climate Normals period and the other. Differences between the reported 1981–2010 Climate Normals and the 1991–2020 Climate Normals may be due to station moves, changes in methodology, changes in instrumentation, etc., that are not reflective of real changes in the underlying climate signal. Rather than inferring Climate Change impacts from Climate Normals, we recommend users instead look at trends in U.S. Historical Climatology Network (USHCN) time series.

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.

Tropical Storm Helene Threats and Impacts

Frequently Asked Questions