About Climate Normals

Climate is an important factor in agriculture, commerce, industry, and transportation. It affects many human activities such as farming, fuel consumption, structural design, building site location, trade, analysis of market fluctuations, and the utilization of other natural resources. The influence of climate on our lives is endless. The National Oceanic and Atmospheric Administration's (NOAA's) National Climatic Data Center (NCDC) has a responsibility to fulfill the mandate of Congress "... to establish and record the climatic conditions of the United States." This responsibility 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).

The mandate to describe the climate was combined with guidelines established through international agreement. The United Nation's World Meteorological Organization (WMO) requires the calculation of normals every 30 years, with the latest covering the 1961-1990 period. However, many WMO members, including the United States, update their normals at the completion of each decade.

The average value of a meteorological element over 30 years is defined as a climatological normal. The normal climate helps in describing the climate and is used as a base to which current conditions can be compared. Every ten years, NCDC computes new thirty-year climate normals for selected temperature and precipitation elements for a large number of U.S. climate and weather stations. These normals are summarized in daily, monthly, divisional, and supplementary normals products.

Current climate normals may be obtained from the National Climatic Data Center's U.S. Climate Normals Product Page.

Frequently Asked Questions

How much will the normals products cost?

For current product prices, please contact NCDC at (828) 271-4800 (phone), orders@ncdc.noaa.gov (e-mail), or by writing to National Climatic Data Center, NOAA/NESDIS, Veach-Baley Federal Building, 151 Patton Ave., Asheville, NC 28801-5001.

What is the period of record of the official normals and how often are they updated?

Normals cover a 30-year period of record, and are updated through the end of each decade ending in zero (e.g., 1951-1980, 1961-1990, etc.).

What is the difference between Cooperative and First-Order stations?

Cooperative refers to weather stations that are part of the U.S. Cooperative Observing Network. This network consists of several thousand temperature and/or precipitation stations that, in general, are manned by volunteer observers. A subset of about one thousand of the longer-term stations from this network make up the U.S. Historical Climatology Network (U.S. HCN), which is a very important and celebrated network in the climate research community.

First-order refers to weather stations that are professionally maintained, primarily through the National Weather Service or Federal Aviation Administration. Modernization of the National Weather Service during the 1990s resulted in the consolidation of many manned weather stations and the introduction of Automated Surface Observing System (ASOS) instrumentation throughout the United States. ASOS instrumentation is now in use at the vast majority of first-order sites, which are primarily located at airports.

Both cooperative and first-order sites are essential to the 1971-2000 normals. In many cases, cooperative sites offer a well-established, consistent observing site with periods of record that are continuous through the normals period. First-order sites are the professionally manned sites with hourly observations.

Is it appropriate to use normals for predictive purposes?

Normals are best used as a base against which climate during the following decade can be measured. Comparison of normals from one 30-year period to normals from another 30-year period may lead to erroneous conclusions about climatic change. This is due to changes over the decades in station location, in the instrumentation used, in how weather observations were made, and in how the various normals were computed. The differences between normals due to these non-climatic changes may be larger than the differences due to a true change in climate.

What are the differences between normals, standard normals, and long-term means?

Normals refer to the official thirty-year normals computed by the National Climatic Data Center.

Standard Normals are official normals computed for the World Meteorological Organization based on submission of data by its members. Standard normals are computed every thirty years (e.g. 1931-1960, 1961-1990, etc.).

Long-term means are mean values of meteorological elements that are computed for a myriad of reasons by organizations and individuals. Even if long-term means are computed for the normals period (e.g. 1971-2000), only the NCDC values are appropriately called normals.

On What Output Media Are the Normals Available?

The 1931-1960 through 1971-2000 climate normals discussed on this home page are available as printed and digital publications, on microfiche, and/or digitally on magnetic tape. Please contact the NCDC Climate Services Division to determine the availability of any particular product, or if you have any questions. The Climate Services Division can be reached by telephone (828-271-4800), fax (828-271-4876), or via the internet: orders@ncdc.noaa.gov

All of the normals files in NCDC's digital archive (through the end of 1996) have been loaded onto one CD-ROM. This USDS - Vol. 1.0 CD-ROM contains only the documentation files and ASCII text data in the NCDC archive tape format; the data are not importable into a spreadsheet and the CD-ROM contains no software or extraction routines that allow users to import the data directly into spreadsheets or other applications. An updated CD-ROM product fo the 1971-2000 normals is planned for early 2002.

Digital ASCII data and PDF-formatted digital publications are available through NCDC's online store.

What is a “Climate Normal”?

The term climatic "normal" has faced a dilemma since its introduction a century and a half ago. A climate normal is defined, by convention, as the arithmetic mean of a climatological element computed over three consecutive decades (WMO, 1989).... a normal value is usually not the most frequent value nor the value above which half the cases fall." The casual user, however, tends to (erroneously) perceive the normal as what they should expect. Dr. Helmut E. Landsberg, who became Director of Climatology of the U.S. Weather Bureau in 1954 and, later, Director of the Environmental Data Service, summarized the dilemma quite well over four decades ago (Landsberg, 1955): "The layman is often misled by the word. In his every-day language the word normal means something ordinary or frequent. ...When (the meteorologist) talks about 'normal', it has nothing to do with a common event..... For the meteorologist the 'normal' is simply a point of departure or index which is convenient for keeping track of weather statistics..... We never expect to experience 'normal' weather."

It might be "normal" for the weather to swing radically between extremes from day to day and year to year, but the "climatic normal" is simply an arithmetic average of what has happened at such a "swinging" place. This is why it's important to use a measure of the variability of climate (such as the standard deviation and extremes) in conjunction with the climatic normal when studying the climate of a location (Guttman, 1989).

In accordance with national and international convention, the official climate normals computed for U.S. stations by NCDC consist of the arithmetic average of a meteorological element over 30 years. The ‘official’ normals are provided solely by NCDC, which should be noted in light of other non-official means computations from a myriad of sources.

Is a "Normal" the Climate You Would "Expect"?

Climate normals are a useful way to describe the average weather of a location. Several statistical measures are computed as part of the normals, including measures of central tendency (such as the mean or median), of dispersion or how spread out the values are (such as the standard deviation or inter-quartile range), and of frequency or probability of occurrence.

Over the decades the term "normal", to the lay person, has come to be most closely associated with the mean or average. In this context, a "climatic normal" is simply the arithmetic average of the values over a 30-year period (generally, three consecutive decades). A person unfamiliar with climate and climate normals may perceive the normal to be the climate that one should expect to happen.

It's important to note that the normal may, or may not, be what one would "expect" to happen. This is especially true with precipitation in dry climates, such as the desert southwestern region of the United States, and with temperature at continental locations which frequently experience large swings from cold air masses to warm air masses.

If the measuring equipment changes at a site, does that change the records? Does it change the normals?

Changes at the site never change what was originally observed, but these observations are subject to edits based on established criteria.

For normals, if the new equipment does not record weather elements in exactly the same way as the old, and causes a change in how weather is recorded relative to the previous instrumentation, then it does change the normals. If the equipment MOVES, this will also often cause apparent changes in climate. Location moves (anywhere from a few hundred feet to a few miles) can, in fact, cause greater changes than do instrumental changes.

The following white paper (United States Climate Normals, 1971-2000: Inhomogeneity Adjustment Methodology) [PDF] is available regarding procedures for adjusting station data to account for inhomogeneities due to changes in station locations, instrumentation, time of observation, surrounding environment, observing practice, sensor drift, etc. The purpose of such adjustments is to produce a time series and normals statistics that are representative of the observing practices as of the end of the normals period (December 2000), since these are the conditions under which future observations will likely be compared.

Since climate fluctuates constantly, there are real changes in "normals" due to climate variations, and there are fake changes in "normals" due to artificial things like sensor changes, equipment moves, methodological issues, and so on. The ONLY way to distinguish between fake and real variations is to have records from another piece of equipment at one site or the other that just keeps measuring the same way.

Are there differences in the values for the same time period between the 1961-1990 and 1971-2000 normals?

Differences in monthly values during the overlap period (1971-1990) in temperature and precipitation for a given station are possible due to differences in adjustment methods between the 1961-1990 and the 1971-2000 normals. Generally, the 1971-2000 time series has been subjected to adjustments with combined spatial and statistical quality control, whereas the 1961-1990 time series was based less on spatial comparisons and more on limited quality control. See the methdology section under the products page for more information on the 1971-2000 methodology.

How are degree day normals computed?

The following white paper (United States Climate Normals, 1971-2000: Degree Day Computation Methodology) [PDF] is available regarding the two-tiered approach to computing degree day normals.

The 1971-2000 degree day normals are computed using a new methodology. Previously, degree days were computed using the Thom rational conversion formulae (Thom, 1954, 1966). The Thom method allows a monthly degree day total to be estimated from input average temperature means and standard deviations.

For the 1971-2000 normals, degree day totals were computed in two distinct ways. For stations that are not first-order National Weather Service locations, the rational conversion formulae developed by Thom (1954, 1966) was modified by using inputs of daily spline-fit (rather than monthly) means and standard deviations of average temperature. This modification improved consistency of the estimated degree day totals by eliminating monthy-by-month 'steps' in the inputs. For first-order stations, where daily data sets are largely devoid of missing values, monthly degree day totals were derived directly from daily values.

What are 'spurious' degree day values in the daily normals?

In the daily normals files, values of '-99' (or an asterisk in non-digital printouts) in the daily heating/cooling degree days represent values of 1 that have been designated to be 'spurious'. Such values are designated as spurious because of their separation from the major rise and fall of non-zero degree day values over the course of a heating/cooling degree day season, yet their presence assures consistency between the monthly total and the sum of the daily total (when values are considered equal to 1).


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