How Normals Are Calculated



The calculation of monthly precipitation normals by NCEI is fairly straightforward, but there are rules that are followed. NCEI follows guidelines established by the World Meteorological Organization (WMO) with some differences. One of the primary changes to calculations involved the adoption of Banker’s rounding, where a decimal value ending in 5 rounds to the nearest even integer. Calculations are performed in the original metric units of the source GHCN datasets. Numbers are retained at machine precision until rounding is performed for the final output. Users may notice small apparent inconsistencies between different normals variables that result whenever any rounding is applied.

Each month is treated equally in calculating seasonal and annual averages; they are not weighted by the length of month. Monthly values of most variables are output to one decimal place in customary units, except for precipitation reported to the nearest hundredth of an inch.

The calculation of monthly, seasonal, and annual counts of threshold exceedances are done as stipulated by the updated WMO guidelines. An average monthly count is calculated using the following procedure: (1) counting the number of exceedances in each year/month, (2) converting these counts to percentages of the total number of observations in each year/month, (3) averaging these percentages to obtain a climatological average percentage for each calendar month, and (4) multiplying the climatological percentage by the maximum number of days in the month.

Here are the precipitation, snowfall, and snow depth thresholds for daily, monthly, seasonal, and annual frequencies of occurrence.

Precipitation: >= 0.01, 0.10, 0.25, 0.50, 1.00, 1.50, 2.00, 4.00, 6.00 inches

Precipitation: >= 1, 5, 10, 50, 100, 150 mm

Snowfall: >= 0.1, 1.0, 2.0, 3.0, 4.0 5.0, 10.0, 20.0 inches

Snowfall: >= 5, 10, 25, 100, 500 mm

Snow Depth: >= 1, 2, 3, 4, 5, 10, 20 inches


More background on the calculation of normals and changes from the 1981-2010 normals can be found at this link:


“Pseudo-normals” are estimates of monthly precipitation totals and month-to-date and year-to-date totals scaled to match them. A station qualifying for “Pseudo-normal” status must have each calendar month complete with daily observations in at least two years. This means that observers who report regularly but have only been observing for a few years can qualify. NCEI uses multi-day totals spanning up to 5 days for the calculation of monthly totals, as long as they are entirely within the month, and as long as there are no more than 10 days involved in them in any one year/month.

“Provisional normal” means that calculations are made of daily and monthly averages, percentiles, and frequencies of occurrence for precipitation and, when sufficiently available, for snowfall and snow depth. A station qualifies for Provisional normal status if each calendar month is complete with daily (not multi-day) observations in at least 10 years. This means zero must be entered on days with no precipitation. NCEI requires daily values for this criterion in order to be able to calculate daily normals. Note that this does not mean you cannot have any multi-day values in your record. If each day in each calendar month has 10 years of data (does not have to be the same ten years), then the station qualifies. If a station, for example, has 13 years of record and several multi-day reports scattered in its record, as long as the dates in the multi-day reports have at least 10 years of daily values in the record, the station qualifies for Provisional status. To demonstrate this further, let’s take an example of an observer who has been reporting every day for ten years takes a vacation from June 10-15 every year and always submits a multi-day report for that period on return from vacation. While all days are accounted for in terms of monthly and longer periods, there are not 10 daily records for June 10 through June 15. Thus, this station would not qualify for Provisional status. However, it would qualify for “Pseudo-normal” status.