Understanding climate normals: What NOAA's 1991-2020 normals are, how they are made and how to read them
Climate normals are 30-year averages of weather observations that NOAA computes every ten years for each station. The current set covers 1991-2020 and was released in May 2021; it holds normals for about 15,492 U.S. stations, and PlainClimate shows 7,247 of them.
First published · revised
What does a normal tell me?
A normal is what the station measured on average. Chicago's July normal high of 85°F at O'Hare means that over 1991-2020 July afternoons at that thermometer averaged that much. It does not say any July day will reach it, and it describes the station, not the whole city.
How does NOAA compute them?
NOAA starts from each station's daily observations, fills short gaps from nearby stations, adjusts for instrument and site changes, and averages the 30 years. Monthly, daily and hourly normals, frost-date probabilities and counts of days over thresholds are all part of the set. Stations with shorter records get normals estimated from their neighbours, marked by NOAA's completeness flags.
Why do normals change from one set to the next?
Each new set drops the oldest decade and adds the newest. Because the 2010s were warmer than the 1980s across most of the country, the 1991-2020 normals are warmer than the 1981-2010 set at most stations. That shift is the climate changing, measured ten years at a time.
Questions
Why 30 years?
The World Meteorological Organization sets 30 years as the standard period: long enough to smooth out single unusual years, short enough to reflect the current climate. NOAA updates the set every decade.
When are the next normals?
The 2001-2030 normals follow the end of 2030; NOAA released the 1991-2020 set in May 2021, about five months after the period ended.
Is a normal the same as an average year?
It is the average over 30 years, adjusted for station changes and gaps. A single year usually differs from it, sometimes by several degrees.
Source: NOAA NCEI U.S. Climate Normals; NCEI methodology.