Weather
This post first appeared on NOAA.gov
Every 10 years, NOAA releases an analysis of U.S. weather of the past three decades that calculates average values for temperature, rainfall, and other conditions.
That time has come again.
Known as the U.S. Climate Normals, these 30-year averages — now spanning 1991-2020 — represent the new “normals” of our changing climate. They are calculated using climate observations collected at local weather stations across the country and are corrected for bad or missing values and any changes to the weather station over time before becoming part of the climate record.
Simply stated: The Normals are the basis for judging how daily, monthly, and annual climate conditions compare to what’s normal for a specific location in today’s climate.
For the past decade, the Normals have been based on weather observations from 1981 to 2010. In early May, climate experts at NOAA’s National Centers for Environmental Information (NCEI) issued an updated collection based on the weather occurring from 1991 to 2020. The data set reflects a “new normal” that takes the most recent 30 years of climate change-influenced weather and climate conditions into account. (More: See our Climate Normals Explainer.)
A warmer normal
The U.S. Climate Normals collection has 10 versions: 1901-1930, 1911-1940, and so on through 1991-2020. In the image below, we’ve compared the U.S. annual average temperature during each Normals period to the 20th-century average (1901-2000). The influence of long-term global warming is obvious: The earliest map in the series has the most widespread and darkest blues, and the most recent map has the most widespread and darkest reds.
A wetter normal?
In the collection of precipitation maps, few places exhibit a precipitation trend that is either steadily wetter or steadily drier than the 20th-century average. Instead, drier areas and wetter areas shift back and forth without an obvious pattern.
And yet, it’s probably not a coincidence that the last four maps in the series — the 1961-1990, 1971-2000, 1981-2010, and 1991-2020 Normals — are nationally the four wettest-looking maps in the collection. At least some of that wetness relative to the 20th-century average is linked to the overall climate warming and “wetting” of the atmosphere that’s occurred as rising temperatures cause more water to evaporate from the ocean and land surface.
What used to be normal
The 1991-2020 Normals tell us what is normal in today’s climate. NOAA scientists conduct other analyses that tell us about what used to be normal.
For example, in NOAA’s monthly and annual climate monitoring reports, temperature averages and precipitation totals are ranked in the climate record dating to 1895; the U.S. and global climate conditions are compared to the 20th-century average.
Visualizing climate is easier now than ever
NCEI has a collection of maps showing both recent and long-term trends in temperature and precipitation. You can also create a custom graph showing monthly, seasonal, or yearly climate conditions for any region, state, and many cities that shows the long-term trend.
The Normals might be shifting, but NOAA scientists and forecasters aren’t losing track of climate change.
