January and now February, parts of the country have faced extreme cold, prompting many meteorologists to note, “This air is much colder than average.”
But how do we define “average,” and have those averages—or normals—changed over time?
Climatologists use 30-year periods to smooth out year-to-year variability and establish baselines, or “climate normals,” for comparison. For example, a high of 45 degrees in January in Missouri might be five degrees above the long-term average.
Why 30 years?
Jared Rennie, a physical scientist at the National Oceanic and Atmospheric Administration (NOAA)’s National Centers for Environmental Information, explains that the 30-year standard better reflects the changing climate and its influence on day-to-day weather.
He adds that NOAA also explores other base periods to meet user needs. In addition to the 1991–2020 normals released a few years ago, NOAA provides a 15-year baseline (2006–2020).
These baselines are updated every ten years to capture ongoing changes—so data from 1980 to 2010 will differ from values averaged over 1990–2020.
Differences in the data
There are regional differences when comparing the most recent datasets (1980–2010 vs. 1990–2020). Annual precipitation has increased 5–10% in the central and eastern U.S. and decreased 5–10% in the Southwest.
Temperatures are generally warmer by 0.3 to 1.0°F across most areas, with the north-central U.S. slightly cooler.
(Courtesy: NOAA’s National Centers for Environmental Information)
More frequent 100-year events
It seems like every few weeks we hear, “This is a once in a 100-year event.” Why are we seeing more of these “once in a lifetime events?”
“Scientifically, this usually refers to the percent chance an event happens in any given year,” he stressed.
Adding, “With extreme rainfall events, NOAA uses data to identify areas that exceeded the 1% or 0.1% chance of occurring in a given year (known as a 1-in-100-year event and 1-in-1000-year event, respectively).”
He says that as for the frequency of these types of events, it depends on the specific event. “When it comes to large-scale events like droughts and heat, we have a better understanding of how these are trending over time.”
“For example, the science tells us that temperatures are increasing, especially at nighttime, which is affecting the number of heat events in the 21st century.”
Smaller-scale events, such as tornadoes and certain floods, are harder to quantify, and ongoing research aims to improve understanding.
Weather vulnerability
How do population densities impact weather vulnerability? “There is lots of research in the socioeconomic space that is attempting to identify not only populations affected by weather and climate extremes, but also their risk.”
The Federal Emergency Management Agency (FEMA) provides resources to help communities assess risk from heat waves, cold outbreaks, hurricanes, and tornadoes. More information can be found in the Dec. 2025 National Risk Index for Natural Hazards index.
FEMA is working to provide information for communities most at risk from weather events, like heat waves, cold outbreaks, hurricanes and tornadoes.
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Meteorologist Stacy Lynn
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