Another Arctic blast is here, but how much colder than ‘normal’ is it really?
A large part of the U.S. is getting hit with another Arctic blast, adding to an already cold January 2025. Forecasters expect temperatures to drop below zero in several states, and to plunge to more than minus 30 in parts of the Dakotas, Minnesota and Wisconsin. More winter storms are expected alongside that cold polar air.
Forecasters are warning that temperatures in some areas could be 30 degrees or more “below normal” at times between Jan. 18 and 22. The high in Washington, D.C., on Inauguration Day is expected to be in the mid-20s, about 20 degrees below “normal”, and the swearing-in ceremony was moved indoors for the first time since Ronald Reagan was in office.
But what does “normal” actually mean?
While temperature forecasts are important to help people stay safe, the comparison to “normal” can be quite misleading. That’s because what qualifies as normal in forecasts has been changing rapidly over the years as the planet warms.
Defining normal
One of the most used standards for defining a science-based “normal” is a 30-year average of temperature and precipitation. Every 10 years, the National Center for Environmental Information updates these “normals,” most recently in 2021. The current span considered “normal” is 1991-2020. Five years ago, it was 1981-2010.
But temperatures have been rising over the past century, and the trend has accelerated since about 1980. This warming is fueled by the mining and burning of fossil fuels that increase carbon dioxide and methane in the atmosphere. These greenhouse gases trap heat close to the planet’s surface, leading to increasing temperature.
Because global temperatures are warming, what’s considered normal is warming, too.
So, when a 2025 cold snap is reported as the difference between the actual temperature and “normal,” it will appear to be colder and more extreme than if it were compared to an earlier 30-year average.
Thirty years is a significant portion of a human life. For people under age 40 or so, the use of the most recent averaging span might fit with what they have experienced.
But it doesn’t speak to how much the Earth has warmed.
How cold snaps today compare to the past
To see how today’s cold snaps – or today’s warming – compare to a time before global warming began to accelerate, NASA scientists use 1951-1980 as a baseline.
The reason becomes evident when you compare maps.
For example, January 1994 was brutally cold east of the Rocky Mountains. If we compare those 1994 temperatures to today’s “normal” – the 1991-2020 period – the U.S. looks a lot like maps of early January 2025’s temperatures: Large parts of the Midwest and eastern U.S. were more than 7 degrees Fahrenheit (4 degrees Celsius) below “normal,” and some areas were much colder.
But if we compare January 1994 to the 1951-1980 baseline instead, that cold spot in the eastern U.S. isn’t quite as large or extreme.
Where the temperatures in some parts of the country in January 1994 approached 14.2 F (7.9 C) colder than normal when compared to the 1991-2020 average, they only approached 12.4 F (6.9 C) colder than the 1951-1980 average.
As a measure of a changing climate, updating the average 30-year baseline every decade makes warming appear smaller than it is, and it makes cold snaps seem more extreme.
Conditions for heavy lake-effect snow
The U.S. will continue to see cold air outbreaks in winter, but as the Arctic and the rest of the planet warm, the most frigid temperatures of the past will become less common.
That warming trend helps set up a remarkable situation in the Great Lakes that we’re seeing in January 2025: heavy lake-effect snow across a large area.
As cold Arctic air encroached from the north in January, it encountered a Great Lakes basin where the water temperature was still above 40 F (4.4 C) in many places. Ice covered less than 2% of the lakes’ surface on Jan. 4. Even after two weeks of cold air, the ice cover remained at less than 10% on Jan. 14.
That cold dry air over warmer open water causes evaporation, providing moisture for lake-effect snow. Parts of New York and Ohio along the lakes saw heavy snow.
The accumulation of heat in the Great Lakes, observed year after year, is leading to fundamental changes in winter weather and the winter economy in the states bordering the lakes.
It’s also a reminder of the persistent and growing presence of global warming, even in the midst of a cold air outbreak.
This article has been updated with details on the latest Arctic blast.
This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Richard B. (Ricky) Rood, University of Michigan
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Richard B. (Ricky) Rood receives funding from the National Oceanic and Atmospheric Administration.
