Large parts of the United States and Europe experienced unusually high temperatures during the Christmas period, prompting climate scientists to describe the phenomenon as a “winter heatwave” and, in some regions, a “winterless Christmas.” The unseasonal warmth has been linked to persistent high-pressure systems in the atmosphere, with climate change amplifying the intensity of these events.
In the days leading up to Christmas, vast areas of the US recorded daytime temperatures as much as 20 degrees Celsius higher than the seasonal average. Regions across the West, Plains, Midwest, South and Southeast were affected, with temperatures running 15 to 25 degrees Fahrenheit above normal, and in some locations soaring 25 to 35 degrees Fahrenheit higher than usual.
Climate experts described the scale of the warming as extraordinary. Peter Carter of the Climate Emergency Institute noted that the warmth extended across most of the country, while Jeff Berardelli, chief meteorologist at WFLA-TV in Tampa, called it the hottest Christmas period on record for the US by a significant margin. Temperature anomaly maps showed widespread departures from normal conditions, alongside record highs being set on successive days through Christmas.
Across the Atlantic, parts of Europe also witnessed exceptional warmth. Iceland recorded its highest-ever December temperature on December 24, 2025, when the mercury reached 19.8 degrees Celsius in the town of Seyðisfjörður, surpassing the previous record set in 2019. Meteorologists attributed the event to a combination of warm air masses and strong winds, which intensified warming as air descended from mountainous regions.
Scientists say these extremes were driven by high-pressure ridges, also known as atmospheric blocking patterns. Under such systems, air sinks and warms, becoming trapped beneath what is often described as a heat dome. These conditions suppress cloud formation and precipitation, allowing temperatures to climb well above normal levels. Forecasters had warned in advance that such patterns would lead to unusually warm conditions across both North America and Europe.
The US National Oceanic and Atmospheric Administration had earlier projected above-normal temperatures across most of the continental US from late December into early January. Similar atmospheric patterns over the North Atlantic helped funnel warm south-westerly winds towards Iceland, Greenland and nearby regions, making Christmas Day among the warmest on record in parts of the far North Atlantic.
While high-pressure ridges are more commonly associated with summer heatwaves, scientists warn that background global warming is increasing the likelihood, intensity and duration of such anomalies during winter as well. Rising greenhouse gas emissions, particularly from historically high-emitting regions such as the US and Europe, are creating conditions where extreme warmth can occur even during what should be the coldest months of the year.
As temperatures return closer to seasonal norms only when storm systems disrupt these heat domes, researchers caution that winter heatwaves may become more frequent in a warming world, further blurring the lines between seasons.