UK experiences unseasonably warm December as London hits 12°C; meteorologists attribute mild spell to Atlantic jet stream

London logged daytime temperatures around 12°C (53.6°F) in December, prompting bafflement and discussion across social media as Britons swapped winter coats for lighter layers. The unusually mild start to the month has raised questions about how this pattern fits into longer-term climate trends, with many residents noting that December has felt warmer than usual this century. While some saw it as a temporary blip, others warned against ignoring what the warmth could mean for future winters.

Across the United Kingdom, the early December period has shown temperatures well above the historical norms. Average December temperatures in the country typically range from 2°C to 8°C (35°F to 46°F), depending on location, yet many regions have remained above those levels. In certain areas, daytime highs have climbed to around 15°C (59°F), with nights rarely dipping below freezing. The warmer spell has sparked a wave of social media posts from people who say they are walking to work in shirts rather than jackets and who fear the waning likelihood of a white Christmas. Some observers also report feeling overheated inside buildings, underscoring how anomalous the pattern feels to those accustomed to colder December days.

Meteorologists say the warmth is linked to a persistent North Atlantic jet stream that has driven a series of low-pressure systems northeast across the Atlantic. The jet stream is a fast-moving river of air high in the atmosphere that steers weather fronts and systems toward the United Kingdom. In recent weeks, it has been unusually energetic over the country, helping push mild air from the south while also maintaining wet, unsettled conditions across the British Isles. The pattern has formed in part because of the cold weather that has settled over northeastern North America and parts of Canada, which interacts with the Atlantic to energize the jet and spawn successive low-pressure depressions.

Jim Dale, a senior meteorologist at British Weather Services, described the situation as a relatively active jet stream that has kept the UK in a period of warm, wet, and windy weather since November. He explained that when cold air sits over parts of the northeastern United States and Canada, it can spill into the Atlantic and interact with mild air to accelerate the jet stream, fueling ongoing low-pressure systems. That sequence helps explain both the warmth and the accompanying rainfall and storminess—the kind of weather that has produced events such as Storm Bram in recent weeks.

The Met Office notes that forecasts anticipate the pattern may ease in the coming weeks as a large area of high pressure builds closer to the UK around December 22. If that high pressure develops as predicted, nights may turn clear, frost and fog patches could form, and Christmas Day could be drier and marginally cooler than the early December spell. The agency cautions that even with an eventual cooldown, the underlying tendency toward milder winters remains a factor in the longer-term climate picture.

A Met Office spokesperson stressed that attributing the current mild period to climate change requires formal attribution study. Still, the spokesperson added that human-induced climate change is associated with a general trend toward warmer winters, and that the pattern observed this December aligns with that broader background signal. The agency also notes historical context: six of the ten warmest winters on record have occurred since the start of the 21st century, dating back to 1884, and 2025 is on track to be one of the three hottest years on record. This does not imply that cold spells cannot occur, but it does reflect a shift toward warmer winter norms on average.

The broader explanation for the current warmth centers on jet stream dynamics. Jet streams are narrow bands of strong winds that circle the globe and flow from west to east in the upper troposphere. They are strongest in winter when there is the largest temperature difference between Arctic and tropical air masses. In recent weeks, the Arctic–tropical temperature contrast in the upper atmosphere has produced a jet stream that has stayed unusually active near the British Isles, reinforcing a sequence of low-pressure systems that bring mild air and substantial rainfall. Forecasters say this pattern can be episodic, with periods of warmth alternating with cooler snaps, but the long-term trend toward warmer winter averages remains likely to influence future seasons.

Looking ahead, meteorologists caution that while a late-December cooldown is plausible, the overall winter climate is expected to continue evolving in a warmer direction compared with historical norms. The combination of a strong jet stream, Atlantic storm activity, and the continuing influence of global warming means that episodes of milder weather are more probable than they were several decades ago, even as cold spells can still occur.

For residents, the immediate takeaway remains simple: monitor updated forecasts, prepare for unsettled conditions through mid-December, and expect the possibility of drier, clearer nights if the high-pressure pattern materializes near the holiday period. Authorities urge people to remain aware of flood watches and wind advisories in areas exposed to heavy rainfall and gusty winds as the season progresses, particularly in regions that have seen repeated storm events.