The experience of 60-degree temperatures in December is a stark departure from the expected seasonal norm for many regions in the temperate latitudes. When the calendar suggests the onset of winter, such warmth signals an unusual atmospheric event that warrants scientific explanation. This unseasonal warmth is not a random occurrence but the result of specific, identifiable processes unfolding within the atmosphere. Understanding this phenomenon requires examining the short-term weather mechanics, the larger-scale climate influences that set the stage, and the long-term changes affecting the entire system.
The Immediate Meteorological Drivers
The primary cause of a short-term warm spell in December is often a persistent atmospheric feature known as a high-pressure ridge. This ridge is a dome of sinking, stable air that settles over a region, acting as a barricade against cold air masses. The presence of this high pressure forces the powerful, high-altitude air current known as the Jet Stream to take a significant detour, pushing its path far northward.
By diverting the Jet Stream, the high-pressure system prevents the natural southward flow of frigid Arctic air from reaching the area. Instead, air circulating clockwise around the high-pressure center draws in warmer air from the south, a process called warm air advection. Additionally, the air within the high-pressure dome is sinking and compressed, which leads to adiabatic warming. This compression further raises ground temperatures, contributing to the unseasonal heat and locking the weather into a warm, clear state.
The Influence of Global Climate Cycles
While the high-pressure ridge is the direct cause, its formation and persistence are often influenced by large-scale, naturally occurring global climate patterns. These phenomena, operating over months or years, create “teleconnections” that shift the probability of certain weather setups across vast distances. The El Niño-Southern Oscillation (ENSO), characterized by fluctuating sea surface temperatures in the equatorial Pacific, is a significant source of this year-to-year climate variability.
During a positive phase of ENSO, or El Niño, the distribution of atmospheric pressure is altered worldwide, frequently favoring warmer and drier conditions across the northern United States and parts of Canada. Another influential pattern is the Arctic Oscillation (AO), which involves changes in atmospheric pressure between the Arctic and the mid-latitudes. When the AO is in its positive phase, lower pressure over the Arctic helps keep colder air masses contained closer to the pole, allowing mid-latitude regions to experience milder winter temperatures. These cycles influence the placement and strength of the Jet Stream, making the formation of a warm-air-advecting high-pressure ridge more likely.
Weather Variability Versus Climate Change
To understand this unseasonal warmth, it is necessary to distinguish between “weather” and “climate.” Weather describes atmospheric conditions over a short period, such as a day or a week, while climate is the average weather pattern measured over long timeframes, typically 30 years or more. A single warm day in December is a weather event, but the increasing frequency of such events signals a changing climate.
Anthropogenic climate change, driven by greenhouse gas emissions, is raising the planet’s overall baseline temperature. This long-term warming means that when natural weather variability occurs, the resulting warm spell starts from an already higher temperature point than it would have historically. This baseline change is analogous to rolling a die on a tilted table: the tilt makes rolling an extreme warm event occur more often.
Warming in the Arctic is weakening the temperature gradient between the pole and the equator. This reduced temperature difference contributes to a slower and wavier Jet Stream, which is more prone to developing highly amplified ridges and troughs. These features can become “stuck” in place for longer periods, a phenomenon known as atmospheric blocking. This blocking allows the unseasonably warm conditions caused by the high-pressure ridge to persist for days or weeks, transforming a temporary fluctuation into a prolonged warm event.