Warm weather in December, when expectations are for cold and snow, has become increasingly common across many mid-latitude regions. Historically, the month has been defined by the deep chill of winter, but recent years have delivered temperatures far above the seasonal average. This shift prompts a fundamental question about the forces driving such unseasonable warmth. Understanding this requires looking beyond the immediate daily forecast to examine both short-term atmospheric dynamics and the long-term changes shaping our climate system.
Differentiating Short-Term Weather from Long-Term Climate
The immediate feeling of a warm December day is a function of weather, which describes the atmospheric conditions at a specific time and location. Weather involves the current state of variables like temperature, precipitation, and wind, changing minute-to-minute or day-to-day.
Climate, conversely, is defined as the statistical average of weather patterns over an extended period, typically calculated over 30 years or more. Weather is what you get on a given day, while climate is what you expect over a season. The increasing frequency of warm Decembers signals a changing climate, while the specific warmth on any particular day is a manifestation of the current weather.
Immediate Atmospheric Mechanisms That Bring Warm Air
The specific cause of unseasonable warmth in December is the direct movement of warm air masses from lower latitudes into typically cold regions. This process is often controlled by the behavior of the Jet Stream, a fast-flowing river of air high in the atmosphere that separates warm, southern air from cold, northern air. The Jet Stream does not flow in a straight line, but instead meanders in large waves, consisting of northward bulges called ridges and southward dips called troughs.
A pronounced northward bulge, or ridge, in the Jet Stream allows warm, subtropical air to be drawn far to the north, effectively walling off cold air that would typically flow down from the Arctic. This pattern is a mechanism for advection, the horizontal transfer of atmospheric properties, bringing mild temperatures to areas that should be experiencing freezing conditions. When these ridges become large and persistent, they can trap warm air over a region for an extended period, leading to unseasonable warmth.
The position of the Jet Stream is also linked to large-scale pressure systems, such as the Arctic Oscillation (AO). During the AO’s positive phase, the Jet Stream is stronger and stays farther north, which helps to contain the coldest polar air near the Arctic. This configuration results in milder winter conditions across the mid-latitudes, as cold air outbreaks are blocked from moving south.
Furthermore, large-scale oceanic patterns, such as El Niño, can indirectly influence the Jet Stream’s track and wave patterns. These periodic warmings of the Pacific Ocean surface can trigger atmospheric ripples that affect the position of the Jet Stream over continents. These influences contribute to the formation of the persistent ridges that steer warm air into northern regions during December.
The Context of Increasing Global Temperatures
While immediate warmth is caused by specific atmospheric configurations, the underlying reason for the increasing intensity and frequency of these events is the long-term trend of rising global temperatures. As the planet’s average temperature increases, the baseline temperature for any given day is already higher than it was decades ago. This rising baseline means that when a typical weather pattern brings warm air, the resulting temperature is more likely to break record highs.
In many locations, winters are warming faster than any other season. For instance, average winter temperatures in many U.S. cities have increased by nearly four degrees Fahrenheit since the 1970s. This overall warming trend has resulted in fewer days below freezing and a general shortening of the winter season.
A significant factor contributing to this trend is Arctic Amplification, where the Arctic region warms at a rate faster than the rest of the globe. This uneven warming reduces the temperature difference between the cold Arctic and the warmer mid-latitudes, a contrast which provides the energy that drives the Jet Stream. A smaller temperature gradient can cause the Jet Stream to slow down and become wavier, leading to more persistent weather patterns.
These slower, wavier patterns increase the probability of persistent ridges that bring warm air, translating the long-term climate shift into short-term weather extremes. The concept of a “warm” December is changing as the meteorological normals, calculated over 30-year periods, are steadily increasing. This means what was once considered unseasonably warm is now closer to the new expected average.