December is traditionally associated with cold temperatures and snow, yet many regions are increasingly experiencing rain instead of the anticipated snowfall. This shift from frozen to liquid precipitation results from a complex interaction of atmospheric dynamics. Understanding why it is raining involves examining the physical conditions required for precipitation, the immediate drivers of weather patterns, and the long-term forces shaping the global climate. The weather experienced in December depends heavily on a location’s geographical position and the current behavior of the atmosphere’s major circulation systems.
The Necessary Ingredients for Winter Rain
For precipitation to fall as rain during a cold month, the atmosphere must satisfy two primary requirements: sufficient moisture and a specific vertical temperature structure. Precipitation almost always begins high in the cloud layer as ice crystals or snowflakes, even when surface temperatures are above freezing. This requires adequate water vapor, which is lifted and cooled to form ice particles.
The determining factor for the type of precipitation that reaches the ground is the temperature profile of the air column below the cloud base. If the entire layer of air from the cloud to the surface remains at or below 32°F (0°C), the precipitation falls as snow. However, when a layer of air with temperatures above freezing exists just above the surface, the falling snowflakes melt into raindrops.
This warm layer, known as the melting layer, must be deep enough to completely convert the ice crystals to liquid water. If the surface temperature is above freezing, the resulting liquid precipitation falls as rain. Even if the temperature at the ground is near or slightly below freezing, a deep warm layer aloft will cause the precipitation to fall as supercooled rain, which then freezes on contact with surfaces.
How the Jet Stream Directs December Weather
The immediate cause of these warm atmospheric profiles often lies with the position and strength of the Jet Stream, a fast-moving, high-altitude river of air circling the globe. This stream acts as the major boundary between cold Arctic air masses to the north and warmer, mid-latitude air to the south. The Jet Stream is not a straight band but features large, wave-like meanders, which meteorologists call troughs and ridges.
A ridge is an area where the Jet Stream bulges northward, allowing warm, moist air from subtropical or maritime regions to be pulled into colder areas. When a strong ridge persists over a region in December, it effectively blocks the southward flow of frigid Arctic air. This brings in a tropical maritime air mass, which contains high levels of water vapor and temperatures warm enough to create a deep melting layer.
This configuration is effective at generating December rain events, as the Jet Stream transports storm systems that tap into this warm, humid air. The resulting low-pressure systems follow the path of the stream, delivering significant precipitation that, due to the overriding warm air, falls exclusively as rain instead of snow. The persistence of this ridge determines whether a region experiences a prolonged period of mild, wet weather.
Global Climate Cycles and Localized Effects
The long-term positioning of the Jet Stream is heavily influenced by large-scale, naturally occurring climate cycles, such as the El Niño Southern Oscillation (ENSO) and the Arctic Oscillation (AO). El Niño, the warm phase of ENSO, involves warmer-than-average sea surface temperatures in the central and eastern tropical Pacific Ocean. This oceanic warming typically shifts the Jet Stream southward and extends it eastward across the southern United States during the winter.
This southern track often results in wetter-than-normal Decembers across the West Coast and the Southern Tier of the U.S., where moisture arrives as rain due to the warmer temperatures associated with the pattern. Conversely, the Arctic Oscillation refers to the difference in atmospheric pressure between the Arctic and mid-latitudes. Its negative phase allows cold air to plunge farther south. However, even when cold air is present, the simultaneous influence of El Niño can deliver enough warm, moist air to override the cold, leading to rain or mixed precipitation near the freezing line.
These cycles create teleconnections, linking conditions in one part of the world to weather patterns thousands of miles away. These drivers determine which areas receive the brunt of the Jet Stream’s influence, leading to localized December weather anomalies. A December with rain in one area may correlate with colder, snowier conditions elsewhere, depending on how these global patterns interact.
The Impact of a Warming Climate
Underlying the natural variability of the Jet Stream and climate cycles is the long-term trend of rising global average temperatures. This warming has a direct impact on the rain-to-snow ratio during December, especially in mid-latitude regions where temperatures hover near freezing. As the climate warms, the altitude at which the air temperature drops below freezing—the freezing level—is pushed higher into the atmosphere.
This higher freezing level means that a greater proportion of precipitation that starts as snow has more time to melt into rain before it reaches the ground. Even a small increase in the base temperature can be the difference between a major snowfall and a heavy rain event. This trend effectively shortens the snow season by causing precipitation at the beginning and end of winter to fall as rain instead of snow.
The result is a documented shift where locations that historically received significant December snow are now observing an increase in December rain events. This change is noticeable in regions near the historical snow-rain boundary, where the average temperature has warmed just enough to consistently favor liquid over frozen precipitation.