Unseasonably warm February weather, often called a “winter thaw” or “warm spell,” feels unusual because February is typically when the most intense cold air masses are still possible. This phenomenon is not random but results from forces acting on three different timescales: immediate atmospheric patterns, broader natural climate cycles, and the long-term influence of a warming planet. Understanding this warmth requires examining the specific mechanisms moving warm air, the natural oscillations that set the stage, and the permanent shift in the climate baseline.
The Atmospheric Mechanics of a Warm Spell
A sudden surge of February warmth is triggered by a specific configuration of air masses and pressure systems high in the atmosphere. The most immediate cause is a major northward bulge in the jet stream, the fast-moving river of wind that steers weather across the globe. When the jet stream develops a strong ridge, it pushes far north, acting as a temporary barrier that blocks frigid Arctic air from moving south.
This ridge allows warm, subtropical air to be efficiently drawn northward from lower latitudes, transporting heat energy into a region that should be cold. Simultaneously, this pattern often leads to the development of a persistent high-pressure system, or anticyclone, over the affected area. Air within a high-pressure system slowly sinks, and as it descends, it compresses and warms through a process called adiabatic heating, further raising surface temperatures.
The combination of this atmospheric steering and warming effect creates a dome of warm air, resulting in clear skies that allow more solar radiation to reach the surface. Even in February, the sun angle is climbing, and clear conditions allow for significant daytime warming. This immediate, synoptic setup determines the location and intensity of the warm spell on a day-to-day basis.
Natural Climate Drivers and Seasonal Fluctuation
While the jet stream sets up the daily warmth, larger, natural climate drivers determine the atmospheric conditions that make this setup more likely in a given year. The climate system naturally fluctuates on multi-year to multi-decade cycles, which predispose certain regions to unusual warmth or cold. One of the most influential is the El Niño-Southern Oscillation (ENSO), a pattern of varying sea surface temperatures in the tropical Pacific Ocean.
In an El Niño phase, the warm waters in the Pacific influence the global jet stream pattern through atmospheric teleconnections, often leading to milder-than-average winter conditions across large parts of the Northern Hemisphere. This global shift can make it easier for the jet stream to form the specific northward ridge necessary for a February thaw. Another influence is the Arctic Oscillation (AO), which describes the fluctuation of atmospheric pressure between the Arctic and mid-latitudes.
The Arctic Oscillation (AO)
A positive phase of the AO corresponds to lower pressure over the Arctic, keeping the coldest air locked near the pole and resulting in milder winters for many areas. Conversely, a negative phase allows frigid air to plunge south. However, a rapid switch to a positive phase can occur in late winter, facilitating a warm spell.
February is also a month when the contrast between the still-cold land masses and the warming sun angle starts to weaken. This makes the atmosphere more susceptible to warm air incursions even without extreme external forcing.
How Climate Change Influences Winter Warmth
The long-term, human-caused warming of the planet acts as a permanent backdrop, altering the nature of these warm spells. While natural variability causes the specific weather event, climate change increases the frequency and magnitude of the warmth experienced. The most direct influence is the rise in global baseline temperatures, meaning that when warm air is pulled north, it starts from a significantly warmer point than it would have decades ago.
This higher starting point means that a naturally occurring warm spell is more likely to break temperature records than in the past. The warming trend has already been observed to shorten winter seasons everywhere, reducing the number of days below freezing. Studies suggest that the frequency of winter warm spells is increasing, while the frequency of extreme cold spells is decreasing.
Furthermore, the disproportionately rapid warming of the Arctic compared to the mid-latitudes may be influencing the jet stream’s behavior, potentially making it wavier and slower. This wavier pattern can lead to more frequent blocking events where weather systems, including warm ridges, stall and persist for longer periods. Climate change intensifies the effects of naturally occurring weather patterns, making February warmth more common and more extreme.