Polar Vortex: How Sudden Stratospheric Warming Sets the Stage for Cold-Air Outbreaks

[Jari Sochorová]

As the strength of the Sun’s rays decreases and the polar night beyond the Arctic Circle expands day by day, winter is approaching in the Northern Hemisphere. Astronomically, it does not begin until the moment of the winter solstice, which this year falls on Sunday, 21 December 2025 at 15:03 UTC. Meteorological winter arrives earlier, on 1 December.

Even though autumn still officially reigns in the calendar, the first wintry days with freezing weather and, in places, a generous blanket of snow have already arrived in the Northern Hemisphere during the 2025/2026 season.

Solar Power: forecast solar radiation at Earth’s surface (ECMWF), 23–25 November 2025; Windy.com

The Northern Hemisphere has already experienced two notably cold episodes. The first came in the first half of November 2025, when Arctic air spread into the eastern two-thirds of the United States. The second affected western and central Europe and parts of southern Europe in the second half of November, bringing an early taste of winter weather with below-average temperatures and snow not only in the mountains but also in some places at lower elevations.

Forecast maximum temperatures and temperature anomalies for 11 November 2025; NOAA National Weather Service

According to current forecasts, another outbreak of cold Arctic air is on the horizon around the turn of November and December 2025, this time for much of Canada and probably part of the United States.

Forecast 2 m air temperature anomaly (°C) over successive 24-hour periods from 22 November to 1 December 2025; ECMWF

With winter on the way, terms like Arctic air outbreaks, the polar vortex, or sudden stratospheric warming will appear more and more often in weather forecasts. In this article, we will explain them in simplified terms, show how they are connected, and why they can influence the weather from North America across Europe to the Arctic.

Polar vortex

At its most basic, the polar vortex is a ring of strong westerly winds that, in winter, circulates the cold polar air near the pole. It forms as part of the general circulation of the atmosphere and, in terms of its size, is incomparable with any other vortical phenomenon on our planet.

Schematic of the stratospheric and tropospheric polar vortex over the Northern Hemisphere; NOAA Climate.gov

In the media, the term polar vortex began to appear frequently during the 2013/2014 winter season, in connection with the freezing winter in North America. However, it has been used as a technical meteorological term for a long time; it appeared as early as 1853 in the magazine Littell's Living Age.

Temperature anomalies in winter 2013/2014 show an extreme east–west contrast across the United States. The central and eastern regions were much colder than average due to repeated Arctic air intrusions, while the West, especially California, was significantly warmer than average; NOAA Climate.gov

In meteorological practice, we often distinguish between the stratospheric and tropospheric polar vortex. They are two layers of the same system, but their evolution over time need not be synchronized.

Stratospheric polar vortex

If only the term polar vortex is used, it usually refers specifically to the stratospheric polar vortex, that is, the circulation in the stratosphere, roughly between 100 and 1 hPa (about 15–50 km). You can think of it as a strong, relatively symmetric band of westerly winds encircling the pole, which is clearly visible, for example, on geopotential height maps at 10 hPa.

Animated map of Northern Polar region showing the polar vortex; NOAA Climate.gov

Tropospheric polar vortex

The tropospheric polar vortex, on the other hand, refers to the circulation of cold air and westerly flow in the troposphere, typically in the 500–200 hPa layer (the jet stream region), and also in the near-surface pressure field as a deep polar trough or a broad area of low pressure.

Schematic of the position of the polar night jet and the jet stream within the polar vortex; Met Office

The boundary between cold polar air and the warmer air of the mid-latitudes in the upper troposphere is formed by the polar jet stream. The polar jet stream (hereafter simply the jet stream) is a narrow, elongated band of powerful westerly winds at altitudes of roughly 8–12 km. It extends approximately between 50 ° and 60° latitude. This strong temperature and pressure contrast between cold and warmer air is the main reason why the jet is so fast: wind speeds in its core can exceed 200 km/h.

On a large scale, the jet stream is one of the main factors controlling weather in the mid-latitudes. It strongly influences the movement of low- and high-pressure systems, thereby determining the overall character of the weather. When it is relatively straight and strong, smooth westerly flow predominates; when it becomes strongly meandering, its troughs and ridges can bring deep intrusions of cold polar air towards the equator and, conversely, carry warm air far into the cold polar regions.

Schematic of a meandering jet stream and the positions of low- and high-pressure systems; NOAA Climate.gov

How the Polar Vortex Forms

In winter, the polar regions are under polar night conditions. The Sun does not rise at all for weeks to months. As a result, the air over polar regions cools strongly through radiative cooling. At lower latitudes, the stratosphere at middle and upper levels, roughly 30–10 hPa (about 25–30 km, or 16–19 miles), is significantly warmer. This creates a sharp temperature contrast between the warmer low-latitude air and the colder air over the polar regions.

The atmosphere tries to balance this difference. Warmer air tends to flow from the tropics toward the pole, but Earth’s rotation deflects it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The closer the flow gets to the pole, the stronger this effect becomes. The result is a band of strong westerly winds (called the polar night jet) blowing from west to east around the cold air over the polar regions. These westerly winds form the stratospheric polar vortex.

As long as the pole remains much colder than the lower latitudes in the stratosphere, the ring of westerly winds is maintained.

Observed (bold blue) and forecast (red) polar vortex wind speeds at 60°N compared with the 1991–2020 average and the range of natural variability. Around 9 March 2025, a significant disruption of the polar vortex occurred; NOAA Climate.gov

Meteorologists most often monitor the zonal-mean westerly wind component at about 10 hPa and 60°N to determine when the stratospheric polar vortex has formed. Once a persistent westerly wind sets in during autumn and its speed stabilizes at values of at least 10–20 m/s (about 36–72 km/h, 22–45 mph) for several days, the stratospheric polar vortex is considered to have formed. Other parameters are also monitored.

Strength and stability of the polar vortex

When the westerly circulation in the polar regions is strong, the stratospheric polar vortex is strong and relatively stable. The jet stream in the troposphere tends to flow closer to the polar areas and has fewer large meanders. As a result, cold air remains confined over the polar regions (the Arctic during the Northern Hemisphere winter and Antarctica during the Southern Hemisphere winter), and winters in the mid-latitudes are usually milder, without pronounced cold outbreaks.

When the stratospheric polar vortex is strongly weakened or breaks down, the wintertime westerly circulation in the stratosphere is disrupted. The westerly winds weaken, or may even temporarily reverse to easterly. The polar vortex can be displaced away from the pole (a displacement event) or split into two or more parts (a split event). As a consequence, the jet stream in the troposphere often becomes highly meandering, facilitating deep intrusions of cold polar air into the mid-latitudes.

For example, when the polar jet stream dips far south over the United States, cold polar air can reach as far south as Texas and the Gulf Coast. Conversely, where the jet stream flows northward into higher latitudes, warm air is carried into those regions.

When the Arctic polar vortex is strong and stable, it keeps the polar jet stream farther north, allowing the coldest air to remain in the Arctic. When the vortex weakens, shifts, or splits, the jet stream becomes highly wavy, allowing warm air to enter the Arctic and polar air to spill into the mid-latitudes; NOAA Climate.gov

Causes of Polar Vortex Disruption

The westerly circulation around the cold polar air in the stratosphere can be disrupted for several reasons.

This regularly occurs due to the seasonal change in radiative conditions. With the onset of the polar day, ozone in the stratosphere above the pole once again begins to absorb UV radiation from the Sun. The polar stratosphere gradually warms, and the temperature difference between the pole and lower latitudes decreases until, at certain levels, the temperature gradient reverses and the westerly flow gradually changes to easterly (the so-called final warming).

During the polar summer, temperatures in the middle and upper stratosphere (roughly 30–10 hPa, that is, from about 25–30 km/16–19 miles upwards) above the pole gradually increase to the point where they eventually become warmer there than over the tropical latitudes. It is precisely this reversal of the temperature gradient in the middle and upper stratosphere that explains why easterly winds prevail there in summer instead of the westerlies typical of winter.

Rossby waves are large bends in the jet stream that turn an otherwise straight west-to-east flow into a wavy pattern; Wikimedia Commons

During the winter season, strong planetary Rossby waves are among the most common causes of disturbances to the stratospheric polar vortex. On upper-level height maps at 500 hPa or 300 hPa, they appear as large meanders of the jet stream in the form of deep troughs and pronounced ridges. They arise primarily as a consequence of Earth’s rotation and the fact that the effect of the Coriolis force changes with latitude. Their shape and strength are further influenced by mountain ranges, land–sea contrasts, and significant differences in heating between different regions, which distort the flow and can lead to the development of pronounced meanders.

When they are strong enough, they can propagate vertically upward across the tropopause into the stratosphere, where they gradually dissipate and transfer their energy and momentum to the surrounding flow. In doing so, they distort the wind field, slow down the zonal (westerly) winds at high latitudes, and thereby enhance the Brewer–Dobson circulation.

Average temperature profile in the lower atmosphere (left) and schematic of the Brewer–Dobson circulation (right); NOAA Jet Stream, EULIAA

The Brewer–Dobson circulation is the main large-scale circulation of air in the stratosphere. It provides a slow but crucial transport of air and trace gases (such as ozone). In this circulation, air rises from the troposphere into the stratosphere above the tropics and then, at higher altitudes, flows predominantly toward the poles (i.e., meridionally), where it gradually descends. This circulation exists year-round but is most pronounced during winter in the respective hemisphere. The concept of the Brewer–Dobson circulation was derived by Gordon Dobson and Alan Brewer from the distribution of ozone and water vapour in the stratosphere in the mid-20th century; it was only later confirmed by direct measurements.

If Rossby wave activity becomes very strong, the downward motion of air over the polar region intensifies. As the air descends, it warms rapidly adiabatically, and a sudden stratospheric warming can occur.

Sudden stratospheric warming

Sudden stratospheric warming (SSW) is a phenomenon in which the winter stratosphere over the polar region warms by several tens of degrees within just a few days. Most often, SSW is discussed in the context of the Arctic.

At levels around 10–30 hPa (about 20–30 km, roughly 12–19 miles above the surface), the temperature can jump by about 20–40 °C (35–70 °F). At the same time, the westerly zonal winds in the polar vortex weaken. During a so-called major SSW, the ring of westerly flow around the pole at 60°N and 10 hPa even temporarily reverses to easterly. The vortex then often shifts off the pole or breaks up into several parts, typically into two lobes.

Evolution of the stratospheric polar vortex: on 4 March 2025 (left) it is compact and more tightly centred over the pole, while by 10 March (right) it has been displaced off the pole, warm air is entering the Arctic, and easterly winds appear around 60°N, a hallmark of a sudden stratospheric warming; NOAA Climate.gov

At the surface, the effects of an SSW usually appear with a time lag of about 1–3 weeks. As the altered circulation in the stratosphere gradually propagates downward, the jet stream in the troposphere often weakens and becomes more wavy, and blocking high-pressure systems become more frequent in the polar regions. This increases the likelihood of significant outbreaks of cold Arctic air into the mid-latitudes (Europe, North America, Asia) and, conversely, intrusions of warm air into the polar regions. It is not the case, however, that every SSW automatically brings severe cold to a specific area in the mid-latitudes; rather, it increases the overall likelihood of cold polar air outbreaks in the weeks that follow.

Suppose an SSW and the subsequent weakening of the polar winds occur during winter, when solar radiation does not yet reach the polar regions directly (for example, in January or February). In that case, the polar stratosphere will cool radiatively again, become colder than the stratosphere at lower latitudes, and the westerly winds and thus the polar vortex will be re-established.

Average daily temperatures in the polar stratosphere (10-millibar pressure level) of the Northern Hemisphere from late 2022 into early 2023 (dark purple line); NOAA Climate.gov

Suppose the disruption occurs after the polar night ends and sunlight returns to the polar regions (for example, in March). In that case, the polar stratosphere is already beginning to warm, and the temperature difference between lower latitudes and the pole is weakening. In this situation, easterly flow tends to persist, and the polar vortex decays. It does not re-establish itself until the following autumn. Such an SSW is referred to as a final warming.

How to identify the polar vortex on meteorological maps and graphs

To monitor and describe the stratospheric polar vortex, several types of meteorological products are used, which together provide a picture of its strength, shape, and stability.

A basic indicator is the time series of the zonal-mean westerly wind at 60°N at 10 hPa. This graph shows how the average strength of the westerly flow in the mid-stratosphere changes over the course of winter: a strong, cold, and compact vortex corresponds to large positive values, while a weakened or disturbed vortex appears as a pronounced drop in wind speed or even a reversal of the wind direction.

Mean zonal wind at 10 hPa - Sub-seasonal range forecast; ECMWF

Equally important are maps of geopotential height and stratospheric flow, most often at the 10 hPa level. They clearly show whether the polar vortex is nearly circular and centred over the pole, or instead deformed, displaced toward lower latitudes, or split into several smaller vortices. Temperature and temperature anomaly maps then show how cold or warm the air inside the vortex is compared to climatology; a cold, low, “compressed” stratosphere indicates a strong vortex, whereas warming and positive anomalies over the pole signal that it is weakening.

For a comprehensive description of the state of the polar vortex, various indices (such as the Northern Annular Mode, NAM) and also “polar-cap” averages are often used. These are averages of quantities (for example, temperature anomalies or geopotential height) over the entire 60–90°N latitude band. These indices and polar-cap averages condense the complex spatial fields of pressure and temperature into a simple numerical time series, from which it is immediately apparent whether the polar vortex is powerful, near normal, or, conversely, significantly weakened.

How temperature affects the height of pressure; NOAA Jet Stream

Vertical cross-sections of the atmosphere are also very useful, often showing anomalies of geopotential height, either as latitude–height plots at a given time or as time–height plots over the polar cap (e.g., 60–90°N). This makes it possible to see how deeply into the troposphere the influence of a strong or weakened polar vortex extends, how its vertical structure changes throughout the winter, and whether a sudden stratospheric warming occurs.

Geopotential height anomalies over the polar cap during the 2024–25 polar vortex season. Positive atmospheric thickness anomalies over the Arctic are gradually propagating downward from the stratosphere into the troposphere; NOAA Climate.gov

When an SSW occurs over the polar region (typically shown on a vertical cross-section as an average over the polar cap 60–90°N), the air column in the middle stratosphere warms significantly, “puffs up,” and the 10–30 hPa pressure levels are shifted to higher altitudes. On vertical cross-sections of geopotential height anomalies, this appears as a column of positive anomalies over the pole, initially strongest in the upper stratosphere. Over time, this signal propagates downward into the lower stratosphere and the troposphere, eventually reaching the surface, where blocking anticyclones often form, allowing frigid air to spill into the mid-latitudes around them.

Daily geopotential height anomalies (17 pressure levels, last 120 days), normalized relative to the 1979–2000 daily climatology and averaged over the polar cap north of 65°N; NOAA Climate Prediction Center

Polar Vortex on Windy.com

You can easily view outbreaks of cold polar air in Windy in the Temperature or Wind (surface) layers. These views show where the cold air is coming from, how it spreads into the mid-latitudes, and allow you to switch on additional layers such as Clouds, Rain, thunder, or New snow. This gives you a much more complete picture of how the cold-air outbreak will affect the weather in your area.

If you are interested in the polar vortex itself, switch to higher levels of the atmosphere and display, in the same layers, for example, 150 or 10 hPa. In the standard atmosphere, the 10 hPa level corresponds to an altitude of roughly 30 km (19 mi) above sea level, i.e., the boundary between the middle and upper stratosphere. For more precise information about the altitude at each pressure level, select Geopotential height in the isoline settings.

Temperature (shading) and geopotential height (contours) at 10 hPa on 24 November 2025 and 8 December 2025. During this period, the displaced and weakened polar vortex undergoes a slight strengthening; Windy.com

Vertical cross-sections of the atmosphere (especially in the troposphere) are also a handy tool for visualizing temperature stratification and airflow. You can find them in the Airgram mode. In Windy, you can display them in two ways. The first option is to view the evolution of temperature and wind above a single point in the detailed forecast for a given location. The second option is a cross-section along a chosen route, which you can find in the Distance & planning tool. Draw a track, then select Airgram mode again.

On Windy.com, you can monitor the polar vortex and how it is forecast to change over time, from the Earth’s surface all the way up to the stratosphere – all in one place and with just a few clicks.

Vertical cross-section of the troposphere along the Distance & Planning route (Airgram mode); Windy.com

[Gkikas LGPZ]

@Jari-Sochorová

Similar situations (Feb 2018)
https://community.windy.com/topic/5459/arctic-cold-outbreak-over-europe

and

Jan 2019
https://community.windy.com/topic/7459/sudden-stratospheric-warming-ssw?_=1764505053015

This year seems SSW happened too early!

Time-series representation of temperatures at the 30-hPa level over the North Pole
The black line shows daily temperatures, and the gray line indicates the normal (i.e., the 1991 - 2020 average).

Source: https://ds.data.jma.go.jp/tcc/tcc/products/clisys/STRAT/

[mistertornado1]

Incredibly well-written and highly informative. Thank you!

[dbutman]

This was excellent, and a great job bringing in the sources. As someone in a closely related field, you had an outstanding balance of technical information and explanation.

[Sharky __]

Very informative!!! Even understandable for me as an absolute newbie to this topic.

[HedgehogInTheFog]

@Jari-Sochorová thank you so much! Before this article I really don't know what happens when the some anomaly in the region is happened. Now I understand it's more. Very detailed and very light to understand, thank you ^^

[Gkikas LGPZ]

Latest data (9 Dec 2025) show that the temperature in the stratosphere is back to normal.