What is the polar vortex?
Quick answer
The polar vortex is a large area of low pressure and very cold air spinning around the North and South Poles, mostly high in the stratosphere. When it’s disrupted, cold Arctic air can spill south and bring cold snaps to the UK, Europe and North America.
The polar vortex isn’t a storm that suddenly appears. It’s a permanent feature of the atmosphere that strengthens in winter and weakens in summer. News headlines tend to mention it when something unusual happens to it.
Strong vortex vs disrupted vortex
| Strong, stable vortex | Weakened or split vortex | |
|---|---|---|
| Cold air | Stays locked over the Arctic | Can spill south |
| Jet stream | Tends to be strong and straight, west to east | Becomes wavy and slower |
| Typical UK weather | Mild, wet and windy westerly weather | Higher chance of cold, dry easterly or northerly winds and snow |
What is a sudden stratospheric warming?
Sometimes, temperatures high above the Arctic rise by tens of degrees in just a few days. This sudden stratospheric warming (SSW) can slow, shift or even split the vortex. Its effects can work their way down to the weather we feel over the following one to four weeks.
What it means for London’s weather
An SSW raises the odds of colder weather in the UK, but it doesn’t guarantee it. The best-known example is February 2018: an SSW was followed by the “Beast from the East,” which brought heavy snow and freezing easterly winds to London and much of Britain. Other SSWs have had little effect on the UK because the cold air went elsewhere, such as North America or Asia.
So when you see a headline about the polar vortex, read it as “a cold spell is more likely than usual,” not a firm forecast. Short-range forecasts from national weather services are the best guide.
Is climate change affecting it?
Scientists are still researching whether a warming Arctic makes vortex disruptions more frequent. The evidence is mixed, and it’s an active area of study.
On the brighter side, Arctic winters also bring spectacular skies: see what causes the northern lights.
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