What is Sudden Stratospheric Warming?

Prolonged cold, wintry, sometimes snowy weather, like the sort we saw in 2009/10, 2013 and 2018's Beast from the East, can be a result of something called a Sudden Stratospheric Warming (SSW). So what is a SSW and how does it form?

Blanket of snow covers the UK on 8 Jan 2010 following an SSW Credit: NASA Worldview

To answer that we have to look at how our atmosphere is made up, starting with the troposphere. This is the lowest 10km and the zone where all our weather sits and where our jet streams are. The next layer spanning between 10km and 50km is the stratosphere, where the ozone layer is and where there are also jet streams.

The first two layers of our atmosphere Credit: Met Office

In winter, the North Pole is tilted away from the sun allowing the air to cool and pool over the Arctic, down as low as minus 80 Celsius! A jet stream with strong westerly winds called the Stratospheric Polar Vortex encircles this cold air, locking it in place.

Sometimes the Polar Vortex will become disrupted and the air within it collapses in on itself, squashing and compressing, and warming up quickly. That's the SSW taking place; there's no heat being put in, simply the action of the air being squished. The polar vortex then starts to weaken and slow down, even reverse its winds to easterlies.

This weakening of the polar vortex eventually extends down through the stratosphere to the top of troposphere and starts to weaken our own jet stream. This in turn alters our weather patterns and stops our usual winter stream of Atlantic low pressure systems and mild southwesterlies. We end up instead with cold easterly winds bringing prolonged periods of low temperatures, ice, frost and snow.

ITV Anglia Meteorologist Chris Page explains a Sudden Stratospheric Warming

It takes a few weeks for us to see and feel the effects of a Sudden Stratospheric Warming and a lot can change as things work their way down from 50km up! Although it usually brings cold weather it doesn't always affect the UK, certainly not to the same degree as our previous memorable cold snaps.