UK weather: The atmospheric ‘seesaw’ partly responsible for this week’s cold snap

Meteorology time. Why the ‘partly’ in the headline? Climate change pokes its nose in at the end of the article, but all that’s offered is uncertainty.
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The “seesaw” is bordered by a high-pressure area west of Portugal and a low-pressure area centred over Iceland.

When the balance changes, so does the weather, says Sky News.

An atmospheric “seesaw” is partly responsible for the snow that much of the UK will likely see this week.

As much as 40cm could fall across central Scotland and the southern Highlands, while northern England, including its cities, may see between 15cm and 20cm.

People in southern England and South Wales can expect to wake up to snow on Wednesday – although it is unclear whether it will settle, the Met Office said.

Temperatures could drop as low as -15C in northern Scotland overnight – making it the coldest night of the year so far. [Talkshop comment – coldest reported was -16C]

What is the ‘seesaw’ – and how is it contributing to cold snap?

The “seesaw” – formally known as the North Atlantic Oscillation (NAO) – has two sides: a high-pressure area over the Azores, west of Portugal, and a low-pressure area centred over Iceland.

When the pressure gap between them changes, so does the strength and location of the North Atlantic jet stream.

The jet stream is a collection of very fast winds high in the atmosphere that influences the movement of storms.

If there is a larger-than-usual difference between the pressures on each side of the North Atlantic seesaw, winds from the west become dominant, bringing with them warm air which – when combined with the position of the jet stream – creates mild, stormy and wet conditions in the UK.

This is called a ‘positive’ North Atlantic Oscillation (NAO), but the opposite is occurring this week.

A negative NAO means the reverse: the gap in pressure between the atmospheric areas above Portugal and Iceland is smaller than normal, changing the position of the jet stream, and allowing Arctic air to create cold, calm and dry winters.

“When this pattern is in its negative phase, the jet stream can become wavier. Currently the NAO is in a negative phase and the jet stream is further south than usual,” explained Helen Dacre, professor of meteorology at the University of Reading.

“Thus, air from the Arctic is moving southwards over the UK resulting in colder weather conditions.”

Full article here.