The questions behind the news

What is going on with the weather?

Between the Met Office warning that a harsh winter is on its way and the positively balmy weather that brought us an unseasonably warm October, people could be forgiven for wondering.

According to a Met Office spokesman, explaining the warm weather is the easy bit. As the end of October drew near, southerly winds brought us warm air off the African continent. Nothing unusual about that: Britain's warming is part of a long-term climate change trend that is seeing the planet as a whole warm up. In the UK, that means the summer increasingly tails into the autumn, causing, among other effects, a gradual shift in animal breeding and feeding patterns.

The forecast of an especially biting winter, as with all long-range predictions, is far more speculative. As the winter draws in, a good indicator of how mild or severe British weather will be is the difference in surface pressure between two parts of the Atlantic ocean. Normally, there is a low pressure over the Atlantic near Iceland and high pressure over the waters near the Azores. Although it does not change the weather directly, a big pressure difference between the two suggests stronger westerly winds are likely to blow across Britain.

Occasionally the pressure difference in the Atlantic, known as the North Atlantic Oscillation (NAO), flips to give a high pressure over Iceland and a low pressure over the Azores. The flip indicates there is a high possibility of a shift in the wind direction from westerly to easterly.

The swing of the NAO brings cold weather for two reasons: first, high pressure over Iceland will often spread as far south as Britain, and high pressures clear the skies of clouds, effectively stripping away a layer of insulation. Secondly, easterly winds bring cooler air from Siberia and the arctic regions.

But how does the Met Office know that the NAO is going to flip? The short answer is that they don't. But recent research by Met Office scientists found that sea temperatures off the eastern seaboard of the US gave a reasonably good indication of whether the NAO will flip. Their measurements indicated there was a 65% chance it would, suggesting a cold winter is on the cards.

"The Met Office are in a tricky position. They have this information, but it's very experimental. If they don't warn of a cold winter, they'll be criticised if it happens; but if they get it wrong, people will think they don't know what they're doing," said Dr Viner.

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