Climate scientists have a diagnosis for the big chill that has made this month probably the coldest December since 1910 – the North Atlantic Oscillation (NAO).
They say it is the hidden hand in our weather in the north-western corner of Europe.
The NAO is an old friend that has swapped its raincoat and galoshes for gloves and a fur hat. Ten years ago, newspapers were full of stories about how it was the cause of a record rainy autumn that caused massive flooding across England. Now it is blamed for the Arctic freeze.
Both are true – and more besides. Go back to the big drought of 1976, or further back to the great winter freezes of 1963 and 1947, and the hidden hand is there every time.
The NAO is a see-saw in weather systems in the North Atlantic. It has two states – positive and negative.
They bring very different weather. The positive phase happens when air pressure is low over Iceland, but high down south over the Azores islands off West Africa.
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