The scale that measures the pressure difference between Iceland and the Azores goes from around plus 5 to minus 5. It is most of the time flitting between plus and minus 2. But sometimes it gets stuck for a few weeks. And when it gets stuck at an extreme – anywhere around plus or minus 4 or 5 – that’s when we get weird weather.

The NAO record in the past century shows this clearly. There have been only occasional winter quarters when the index has averaged below minus 4. They were the 1963 big freeze, another pipe-bursting winter in 1969 and early 2010, when we again got blanketed in snow.

Since mid-November this year, the NAO has been way down again, approaching minus 4.

The NAO has been in a generally positive phase for the past 25 years. As a result, winters have usually been mild since the late Eighties, encouraging one climatologist to predict an end to winter snow in Britain.

But in the middle of summer 2009, it slipped back into a negative phase that has persisted month after month since, bringing us last winter’s snow and now our current record-breaking December freeze.

This year, the index has been in strongly negative territory since mid-November, reaching lows of approaching minus 4 as the snow set in during late November and again on December 18 – the Saturday that brought chaos to southern England.

The NAO influences British weather most strongly in winter. But it can have a summer effect too. During a positive phase, those winds coming in off the Atlantic keep us cool (and damp) in summer.

But when the NAO goes negative, we get hot, dry summers with droughts. The NAO has gone below minus 5 in the summer only once in the past century. It gave us the notorious drought of 1976.

Can we forecast it? Not very well, even short-term. Though when it gets ‘stuck’ at an extreme point, there is usually a reasonable chance it will stay stuck for a bit. As it has over the past month.

The Met Office is being cautious about what happens next. Its best forecast is a likely ‘return to near normal temperatures’ next month. But some analysts say the big chill could continue for weeks. When it ends, there could be a flurry of storms as the wet westerly winds reassert themselves.

The Government has asked its chief scientific adviser John Beddington to report on whether this year is the start of a long-term trend to colder winters. I can predict caution will ensure he doesn’t come up an absolute answer.

But often the long-term swings of the NAO bring clusters of warm or cold winters. So the odds must be shortening.

What about global warming? Doesn’t this all make those theories about warming look a bit stupid? Well, no. The NAO is only cooling a small area round northern Europe. Elsewhere in the world, 2010 has seen near-record warm temperatures.

In November, northern Europe was a most unusual 4C colder than normal. But according to Nasa, last month was the warmest-ever November worldwide. Right now, there are record temperatures west of Greenland and into Canada, where they are wondering when winter will start.

We seem to have got the weather they usually have around Hudson Bay in Canada – and they have got ours. Blame that on the NAO.

Many climate scientists want to link up the recent wild swings of the NAO with global warming. But they can’t work out what the link is – or if there is a link.

Last year Mojib Latif at the Leibniz Institute at the University of Kiel in Germany argued that the long positive phase of NAO from the Eighties to 2009 had helped to warm the world in the past 30 years. It was responsible for at least part of global warming.

But others argue that the boot is on the other foot – that it was global warming that pushed the NAO into its long positive phase. Another bunch of researchers, led by James Overland of the U.S. government’s National Oceanic and Atmospheric Administration, has been suggesting in recent weeks that global warming is kicking the NAO into a kind of super-negative state and causing our bad winters.

Overland points out that global warming is causing massive summer melting of Arctic ice and he says this is playing havoc with the old weather patterns.

If that is right, then we have the weird prospect of global warming unfreezing the Arctic and kicking northern Europe into a run of colder winters without end.

But others are dubious about this theory, including the legendary Nasa climate guru Jim Hansen. Frankly, they don’t know.

So what can we say? The big long-term driver of climate change still looks like the greenhouse effect of gases such as carbon dioxide we put into the air every time we burn fossil fuels.

But global warming is only about global averages. What it means for real weather, place by place, day by day and season by season, is still impossible to predict.

And, in our corner of the world, the NAO is likely to carry on determining a lot of the ups and downs on our way to a warmer world.

Right now, I am going out to buy a shovel.