View Article

Never mind the failed putsch against Gordon Brown and England’s Test match heroics, the matter of the moment is the white stuff. We haven’t had snow all over the country like this for decades. What’s changed and why?

The first part — what’s changed — is relatively straightforward: a freezing draught has started coming in through the UK’s door from the North Pole and Siberia.

As Robin Thwaites of the Met Office describes it: “We have a large area of high pressure sitting over Greenland and Iceland and that is a block of cold air. Cold air is very dense and doesn’t like to budge very quickly, so that is acting as a barrier to the normal westerly flow of [milder] weather that we get off the Atlantic.”

The block has forced our typical weather south: our rain has instead been falling in Spain, which is having an unusually wet winter. “They’ve had the rain and instead we’ve had winds either from the Arctic or from Siberia via Scandinavia,” said Thwaites.

Updated below with comments by Piers Corbyn
This phenomenon, known as the Arctic Oscillation and North Atlantic Oscillation, flips around quite a lot but has not been as persistent as it is now since 1981 and before that the winter of 1962-3.

The resulting weather across the whole of the northern hemisphere is “quite a long way from its normal patterns”, said Peter Inness, a lecturer in meteorology at Reading University. “What we would normally expect at this time of year on average is a jet stream that flows from west to east across the Atlantic.”

This high-altitude air current brings the wind and rain and fairly warm temperatures that the UK has seen in recent winters.

“But at the moment the whole of the northern hemisphere patterns are distorted,” said Inness. “All the jet streams are away from their normal positions. They are waving about ... and the jet stream near us, instead of coming from the west, is coming down from the north.”

Perhaps it is easiest to imagine looking down on the North Pole and seeing a rough circle of dense cold air sitting on top of it. What’s happened now is that bulges of cold air have spread south over the UK, parts of China and parts of the US.

After the initial fun of winter wonderland wears off, it’s tempting to ask who is to blame for these “oscillations”. The Icelanders? Are they making off with our weather as well as our bank bailout billions?

Tempting, but nobody is sure what causes the Arctic Oscillation. Some experts, including the Met Office, do not rule out a link to the Pacific current known as El Nino, but say the evidence is patchy. “There may be some correlation,” said Thwaites, “but it’s not very strong.”

Inness agrees. “El Nino does cause the weather patterns in the northern hemisphere during the winter to be quite disturbed when it’s happening, and there is one happening at the moment,” he said.

And it’s an unusual El Nino. “The warming of the Pacific is near the dateline, and normally it’s near the coast of South America. You’d expect that to drive different patterns.

“It might be one factor, but it would be wrong to say this winter is being caused by El Nino.”

Is it related to global warming?

Climate change sceptics love a cold winter: how can the Earth be warming up if much of the northern hemisphere is having a freezing winter? On the other hand, some proponents of climate change say unusual weather patterns may be the result of global warming.

Experts tend to say: hang on, there’s a difference between weather and climate. Weather is local and short term; climate change is global and long term.

As Liz Bentley of the Royal Meteorological Society says: “The thing with climate change is we look at things on a much larger scale, globally, and on a much longer timescale. To take one event over, say, western Europe, in a short time space, say a month — it will have no impact on the global climate statistics.

“If we look back at the end of the year we will probably still find that 2010 will be one of the warmest on record.”

What happens next? When will temperatures rise?

According to the Met Office this week “looks set to remain cold or very cold across the UK”. Though it may be dry in the north and west, “occasional wintry showers” are forecast in the east and the south.

Netweather, an online forecaster which predicted in November that we were facing a cold winter, forecasts bitter winds for today, more snow and another freezing week.

And the man who claims to have been the only person to predict the severity of the cold snap has another warning. “Yes, there will be more snow this weekend and the cold will persist this week,” said Piers Corbyn, founder of WeatherAction. “Then change will come around the 25th of the month.It’s going to be exceptionally cold in the first half of January. But then there will be a very mild, significant respite and the snow will melt rapidly.”

After the big freeze, he says, will come floods before the cold returns in February. Prepare to swap the sledge for a canoe, and back again.