in

　　the

　　long­term heating trend.“It’s part of natural variability，” said Gerald Meehl，a senior scientist.With global warming，he said，“we’ll still have record cold temperatures.We’ll just have fewer of them.”

　　Scientists say man­made climate change does have the potential to cause more frequent and more severe weather extremes.But experts did not connect the current cold snap to climate change.

　　So what is going on?

　　“We basically have seen just a big outbreak of Arctic air over populated areas of the Northern Hemisphere”，Arndt said.

　　In the atmosphere，large rivers of air travel roughly west to east around the globe between the Arctic and the tropics.This air flow acts like a fence to keep Arctic air restricted.But recently，this air flow has become bent into a zigzag (之字形) pattern，wandering north and south.If you live in a place where it brings air up from the south，you get warm weather.In fact，record highs were reported this week in Washington state and Alaska.

　　But in the eastern United States，like some other unlucky parts of the globe，Arctic air is coming from the north.And that’s how you get a temperature of 3 degrees in Beijing，a reading of minus­42 in mainland Norway，and 18 inches of snow in parts of Britain.The zigzag pattern arises naturally from time to time，but it is not clear why it’s so strong right now.

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