By Pamela Rutherford
Reporter, BBC News
Scientists have shown that a severe snowfall in North America and Northern Europe in the winter of 2009-2010 was caused by a rare, once in a century, collision of two weather systems.
They concluded the harsh winter and heavy snow was an example of hard to predict weather events, not a change in climate.
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In the winter of 2009-2010 much of Northern Europe experienced heavy snow and temperatures were at the lowest they had been for nearly 30 years. At the same time, record snowfall hit Washington DC and other parts of America's "Mid-Atlantic states".
Some news reports took the extreme cold weather as evidence against climate change.
By analysing 60 years of snowfall measurements and satellite data, researchers concluded the anomalous weather conditions were caused by an unusual combination of an El Nino event and the rare occurrence of a strongly negative North Atlantic Oscillation (NAO).
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more:
http://www.bbc.co.uk/news/science-environment-11152077