Almost the whole of the United Kingdom saw snowfall at some point during February 2009. The cold spell was caused by a weather anomaly that occurs on average every 2-3 years.
Scientists at the Met Office first spotted this anomaly in mid-January when an unusual situation in the stratosphere (some 20-30 miles up in the atmosphere) caused the temperature there to rise by 50°C over the Arctic. This reversed the wind direction, from a normal westerly, to a cold easterly across the UK. It normally takes about two weeks for the change in wind direction in the stratosphere to reach the surface.
The change in wind direction brought very cold air from Siberia on easterly winds, and this combining with weather systems from the Atlantic led to heavy snowfall in many places. The last time this anomaly occured was during the winter of 2005/2006, however the majority of the cold air went into continental Europe, narrowly missing the UK.
If the regular occurrence of this anomaly holds true, then the winter of 2011/12 could yet again be a cold and snowy one.
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