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Cold snap — February 2009

Exceptional levels of snow fell, especially in London

When cold air from Siberia collided with weather systems coming off the Atlantic, it produced a sharp drop in temperature and heavy snowfall in parts of the UK. Accurate forecasts in January and early warnings in February helped our customers prepare.

In mid-January our forecasters realised there was an anomaly in the stratosphere, between 20 and 30 miles up, with temperatures forecast to rise rapidly. Over just a few days, a leap of 50 °C was observed in the Arctic stratosphere due to a sudden inflow of air. This has the effect of reversing the usual direction of winds, causing them to blow from east to west.

Over the next two weeks, this change in wind direction moved down through the atmosphere. When it reached ground level, the easterly winds brought cold air from Siberia and the continent over the UK, producing a sharp drop in temperature. When this cold air collided with weather systems coming off the Atlantic it led to heavy snowfall in February.

Graphics showing change in wind direction overlaid over a map of the UK

What we did

  • Forecasters realised temperatures were rising rapidly in the stratosphere in mid-January.
  • Risk of extreme weather assessed in early February.
  • Warnings communicated well in advance to our customers.
  • More specific details issued on where the heaviest snow was expected.

The result

  • Understanding how this anomaly occurs means we can provide advanced predictions of its effects and tell in which years it's most likely to occur.

  • The last time an event like this happened was in winter 2005/6, when large areas of Europe had a very cold end to the season and record snowfall. The UK avoided the worst effects as the cold just missed the country.

The future

  • There is no link between this particular event and climate change — it's just part of the natural variability of the weather.

  • In fact, this winter pales in significance to the very harsh winters of 1946/7 and 1962/3. As the climate warms, we expect fewer cold winters and less snowfall in future.

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