February 2009 — cold snap explained

The freezing temperatures and heavy snowfall which hit many parts of the UK in February 2009 was caused by a weather anomaly which, because of our scientists’ understanding, we were able to foresee and warn about.

What happened?

In mid-January our forecasters realised there was an unusual situation in the stratosphere, between 20 and 30 miles up, when temperatures were 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. Our scientists realised an unusual event, which has the effect of reversing the usual direction of the wind, was taking place.

Adam Scaife, Head of Seasonal to Decadal forecasting, said: "We know this happens when winds in the stratosphere reverse direction and blow from east to west. The change in wind direction then burrows down through the atmosphere over the course of about two weeks, at which point easterly winds appear at ground level.

"This change to 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 the heavy snowfall in February."

What does it mean?

Helping to explain how this anomaly occurs means we can provide improved advanced predictions of its effects and also predict which years it is most likely to occur.

Nick Grahame, Met Office Chief Forecaster, said: "There is no doubt advances in our forecasting capability allowed us to assess the risk of extreme weather in early February well in advance and communicate this to our customers. We then provided more specific details on where the heaviest snow was expected to help with effective mitigation plans."

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

Is it a sign of climate change?

Dr Scaife explains: "There is no link between this particular event and climate change — it is just a part of the natural variability of the weather.

"In fact, this winter pales into insignificance compared 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."