Britain – What caused the great freeze?

Posted: January 25, 2010 in Mentalism

Through mid December, and indeed the first half of January, Britain and Europe was blanketed in ice and snow. This was all thanks to a huge area of high pressure sitting to the North of the UK which acted as a block to our usual mild Atlantic driven weather, and also pushed the Jet Stream Southwards into Spain and Southern France, allowing freezing air to head Westwards across Europe and eventually over the UK.

This was all done thanks to something called the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO). Usually in winter we see a positive AO. When the AO is positive we see low pressure across the Arctic region, we usually see a positive NAO. When these are positive it means high pressure is less likely to develop, and this means that Atlantic weather systems are free to push across the UK keeping us fairly warm. When the AO and NAO turn negative, like they did this year, high pressure becomes more likely. The AO dropped into a very strong negative, which meant a strong area of low pressure developed to the North of us (in the Arctic Regions) which pushed the Jet Stream to the South, blocking the Atlantic weather systems, and allowing our weather to come from the very cold East.

Usually when this happens in winters, the block doesn’t last very long because the NAO and AO usually rise again, which weakens the blocking and allows the Atlantic to smash through and warm things back up again.

Once the block became established, freezing air travelled Westwards from Scandinavia, down across Europe and then across the North Sea into the UK. Now usually Easterly winds are very dry and often only bring snow showers to Eastern Coastal Counties, but because of the time of year the Easterly wind developed, the North Sea was still relevantly warm, and the freezing air crossing it meant there was a lot more convection than usual, which in tern helped aid those heavy snow showers, and because of the length of this cold spell, the air became unstable allowing troughs to develop (a more organised area of snow showers) and push across the country.

As we moved into Mid January, a warm front developed over Europe and pushed Westwards across the UK, despite initially bringing hours of light persistent snow to many Eastern areas adding greatly to accumulations, it also warmed things up slightly to allow a slow thaw to develop, and thanks to the NAO and AO beginning to rise again, the colder air was unable to re-establish itself and the slow thaw began and the block began to break down.

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