Page last updated at 14:58 GMT, Wednesday, 7 January 2009

High pressure to blame for big freeze

As parts of England and Wales struggle in sub-zero temperatures, BBC weatherman Liam Dutton explains the high and lows of Britain's winter weather.

Europe cold weather map

Having worked at the BBC weather centre for about six years, I'm finding it hard to remember the last time I talked about cold weather constantly for such a long period of time - and that's saying something, as I'm a bit of a winter weather fan.

The start of the meteorological winter is 1 December and last month proved to be the coldest December since 1996, with the average temperature at 3.1C (38F), compared with the long-term average of 4.2C (40F) for the first part of the month.

So where did this cold air come from?

The current cold spell is the result of an area of high pressure that has been lying close to or across the United Kingdom.

Initially it lay to the west of Ireland, and as winds travel around an area of high pressure in a clockwise direction, this meant that cold Arctic air was dragged across the country from the north.

The high pressure then became stuck over the UK and, as under high pressure the air beneath doesn't tend to move very much, the cold weather has lingered.

Why doesn't this happen every winter?

In recent winters, the weather has often been unsettled, with low pressure frequently affecting the UK.

The winds have frequently blown from the west or south-west, bringing mild air from the Atlantic Ocean.

With so many of our recent winters being milder than average, it's easy to forget how cold UK winters can potentially be

This has meant winter temperatures have often been above the seasonal average, and any cold spells fairly short-lived.

This winter has been completely different, with areas of high pressure close to or across the UK, meaning the weather has been more settled and less changeable - allowing the cold weather to persist.

Why are winters in continental climates colder?

The main reason that continental winters are colder than those experienced here in the UK is that they don't benefit from the Gulf Stream.

The Gulf Stream is a warm ocean current that has the effect of keeping UK winters milder than they should be for somewhere at our latitude and moderates the intensity of any cold weather.

On the near continent, the land cools very quickly in winter and without the moderating effect of surrounding warm waters, temperatures regularly fall to minus double figures.

Has the current cold spell broken any records?

This has probably been the most commonly asked question in recent weeks, more especially over the past few nights when temperatures have dropped as low as -12C (10F) in rural parts of Oxfordshire. Despite -12C sounding terribly cold, it is nowhere near record breaking.

The current UK lowest temperature record stands at -27.2C (-17F) at Altnaharra in Scotland, which was last reached in December 1995, and before that the same temperature was recorded in Braemar back in 1982.

With so many of our recent winters being milder than average, it's easy to forget how cold UK winters can potentially be.

If you're sick and tired of this cold weather and de-icing your car windscreen in the mornings, then south-westerly winds will bring less cold weather during this weekend.

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