Just as the Daily Telegraph letters page reliably has an April entry from someone who has heard the first cuckoo of Spring, so also we should shortly expect letters happily claiming that the current cold weather disproves man-made global warming.
The fact is that despite the abnormally cold weather in parts of the Northern Hemisphere last winter (and now), global data suggests that 2010 is likely to be warmer than 1998, the previous record. Obviously, the full data is not yet in, and a volcano (causing cooling) could cause 2010 to lose out.
The current cold weather in the UK is balanced by abnormal warmth in North America.
Met Office for the graph]
We see a band of abnormal cold blue across Northern Europe and Russia, with warm reds counterbalancing it in other areas.
The Met Office does not mention the Northern Polar Jet Stream, but it is well south of its normal position, north of Scotland. In fact, it is today shooting across the Bay of Biscay. This downward displacement may be due to low solar activity in the last few years, because higher solar activity (and long term global warming) tends to push the jet stream northwards.
Here's the current position of the jet stream.
So that's why it is cold.
Which illustrates the distinction between weather and global climate. Weather is what is experienced in any specified part of the planet. Global climate is the sum total of temperatures measured all over the planet.
Take a look here at what happened last year: abnormal cold in normally temperate areas, and abnormal warmth in normally cold areas.