Over the last few years, our weather in Britain has become more extreme.Last winter was the wettest ever recorded, as deadly storms battered the country for weeks on end. But previous winters have seen bitter lows of -22, as Britain was plunged into a deep freeze.What everyone wants to know now is: why is our weather getting more extreme, can we expect to see more of it in the future, and has it got anything to do with climate change?In this episode of Horizon, physicist Dr Helen Czerski and meteorologist John Hammond make sense of Britain's recent extreme weather and discover that there is one thing that connects all our recent extreme winters - the jet stream, an invisible river of air that powers along 10 km above us. What's worrying is that recently it has been behaving rather strangely.Scientists are now trying to understand what is behind these changes in the jet stream. Helen and John find out if extreme winters are something we may all have to get used to in the future.