Winter made an outrageous return yesterday as much of Britain woke up to thick snow-falls, including the south coast. So how did we lurch from balmy springtime to freezing misery in only a day or two?

Last week’s warmth came from a block of high pressure over the UK. But that anticyclone gave way to a depression passing over Norway. That dragged raw winds straight down from the North Pole. As that airflow swept over hundreds of miles of Arctic ice and cold seas, it brought freezing temperatures to Britain.

April frequently blows hot and cold, though. A hundred years ago, on April 25, southern England was struck by a blizzard that paralysed roads and left villages marooned.

Such erratic weather often comes from the path of the jet stream, a ribbon of fast winds that circles the globe. The jet stream marks the battlefront between Arctic air and warm southerly air. In springtime, as sunshine grows stronger, the warmer air pushes further north. But the Arctic is still frozen, and so these two air masses are locked in a titanic struggle that sends the jet stream careering north and south, often sending the UK’s weather haywire.