St Swithin’s day if thou dost rain’For forty days it will remain;St Swithin’s day if thou be fair,For forty days will rain na mair. Early sources suggest that St Swithin himself requested the removal of his bones to the cathedral. Summer weather in the UK is shaped by the position of the jet stream. St Swithin’s day could well be a useful barometer. St Swithin’s legend is just one part of a complex picture of human interaction with the weather.