St. Swithun, Bishop of Winchester, was born around the year 800 and died on 2nd of July 862 at Winchester (Hampshire). He was, say the chroniclers, a diligent builder of churches in places where there were none before and a repairer of those that had been destroyed or ruined. St. Swithun was buried, according to his own desire, in the churchyard of the Old Minster (Cathedral) at Winchester, where passers by might tread on his grave and where the rain from the eaves might fall on it.
His reputation as a weather saint is said to have arisen from the translation of his body from this lowly grave to its golden shrine within the Cathedral, having been delayed by incessant rain.
Hence the weather on the festival of his translation (15th July) indicated, according to the old rhyme, what it would be for the next forty days:
St. Swithun's day, if thou dost rain,
For forty days it will remain;
St. Swithun's day, if thou be fair,
For forty days 'twill rain na mair.
Whoever told the story about the St. Swithun's day saying was obviously well aware that summer weather patterns establishing by the beginning to the middle of July tend to be persistent throughout the coming few weeks. In fact this is statistically true in 7 to 8 out of 10 years.
The meteorological interpretation is quite straightforward. The position of the frontal zone around the end of June to early July, indicated by the position of the jet stream, determines the general weather patterns (hot, cold, dry, wet) for the rest of the summer. Like a little stream in its bed, the frontal zone tends to 'dig in' shortly after the summer solstice.
As the path of our weather systems is controlled by the jet stream, a more southerly location of the frontal zone is likely to bring rather unsettled, wet and cool weather. On the other hand, a frontal zone shifted further to the north will help the Azores high to build over western Europe, thus bringing dry and pleasant weather to the UK.
Try and prepare your own summer forecast with our expert maps. The 500mbar maps usually give a good idea about the position of the frontal zone. Have a look at them over the next two weeks and produce a DIY summer forecast valid until mid-August with a confidence of 70 to 80%.
No wonder the St. Swithun's day rule is also know in other western European countries. In France they say Quand il pleut a la Saint Gervais Il pleut quarante jours apres - If it rains on St. Gervais' day (19th of July), it will rain for fourty days thereafter.
In Germany the Siebenschlaefer or seven sleepers day (July 7th, after the Gregorian calendar) refers to the weather patterns of the following seven weeks.