St Swithin's Day is on the 15th July. It is known that this isn't a Pagan festival, or indeed a festival at all. However, it is interesting for the folklore that surrounds it.

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 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 nae 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.

This means that if it rains on St Swithins Day it will rain for forty days. If it doesn't rain on St Swithins Day, it will not rain for forty days. However, according to the Met Office, this old wives' tale is nothing other than a myth. It has been put to the test on 55 occasions, when it has been wet on St Swithin's Day and 40 days of rain did not follow.

The summer of 2003 was particularly warm. and the few weeks prior to St Swithin's Day was completely dry. The day after, Lyneham had less than 1mm rainfall in the 24 hour period, and the weather was unsettled until the end of the month. Then we had the record breaking temperatures in early August 2003. You might want to check one year and see if the rhyme is right.

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.