Experts say there is no truth to the old St. Swithin’s Day superstition and are predicting a summer heatwave. The weather lore proverb states that if it rains on 15 July, it will rain for the next 40 days, and similarly, if doesn’t rain, it won’t rain for 40 days.
Legend has it that St. Swithin, the 9th century miracle-working Bishop of Winchester, requested to be buried outdoors where “it might be subject to the feet of passers-by and to the raindrops pouring from on high.” Years later, his body was moved to an indoor shrine, and on that day there was a heavy shower.
Peter O’Donnell of Irish Weather Online rubbished the superstition, saying, “There is no obvious reason why July 15 would have any more or less predictive significance than any other day.” He advised “cautious optimism” for a warm second half of July. “I think there’s a good chance of seeing some very warm or even hot days eventually in this spell,” he said, adding, “The complicating factor in this warm spell speculation is that a fairly decent warm spell has developed this week and compared to the weather in most of June... I would say this week is fairly close to being a warm spell.”
He continued, “The temperature trend after this upcoming rather cool weekend spell would be upward through the 19 to 22ºC range and possibly into the mid-20s by the early part of the last week of July.”
However, some experts disagree. Around the middle of July, the jet stream settles into a pattern which, in the majority of years, holds reasonably steady until the end of August. Often the weather comes very close to fulfilling the 40 day forecast, with upwards of 30 days of sunshine after a sunny St. Swithin’s Day.