St Swithin warned that showers on July 15 would herald 40 days of rain, whereas sunshine meant a blissful 40 days. Though the 9th-century Bishop of Winchester had a good reputation for long-range weather forecasting, he never got his prediction quite correct. He hadn’t factored in El Niño.

The St Swithin’s Day legend does have a grain of truth because the weather at this time of the year often gets stuck in a rut. But don’t despair. A surge of warm water emerging in the tropical seas of the Eastern Pacific is already making this a far better summer than our past two.

This warming is known as El Niño, a vast climactic phenomenon recurring every few years that carries warm surface waters across the Pacific Ocean and creates chaos in global weather patterns.

The eastern tropical Pacific swings between bouts of warming and cooling every few years. The summers of 2007 and 2008 were plagued by a cooling of the Pacific seas, called La Niña, but this fizzled out this springtime.

Times Archive, 1879: St. Swithin's Day

'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, twill rain no mair'

• Letter: St. Swithin's Day

• Leading article: Popular beliefs and superstitions

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• El Niño is back, bringing floods and famine

• Britain braced for even higher temperatures

For much of the world El Niño, which was announced last week, can lead to droughts and heavy rains. Meteorologists in Britain are optimistic, however, that it will allow us to wallow in al fresco dining. “The pattern we are seeing this summer over the UK is very much like an average El Niño year, with warmer and drier weather than in the La Niña years,” said Adam Scaife, at the seasonal forecasting unit in the Met Office Hadley Centre.

Forecasters are wary of betting the house on sunshine, however. The “barbecue summer” trumpeted by the Met Office in April helped to give an impression of a blistering hot summer all the way through. In truth, the forecast was for a warmer and drier summer than average, and the key word was “average” — the heatwave of June is now being balanced out by the cooler, showery weather of July. By the same law of averages, though, we should expect more hot weather to come, and there are tentative signs that heat might build up again towards the beginning of August.