This is St Swithin’s Day, when any rainfall means that we will be cursed with wet weather for the next 40 days. But if it stays dry then we can expect to bask in 40 days of glorious weather. And the forecast for today is looking extremely fine and dry, apart from a few scattered showers.

Although it is only folklore, and we never strictly get 40 days of rain or dry as a result, St Swithin’s has a surprising grain of scientific truth to it.

Some years are desperately close to fulfilling the 40-day forecast: in the summers of 1983, 1989, 1990 and 1995 there was dry weather on July 15 over southern England followed by fine conditions over about 38 out of the following 40 days — and the two days when rain fell had only light showers.

As for a rainy forecast, it rained on St Swithin’s in 1985 and then for 30 of the following 40 days.

It may seem strange that a 9th-century bishop whose saint’s day happens to fall close to the start of the school holidays could predict most of the weather for six weeks ahead.

But the reason why it seems to work is that around mid-July our weather patterns tend to settle into a pattern that lasts until late August, and this is true for about seven out of ten years. In fact, a better version of the St Swithin’s prophesy may be: “If it rains today, expect some unsettled conditions for several weeks, but fine weather will probably bring spells of warm, sunny weather.”

We can partly thank the jet stream. This ribbon of wind flies at up to 200mph, several miles above ground around the globe, forming a front between the cold Arctic air from the north and warm sub-tropical air from the south. At this time of year the position of the jet stream tends to dig in.

If it takes a southerly track, cool air can sweep down from the Arctic and a jet stream passing over Britain also drags in depressions off the Atlantic, making for a particularly grotty summer. But a more northerly jet stream allows warm air to invade from the south, often in a high pressure system from the sub-tropical Azores — the hallmark of a gloriously sunny, warm summer.

At the moment a ridge of high pressure is in control over Britain and the jet stream is passing to the north and south of it, helping to hold the high pressure in a pincer movement. This type of “blocking” pattern helps to hold the fine weather and will keep the depressions away over the next few days at least, especially in the South.

Although we do not fully understand how these overall weather patterns work, in springtime the jet stream over Asia switches from south of the Himalayas to the north of the mountains, and over the Pacific it switches from the south of Alaska to the north.

One reason that the St Swithin’s prediction never holds true for an entire 40 days is because the jet stream wobbles around from day to day. So this summer we can expect a brief invasion of the jet stream from the North, tending to bring cooler, wetter weather. But the forecast is looking bad for the drought in most parts of Britain.

SUNNY SPELLS

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