Occasional sun, sporadic rain; nothing too sweltering, nothing too sharp. For a country becoming increasingly used to storms, floods and the odd 30C (86F) heat wave, this year is a surprising throwback to the classic British summer.

A younger generation accustomed to temperatures closer to those in Barbados than Bognor may be disappointed. Stay-at-home Britons buffeted by the credit crunch must quickly learn that a trip to the seaside involves a Thermos as well as a cooler and an umbrella as well as sunscreen.

The first indication that there will be a long wait for serious sunshine comes today, St Swithin’s Day, with a little bit of everything forecast.

The Met Office has apparently aligned itself with the law of St Swithin’s, which asserts that the weather today will be the norm for the next 40 days — a mixture of sunshine and clouds, average temperatures, lots of rain and no heat waves, a summer combination seen rarely in recent years. The sun will come, eventually, the preliminary forecasts suggest, in September when there will be dry, warm weather.

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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According to legend, St Swithin requested to be buried outside so that his grave would be exposed to the patter of rain and feet of parishioners. His wish was granted upon his death in AD862, but his remains were later moved inside Winchester Cathedral. The move was hampered by prolonged wet weather, which caused superstitious parishioners to conclude that they had been hexed.

Though the curse of St Swithin may be considerably more myth than meteorology, it does contain some truth. Summer weather patterns tend to settle so that weather is consistent between mid-July and late August.

If longer-range forecasts are proved right, then so will be St Swithin. The forecast for today is indecisive — showers across northern parts of Britain and dry weather in the South — so meteorologists have based their predictions on the influence of La Niña.

La Niña, the cooling of the tropical seas of the Pacific, has disrupted the jet stream, which is the high-altitude wind that meanders around the globe. This has driven the stream farther south than usual for the summer, dragging stormy weather over Britain in a depressing rerun of the summer last year, which had gales, floods and plagues of slugs and snails.

The prospect for the autumn is better because La Niña will have died out by September, when normal conditions are expected to return to the Pacific. Preliminary forecasts by the Met Office hold out some hope that the autumn could be dry and warm. Such periods of sunny, warm weather are known as “Indian summers”, which occur usually after the first frost and can be in September, October and early November.

The Met Office also announced its preliminary forecast for December to February. It is expecting normal conditions that are not too cold but not too mild either, “a typical sort of winter we have become used to”, according to Dave Britton, of the Met Office.

The winter prediction is based on how the Atlantic behaved in May. This is a key time of the year when the seas have calmed down after the winter and spring storms but before they have warmed up over the summer. The pattern of Atlantic temperatures in May provides a good idea of how the atmospheric pressure systems over Iceland and the Azores will behave in the winter. These pressure systems tend to drive mild and wet weather towards Britain, or leave the country dry but cold.

After a run of magnificent summers for the past 15 or so years, it’s easy to forget what a British summer should be, but last year’s and this year’s are reminders of the days when you always packed waterproofs with the bucket and spade.

— Gordon Brown is to spend his summer holiday in Britain, eschewing the lavish foreign breaks favoured by his predecessor. The Prime Minister, his wife Sarah and their sons John and Fraser will spend a week in the Suffolk seaside town of Southwold. They will hope that this holiday lasts longer than last year’s trip to Dorset, which was cut short by emergency meetings over foot-and-mouth disease. The decision to stay in Britain is designed to set an example amid the economic gloom. David Cameron, the Tory leader, is expected to travel to Cornwall.

On St Swithin’s Day

1808 Storms, in which a fireball hit Gloucester Cathedral. Hailstones the size of cricket balls in Somerset

1913 London endured 15 hours of heavy rain but it rained on only 9 of the following 40 days

1924 There were 13.5 hours of sunshine: it then rained on 30 of the next 40 days

1976 Storms on the evening of the 15th: 38 of the next 40 days were dry in the South

1990 Hot and sunny. In the next 40 days it rained only 3 times in the South. Almost a St Swithin that came true