There’s a tradition that if it rains on St Swithin’s Day (15th July) then it will rain for the next forty days, and there’s also the converse tradition that if it’s fine then the next forty days will also be fine.
This is very much an “old wife’s tale” and records actually show that if it does rain on the 15th July then the next forty days are likely drier than average! In fact there’s never been a forty day recorded period of rain between July 15th and August 24th…
So where did this story start? St Swithin was an Anglo-Saxon bishop of Winchester and just before he died c. 862 he ordered that he should be buried outside the church “ubi et pedibus praetereuntium et stillicidiis ex alto rorantibus esset obnoxious” (where it might be subject to the feet of passers-by and to the raindrops pouring from on high). The legend says that a few years later when it was decided to move St Swithin’s body to a shrine inside the church there was a huge rainstorm and the move had to be delayed for a period.
You have to take this story with a very big a pinch of salt, 
especially when you consider that in France there’s a similar tradition about St Gervais (19th July) and Germany has “Seven Sleeper’s Day” (7th July).
Actually there is a scientific basis to these traditions as our weather is affected by the Jet Stream. By mid-July it has usually settled into a specific weather. The further North a frontal zone forms the warmer and sunnier the weather is for the next few weeks but if the frontal zone moves South we have cooler and wetter weather. Statistically 8 out of 10 years this pattern holds true.
St Bartholomew’s Day, August 24th when the Swithin forty day period ended has a few weather traditions too. “If August 24th be fair and clear, then hope for a prosperous autumn that year”. Another saying suggests that should the morning of the 24th begin with a hoar frost then cold weather can soon be expected and a hard winter.
September has a few weather pointers to look out for: “Fair on the first of September, fair for the whole month” and “St Matthew’s Day (Sept 22nd), bright and clear brings good wine in next year”. Also check the wind direction on the 21st as it is said that a south wind on September 21st means that the rest of the autumn will be warm.
Whether these sayings are accurate I’ve no idea, I can’t find that anyone has checked the old weather reports except for disproving the St Swithin’s tradition. Incidentally, whilst we’re talking about St Swithin – yes it did rain on July 15th this year! No we didn’t have rain for the next forty days
There’s only one piece of weather lore that I trust, and that’s one from the late Kenneth Williams whilst playing “Rambling Syd Rumpo” in the radio series “Round the Horne”. He said; “When cows lay down in the field during a rainstorm, it a sure sign of…. Wet Cows” 
