The answer is around 1993. A look back at mean temperature and rainfall statistics for east London over the past hundred years reveals that the ninth month has, since that date, slowly become warmer and drier.
Putting all the arguments of meteorological and astronomical summer aside, many people of a certain age regard September as an autumnal month, but as recent years have shown it can very often be an extension of summer; September 2016 was the second warmest on record in the local area, warmer than many previous summer months!
Looking back even further, over the past 100 years, the September mean has trended upward, though many peaks and troughs reveal how the month has ebbed and flowed from being summery to autumnal.
The prognosis to the end of this month suggests that air pressure will be anomalously high – so the pattern in the south-east for settled, summery weather in September doesn’t look like ending any time soon.
September in recent years has often been a more summery month than August. But this month in some years can see a rapid onset of autumn.
I’ve put together a few top 10s of stats for Wanstead, St James’s Park and Heathrow for the month of September.
Some national UK September values according to TORRO
Hottest: 35.6C Bawtry, South Yorkshire – 2nd 1906
Coldest: -4.5C Dalwhinnie, Highland, – 26th 1942
Wettest: 190.7mm West Stourmouth, Kent – 20th 1973
In terms of climatology September maxima, considering the 1981-2010 average, shows a slight decrease through the month, though around the 1st and the 22nd there is often a spike. This would reflect the September singularities; early September warmth occurs in 82 per cent of years while the ‘Old Wives Summer’ has a 64 per cent probability.
The average rainfall graphic shows that downpour amounts are variable through the month. A tendency for dry weather to the 8th prevails before the trend increases, the wettest days usually 13th and 19th.