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.
The first month of autumn continued in the same vein as the last month of summer .
The mean temperature finished 14.8C, 0.6C below average and very similar overall to September 2013.
Some 53.5mm of rainfall was recorded, 103% of the 1981-2010 average. Some 114 hours of sunshine were recorded, 81 per cent of average, so on the dull side.
Overall the month was typical for early autumn – though possibly felt more autumnal because it was around 3C cooler than September 2016 and much wetter and duller than that month.
As usual at this time of year there is already much speculation about how cold the coming winter may, or may not, be. Winter has arrived over Siberia and early season snow cover is predicted to increase over the next week. However, the factor of early snow over Siberia being a sign of a cold winter to come in NW Europe is is only one piece of the jigsaw and I have seen it proved wrong a few times.
Snow cover: 2/10/2017
Modelled snow cover: 9/10/2017
Summary for September 2017
Mean (1 minute) 14.4
Mean (min+max) 14.8
Mean Minimum 10.7
Mean Maximum 18.9
Minimum 5.3 day 21
Maximum 22.7 day 04
Highest Minimum 16.7 day 04
Lowest Maximum 15.2 day 19
Air frosts 0
Total for month 53.5
Wettest day 16.1 day 27
High rain rate 16.1 day 13
Rain days 19
Dry days 11
Highest Gust 28.9 day 12
Average Speed 2.3
Wind Run 1670.8 miles
Gale days 0
Maximum 1022.7 day 01
Minimum 989.1 day 12
The final month of autumn was most notable for its rainfall with 19 consecutive days of measurable precipitation and just four completely dry days .
The month was warmer than average; the mean temperature of 8.9C was 0.9C above average, making it the 19th warmest November since 1797: 2.2C warmer than last November, though only the warmest since 2011.
The sunny and start on the 1st was quickly replaced with an unsettled, cooler regime week which lasted for nearly three weeks. A brief respite on the 20th was followed by more rain – with 12mm and 15.2mm on the 22nd and 23rd. It was wetter than average: 97.5mm represents 165 per cent of what we’d expect to fall in a normal November.
Despite it being wet it was only the 40th wettest November in the series back to 1797.
The warmest day occurred on the 1st with 17.2C recorded. The first frosts of autumn were recorded. The lowest temperature occurred on 24th when the mercury fell to -1.5C.
Sunshine was average with 60.5 hours recorded – that’s 103 per cent of mean. The sunniest day was on the 4th when 6 hours of sunshine were recorded.