September got off to a blazing start, with heat building over the first ten days, and records smashed. The hottest day was the 9th, when the maximum reached 34.2°C, which was way above the previous September record of 31.4°C set on 15th September 2020. Even more unbelievably, that was the only previous September day in our 39 years of records that the temperature topped 30°C, and this September we actually recorded five consecutive days over 30°C, the longest such stretch apart from 6 consecutive days in July 2006 and August 2022. And there’s more: this was our fourth hottest day ever, after two consecutive days of 38.8°C on 18th and 19th July 2022, 37.9°C in August 2003 and 36.6°C in August 2020. It’s hard to remember we are talking about mid September here!

Unsurprisingly we also set records for the highest mean monthly temperature for September, at 18.2°C (the previous record being 17.8°C in 2006), the highest average daily maximum at 23.8°C (previously 21.6°C in 2006), the highest overnight minimum of 18.9°C – actually higher than the long term average daily maximum (previously 17.8°C again in 2006), and equalled the highest average daily minimum of 13.9°C (once again in 2006).

With all this heat, you’d think it had been a dry month. Actually it was our 7th wettest September on record, with a total of 93.0mm, which is 156% of our long term average, thanks to a breakdown in the weather patterns to warm but moist south-westerly air flows in the second half of the month. And yet we had rain on only eight days in the month – only in 2002 have we had fewer! The reason is that when it did rain, boy did it rain. The 20th September was our wettest day with 29.0mm, our 5th wettest September day, and the four-day period of 18th to 21st delivered 66.6mm, or 2⅔ inch of rain.

The barometer reflected these extremes, with high pressure dominant over the first half of the month, and successive thundery lows passing by over the second half. The lowest pressure was 992.7 hPa, our second lowest recorded September pressure (from just 15 years of such detailed records), which came on 20th and delivered our biggest downpour.

Similarly, the highest winds were also associated with that same depression. The highest gust of the month was on 20th, a blow of 27 mph from SW on the 19th.

Overall, the main wind directions were from a south-easterly quarter predominantly during the warmer weather, and from a south-westerly quarter during the wetter weather – nothing unusual about that, but well illustrated by the data.

Measure	Unit	Minimum	Average	Maximum	Average min	Average max	Total
Relative humidity	%	41	78	96
Temperature	°C	8	18.2	34.2	13.9	23.8
Wind source (most common/highest gust)	Sector		ESE	SW
Wind speed	mph		2.5	27		15.6
Rain daily accumulation	mm		3.1	29.0			93
Barometric pressure	hPa	992.7	1013.5	1027.3
Solar irradiance during daylight	W/m²		229	860		635
Sunshine (sunny if irradiance > 120 W/m2)	Hours		8.2	10.5			246

October is on average the wettest month of the year. How will this October measure up? And will the weather deliver another month of above average temperatures? Check next month’s report to find out!