September 2021 was the second-warmest on record in the UK and the warmest ever recorded in Northern Ireland, provisional Met Office figures show.
The mean temperature of 14.7C for the UK was only slightly cooler than the 15C figure that was recorded in August.
The UK’s warmest September was in 2006 when the mean temperature reached 15.2C, the Met Office said.
Dr Mark McCarthy, head of the Met Office’s National Climate Information Centre, described this September as being “an exceptionally warm month for the UK”.
He said the month saw “persistent above-average temperatures” until weather conditions changed right at the end.
Warm nights helped Northern Ireland to its joint-warmest September on record, with mean temperatures of 14.2C, which matched those recorded in 2006.
The highest average minimum temperature in Northern Ireland in September was a record figure of 10.7C, topping the 10.5C reached in 2006.
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The Met Office said that last month, which came on the back of a more subdued August, saw the UK as a whole enjoy its second-warmest September based on figures dating back to 1884.
The mean temperature in September was 13C in Scotland, 14.8C in Wales and 15.8C in England.
Dr McCarthy said: “Interestingly, the exceptionally warm start of the month at one time threatened to push September’s average temperature figures above August’s, but the recent unsettled weather and drop to more average conditions have driven the figures slightly below August’s, but still well above the long-term average.
“High pressure dominated for much of the early part of the month, which brought with it warm, sunny spells and calm conditions.
“Although that particularly warm heat subsided, the temperature remained above average for a good portion of the month, before a more unsettled breakdown late in the month and more low-pressure systems moving in to the UK.”
Glasgow, which is hosting the COP26 climate change talks in November, recorded its joint hottest September with a temperature of 14.9C, matching the top figure dating from 2006, the Met Office said.
Despite last month being Northern Ireland’s joint warmest, it was also a particularly dull time as the hours of sunshine were the lowest since 1965. There was an average of just 74.3 hours.
Temperatures of more than 30C were recorded in some places early in September.
Wales topped 30C in September for the first time since 1961, when Dyfed was basking in sunshine on 7 September.
This was still shy of the all-time September temperature record in Wales of 32.3C set at Hawarden Bridge on 1 September 1906.
Other hot days included 7 and 8 September when 27.2C was recorded in Plymouth, 27.1C in Eskdalemuir and 25.8C in Balmoral.