SEPTEMBER WARMER, WETTER AND SUNNIER THAN AVERAGE. THIRD WARMEST SEPTEMBER ON RECORD
Armagh Observatory reports that September 2023 was much warmer than average, the third warmest September on record at Armagh. It was also wetter and sunnier than average.
Total precipitation was 91.8 mm (3.61 inches) with no trace values. This is approximately 33% more than the 183-year long-term (1838-2020) average September rainfall at Armagh (68.9 mm) and 41% more than the most recent (1991-2020) 30-year average (65.0 mm).
There were several very wet days but the wettest, associated with Storm Agnes, the first named storm of the 2023/2024 season, was the 27th with 22.3 mm (0.88 inches). The second and third wettest days were the 18th (10.4 mm) and 24th (9.9 mm).
September 2023 was much warmer than average, with a mean monthly temperature of 15.2 degrees Celsius, approximately 1.7C warmer than the most recent (1991-2020) 30-year September average at Armagh (13.5 C).
In the Observatory’s nearly 230-year long daily temperature series beginning around 1795 this was the third warmest September on record. It continues a series at Armagh where the last few Septembers, now five in a row, have been warmer than the most recent (1991-2020) 30-year average.
The five warmest Septembers at Armagh are now 2021 (15.5 C), 1865 (15.4 C), 2023 (15.2 C), 2006 (15.0 C), and 1846 and 2016 (both 14.8 C).
A monthly mean can hide large variations, and this September was no exception. The high average temperature was due to an exceptional warm spell near the beginning of the month lasting for a week from the 4th to the 9th. During this period every day except the 9th (maximum temperature 24.8 C) exceeded the official threshold, 25.0 C, for a heatwave in Northern Ireland.
Heatwaves are relatively uncommon in Northern Ireland and September heatwaves are very rare. There is only one earlier example at Armagh in the nearly 180 years since measurements of daily maximum and minimum temperatures began in August 1843, and one other which spanned the end of August and early September.
The previous September heatwave, in 1959, lasted for just three days from the 9th to the 11th, and the other, equally short, lasted from 31st August to 2nd September 1906.
The September 2023 heatwave provided a respite from an otherwise wet, albeit warm and sunnier than average summer. It was noteworthy for including the warmest September day on record at Armagh, namely 28.5 C on the afternoon of the 8th. This equalled the then hottest day of the year, the 13th of June, and surpassed the previous hottest September day at Armagh by nearly 1 degree Celsius. It is very unusual that the hottest day of the year should occur in both June and September.
The half-dozen warmest September days at Armagh are now 28.5 C (8th September 2023), 27.6 C (1st September 1906), 26.6 C (11th September 1959 and 7th September 2023), 26.2 C (7th September 2021), 25.8 C (9th September 1959 and 5th September 2023), and 25.7 C (6th September 2023).
The highest minimum temperature, usually the warmest night, was 16.1 C on the 9th, followed by 15.2 C on the 7th and 14.8 C on the 10th. The lowest minimum temperature or coolest night was 4.4 C on the 13th followed by 6.5 C on the 23rd (though this in fact occurred shortly before midnight on the 22nd) and 6.6 C on the 21st.
The coolest day or lowest maximum temperature was 13.0 C on the 15th, followed by 14.2 C on the 16th and 14.7 C on the 21st. There were three ground frosts, namely -2.1 C on the 13th, -1.6 C on the 23rd, and -0.3 C on the 12th.
There was a total of 123.3 hours of strong sunshine during the month. This is approximately 9.5% more than the 140-year long-term (1881-2020) average strong sunshine at Armagh and 7.8% more than the most recent (1991-2020) 30-year average. The sunniest days were the 4th, 6th, and 5th, with respectively 9.4, 9.2 and 9.0 hours of strong sunshine.
These data refer to observations at Armagh Observatory, which has been recording the weather at Armagh since 1795.
For further information, please contact:
Professor Mark E. Bailey
Emeritus Director of Armagh Observatory
Armagh Observatory and Planetarium