COOL SEPTEMBER, DRIER AND MUCH SUNNIER THAN AVERAGE

Armagh Observatory reports that September 2024 was cooler than average but drier and much sunnier.  It was the coolest September at Armagh for six years, the driest for four years, and the sunniest for 27 years.

The average temperature was 13.1 degrees Celsius (55.6 degrees Fahrenheit). This  is approximately 0.6 C warmer than the 225-year long-term (1796-2020) average for  September at Armagh (12.5 C) and 0.4 C cooler than the most recent (1991-2020) 30-year average (13.5 C).  This was the coolest September at Armagh since September 2018 (12.6 C).

Despite this there were several relatively warm days, notably a brief period of ‘summer’ from the 5th to the 7th and again during the week beginning the 14th.  The three warmest days (highest maximum air temperature) were the 7th, with an unremarkable 25.0 C but nevertheless the warmest day of the year, followed by the  6th (23.9 C) and the 17th (23.5 C).

It is rare that the warmest days of the year should occur in September and June, though this happened both last year and this. Last year, the three warmest days were the 13th of June and the 8th of September, both 28.5 C, followed by the 16th of June (26.9 C).  This year, the three warmest days were the 7th of September (25.0 C), followed by the 24th of June (24.8 C) and the 19th of July (24.3 C).

The highest minimum air temperature, usually the warmest night, was 15.3 C. This  was attributed to the 2nd despite the value occurring at the beginning of the 24-hour ‘meteorological day’ running from 09:00 GMT on the 1st to 09:00 GMT on the 2nd, the day of observations.  The second highest minimum air temperature was 13.4 C on the 17th; and this was followed by 13.1 C, attributed to the 6th.  Again, this value occurred at the beginning of the meteorological day, that is, at 09:00 GMT on the 5th.

The lowest minimum air temperature, or coolest night, was 3.1 C on the 25th, followed by 3.2 C on the 13th, and 5.2 C on the 28th.

The lowest maximum air temperature, or coolest day, was a cold 9.9 C on the 26th, followed by 13.1 C on the 25th and 13.6 C on the 27th.  The 26th was the coldest September day at Armagh for 12 years, that is, since the 24th of September 2012 (lowest maximum temperature also 9.9 C).

There were five ground frosts, that is, nights with minimum grass temperatures less than or equal to zero Celsius.  The three lowest grass temperatures were -3.2 C on the 25th, -2.3 C on the 13th, and -1.6 C on the 28th.

Total precipitation this month was 57.0 mm (2.24 inches) including six trace values, that is, 56.7 mm if trace values are ignored.  This is approximately 83% of the 183-year long-term (1838-2020) average September rainfall at Armagh (68.9 mm) and 88% of the most recent (1991-2020) 30-year average (65.0 mm). It is noteworthy that the (1991-2020) 30-year average September rainfall is slightly less than the 183-year long-term average although in this case the difference is not statistically significant.

Although September rainfall at Armagh shows the usual apparent decadal oscillations and can vary significantly from one year to another, it seems that to a first approximation it has not significantly changed over the nearly two hundred years of observations at Armagh.

The wettest day was the 25th with 24.5 mm of precipitation, followed by the 26th with 13.7 mm, and the 30th with 6.1 mm.  In contrast to the very wet September in parts of southern England this month, where according to the UK Meteorological  Office ten English counties experienced their wettest Septembers on record, the first three weeks of September at Armagh supplied total precipitation amounting to just 10.6 mm and only seven days had total rainfall greater than or equal to 0.2 mm.  Conversely, the last week supplied 46.1 mm of precipitation, three times the corresponding (1991-2020) 30-year average.  Despite the wet end to the month, this September was the driest at Armagh for four years, that is, since September 2020 (49.2 mm including 4 trace values).

There were no named storms this September, but hail was reported on the 11th and a rainbow was seen on the 27th.   A rook and a jackdaw were seen chasing off a buzzard on the morning of the 25th.

There were 137.8 hours of strong sunshine during the month.  This is approximately 23% more than the 140-year (1881-2020) long-term average strong sunshine at Armagh (112.1 hours) and 21% more than the most recent (1991-2020) 30-year average (114.4  hours).  This was the sunniest September at Armagh for 27 years, that is, since 138.8 hours of strong sunshine was recorded during September 1997.  The sunniest days were the 18th (11.2 hours), the 12th (10.3 hours), and the 11th (9.9 hours).

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

College Hill

Armagh

BT61 9DG


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