Armagh Observatory reports that September 2021 was much warmer than average, the warmest September on record at Armagh.  It was also wetter and much duller than average, the dullest September at Armagh since 1983.

The mean temperature was nearly 15.5 degrees Celsius (59.9 Fahrenheit), approximately 2.9 C warmer than the long-term (1796-2020) average September temperature at Armagh (12.54 C) and 2.0 C warmer than the most recent (1991-2020) 30-year September average (13.50 C).

This was the warmest September at Armagh for more than 226 years, that is, since daily measurements of temperature began at the Observatory around 1795. Remarkably, this September’s record average temperature was the same as the most recent (1991-2020) 30-year average for August.

As with August, average September temperatures at Armagh have increased significantly over the last 30 years or so.  For example, the (1981-2010) 30-year average was 13.25 C and the (1961-1990) 30-year average was 12.78 C.  The six warmest Septembers at Armagh are now 2021 (15.5 C), 1865 (15.4 C), 2006 (15.0 C), 1846 and 2016 (both 14.8 C), and 1949 (14.7 C).

The warmest day (highest maximum air temperature) was 26.2 C (79.2 F) on the 7th. This was the warmest September day at Armagh for 62 years, that is, since 11th September 1959 (26.6 C), and the third-warmest September day at Armagh in a 183 year record of daily maximum temperatures extending back to 1838.  This very warm day was followed by two further balmy days: 23.9 C on the 8th and 22.8 C on the 9th. The three warmest September days at Armagh are now 1st September 1906 (27.6C), 11th September 1959 and 7th September 2021.

The coldest day (lowest maximum air temperature) was 14.1 C on the 29th, followed by 14.9 C on the 30th.

The highest minimum air temperature, usually the warmest night, was 16.2 C. This happened during the small hours of two nights, both the 8th/9th and the 9th/10th, and was preceded by a third warm night (15.3 C) on the 5th/6th.  The 8th/9th and 9th/10th were the warmest September nights at Armagh for five years, that is, since a pair of even warmer highest minimum air temperatures occurred during September 2016, namely the 5th/6th and 6th/7th September 2016 (18.0 C and 17.0 C respectively).

The lowest minimum air temperature, or coldest night, was 5.3 C on the 29th followed by 7.1 C on the 28th.  There was just one ground frost this month, when the lowest minimum grass temperature touched zero Celsius on the 29th.

Total September precipitation was 76.35 mm (3.01 inches), including 7 trace values (i.e., 76.0 mm if trace values are ignored).  This is nearly 11% more than the long-term (1838-2020) average September rainfall at Armagh (68.9 mm) and 17% more than the most recent (1991-2020) 30-year average (65.0 mm).  The wettest day was the 8th with 16.7 mm (0.66 inches) of rainfall, closely followed by 16.6 mm on the 26th and 14.4 mm on the 30th.

A double rainbow was seen late afternoon on the 19th, and a colourful lunar halo on the evening of the 20th.  The observer also recorded a buzzard flying low over the meteorological station on the morning of the 5th and, on the morning of the 26th, a rook harrying a kestrel.

A total of 75.7 hours of strong sunshine were recorded during the month. This was close to 68% of the long-term (1881-2020) average number of hours of strong September sunshine at Armagh (112.0 hours) and approximately 66% of the most recent (1991-2020) 30-year average (114.4 hours).

This was the dullest September at Armagh for 38 years, that is, since the exceptionally cloudy September 1983 (67.1 hours), and the seventh-dullest September at Armagh in a record of daily sunshine measurements at Armagh extending back to 1880.  The sunniest day was the 7th with 10.6 hours of strong sunshine, followed by the 27th with 7.9 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


BT61 9DG


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