WETTEST OCTOBER ON RECORD AT ARMAGH, SUNNIER THAN AVERAGE AND VERY MILD

Armagh Observatory reports that October 2023 was very much wetter than average, and both sunnier and warmer than average.  This was the wettest October at Armagh since daily rainfall records began at the Observatory in 1838, making this the wettest October, and indeed the wettest calendar month at Armagh, for at least 185 years.

Total precipitation was 195.35 mm (7.69 inches) including nine trace values, that is, 194.9 mm (7.67 inches) if trace values are ignored. This is more than 2.4 times both the 183-year long-term (1838-2020) average October precipitation at Armagh (80.4 mm) and the most recent (1991-2020) 30-year average (81.0 mm).  The half-dozen wettest Octobers at Armagh are now 2023 (195.35 mm), 1870 (193.75 mm), 2011 (182.75 mm), 2002 (158.1 mm), 1865 (150.0 mm), and 2022 (149.45 mm).  Four of these six wettest Octobers since 1838 have occurred in the last 20 years.

The half-dozen wettest calendar months at Armagh are now 2023 October (193.35 mm), 1870 October (193.75 mm), 1852 November (188.4 mm), 2015 December (186.1 mm), 2011 October (182.75 mm), and 1914 December (181.35 mm).  Three of these wettest months have also occurred in the last 20 years.

The wettest day was the 30th with a total rainfall of 30.8 mm, followed by the 29th with 29.0 mm and the 19th with 28.0 mm.  Although this month was exceptionally wet, there were still only 20 days with measurable precipitation.  This suggests that, rather than rainfall becoming more frequent, when it falls it comes in larger amounts. This month, the high cumulative rainfall total can be largely attributed to two very wet days associated with the passage and aftermath of storm Babet (20.0 mm and 28.0 mm of precipitation on the 18th and 19th of October respectively) and to a very wet spell at the end of the month with five consecutive days from the 27th to the 31st each with high daily rainfall totals.

The moisture-laden storm Babet, which had arrived from the Bay of Biscay on the 16th, was noteworthy in bringing strong winds and very heavy rainfall to central and eastern areas of Britain from the 18th to the 20th.  Predominantly high pressure during the first half of the month changed to low pressure with the arrival of this storm, which produced an exceptionally low atmospheric pressure at Armagh of approximately 978 mbar (reduced to mean sea level) on the 19th.  This was the lowest October atmospheric pressure recorded at 09:00 GMT at Armagh for 23 years, that is, since 970 mbar was recorded on both the 11th and 30th of October 2000.

The five very wet days from the 27th to the 31st at the end of the month produced respective rainfall totals at Armagh of 27.8 mm, 8.2 mm, 29.0 mm, 30.8 mm, and 15.4 mm, totalling 111.2 mm.  This is nearly 40% more than the average total precipitation for the whole of the month.  As was widely reported, this led to significant flooding across Northern Ireland including County Armagh, a result exacerbated by further light rainfall on the 1st of November.

Northern Ireland was fortunate in finding itself located nearly centrally between two very deep areas of low pressure during the first two days of November, leading to comparatively quiet conditions and only limited precipitation on the 2nd.  Thus, Northern Ireland escaped the worst effects of the violent storm Ciaran on the 1st and 2nd of November.  This third named storm of the season caused havoc to northern France and southern Britain as it moved into the English Channel from the southwest approaches before swinging north and weakening as it passed into the North Sea.

Whereas the 2022/2023 storm season provided only two named storms (in August 2023), the 2023/2024 season beginning in September has already provided three.  Rainbows were seen at Armagh on two days, the morning of the 3rd and the afternoon of the 28th.

With an average temperature of 11.7 Celsius (53.0 Fahrenheit) this was another very mild October, only slightly cooler than the two previous Octobers, namely 2022 (mean temperature 11.9 C) and 2021 (12.0 C). This October was 2.20 C warmer than the 183-year long-term (1796-2020) average October temperature at Armagh (9.48 C) and 1.31 C warmer than the most recent (1991-2020) 30-year average (10.37 C).

The monthly average of the daily maximum temperatures was 15.1 C.  This is approximately 1.4 C warmer than the corresponding most recent (1991-2020) 30-year average (13.7 C) and the ninth warmest such record at Armagh.  The monthly average of the daily minimum temperatures was 8.2 C, approximately 1.2 C warmer than the corresponding most recent 30-year average (7.0 C).

The three warmest days (highest daily maximum air temperatures) were 21.7 C on the 9th, bracketed by 20.9 C on the 8th, and 19.2 C on the 10th.  Each of these days was warmer by more than 5 degrees Celsius than the corresponding most recent (1991-2020) 30-year average (13.7 C), suggesting that these very warm October days were an example of a rare October heat wave, which in Northern Ireland may be defined as three or more consecutive days with daily temperature maxima exceeding 19.0 C.  Such October heat waves occur at Armagh roughly once every 25 years on average, the most recent examples being the periods 9th to 11th October 1969 and the 1st to 5th October 1959.

The two warmest days this October, namely the 9th and 8th, were respectively the third and ninth warmest October days on record at Armagh in a 180-year daily series of temperature maxima and minima extending back to 1844.  The 9th (maximum temperature 21.7 C) was the warmest October day at Armagh for 97 years, that is, since 22.3 C occurred on the 3rd of October 1926.

The few warmest October days at Armagh are now 1908 October 3 (22.7 C), 1926 October 3 (22.3 C), 2023 October 9 (21.7 C), 1908 October 1 (21.6 C), and 1959 October 4 (21.6 C).

By contrast, the coolest days (lowest maximum daily temperatures) at Armagh this month were the 30th (10.5 C), followed by the 21st (11.3 C) and the 14th (11.4 C).

The highest daily minimum temperatures, which are usually the warmest nights, were 15.2 C on the 7th followed by 15.1 C, which occurred around the time of observations on the 8th and again shortly after midnight on the night of the 9th/10th October (and so associated with the 10th).  These warm nights were respectively the eighth and equal ninth warmest October nights on record at Armagh.  The coolest nights (lowest minimum air temperatures) were 0.9 C on the 17th, 1.4 C on the 15th, and 1.9 C on the 12th.

There were nine ground frosts (minimum grass temperatures less than or equal to zero Celsius) this month, of which the three coolest were -5.4 C on the 12th, -4.5 C on the 15th and -4.3 C on the 17th.  With a lowest minimum air temperature of 0.9 C there were no nights with air frost.

This year’s Halloween, as with 31st October last year (2022), was very wet with 15.4 mm of precipitation, almost no sunshine, and a mean temperature of approximately 9.9 C.

The total number of hours of strong sunshine this October, namely 98.4 hours, was approximately 12% more than the 140-year (1881-2020) long-term October average at Armagh (87.3 hours) and 7% more than the most recent (1991-2020) 30-year average (92.0 hours).  The sunniest day was the 12th with 9.3 hours of strong sunshine, followed by the 14th with 9.1 hours and the 22nd with 8.7 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

Tel: 028-3752-2928

E-mail: mark.bailey@armagh.ac.uk


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