WARMEST MAY ON RECORD, DRIER AND DULLER THAN AVERAGE. EQUAL WARMEST SPRING ON RECORD, WETTER AND DULLER THAN AVERAGE
Armagh Observatory reports that May 2024 was much warmer than average, the warmest May on record. It was also drier than average, with approximately half the average May rainfall at Armagh, and much duller, with approximately three quarters of the average number of hours of strong sunshine.
Taking March, April, and May together, the meteorological spring was the equal warmest on record at Armagh, shared with spring 2017, and both wetter and duller than average. This was the dullest spring for five years, that is, since the exceptionally dull spring 2019.
The mean temperature this month was 13.9 degrees Celsius (57.0 Fahrenheit), almost 3.3 C warmer than the 225-year long-term (1796–2020) May average at Armagh (10.62 C), and 2.5 C warmer than the most recent (1991–2020) 30-year average (11.42 C). This was the warmest May at Armagh on record, that is, since daily temperature measurements began at the Observatory around 1795. The half-dozen warmest Mays at Armagh are now 2024 (13.9 C), 1809 (13.8 C), 1848, 2017, and 2008 (all 13.2 C), and 2023 (13.1 C).
Although the instrumental record clearly shows that this month was conventionally the warmest May on record, it is interesting to ask why this is at odds with most people’s perception. Many people have remarked that they find the result surprising. Although there were some short-lived warm spells around the middle of the month, unlike some other very warm months, for example June or September 2023, there were no heatwaves nor any very hot days. Despite its unusually high average temperature, the record-breaking May 2024 was not associated with paddling pools or summer BBQs!
Instead, the exceptionally warm month appears to have arisen because most days were warmer than the most recent 30-year average and most nights much warmer. In fact, there was an overwhelming number (25) of days with maximum temperatures greater than the corresponding most recent (1991–2020) 30-year average (15.8 C), and similarly 28 days with minimum temperatures greater than the corresponding lowest temperature average (7.0 C). This month there were no very cold days nor any exceptionally cold nights. Considering that the month was much duller than average, it therefore seems most likely that these results leading to a record-breaking monthly average temperature, conventionally defined to be half the sum of the monthly average maximum and minimum, is linked to cloudiness.
This is because greater daytime cloudiness will tend to suppress daytime temperatures whereas the warmest moments of the day will tend to occur during short sunny spells between clouds. In this case, the day’s highest recorded temperature may exaggerate the overall warmth of the day. Conversely, cloudy nights tend to be much warmer than clear ones, so greater nighttime cloudiness will produce higher temperature minima. The net result is a “cloudy bias” towards higher measured average daily temperatures. This provides a possible explanation for the difference between people’s perception of the month’s weather and the instrumental record.
This May the mean maximum temperature was 18.1 C, which is approximately 2.3 C warmer than the most recent 30-year average at Armagh and the fourth warmest such average on record. Similarly, the mean minimum temperature was 9.6 C, approximately 2.6 C warmer than the corresponding most recent 30-year average and the highest such result on record at Armagh. This average minimum temperature surpassed the previous record (8.9 C) set in May 2022 by the surprisingly large margin of 0.7 C. The three warmest days this month were the 10th, 11th, and 12th, with respective highest
temperatures of 22.9 C, 22.8 C, and 22.6 C. The 10th, with a maximum temperature of 22.9 C, was the warmest day of the year so far, and is projected to remain the warmest day of 2024 until at least the middle of June. Even so, this warmest day ranks only among the top 90 hottest May days observed at Armagh.
The coolest day (lowest maximum air temperature) was 11.5 C on the 23rd, followed by 12.9 C on the 4th, and then by 13.7 C shortly before the daily readings were taken at 9.00am (GMT) on the 25th, and therefore attributed to the 24th.The highest daily minimum temperature, notionally the warmest night, was 12.1 C. This occurred during the early morning of both the 12th and the 22nd and was followed by 11.8 C on the 13th. The lowest minimum temperature, or coolest night, was 4.2 C on the 7th, followed by 6.2 C on the 1st, and 6.4 C on the 19th.
There were two nights with ground frost, namely -1.3 C on the 1st and -1.0 C on the 7th. There were no air frosts this month.
As for rainfall, total May precipitation was 28.85 mm (1.14 inches) including 5 trace values (that is, 28.6 mm if trace values are ignored). This was approximately half the May average and the driest May at Armagh for four years, that is, since May 2020 (17.80 mm). The wettest day was the 21st with 8.1 mm of precipitation, followed by the 26th with 5.7 mm and the 13th with 3.2 mm.
The observer noted the first sighting of a peacock butterfly close to the weather station on the 3rd, and a rare sighting of a jay close to the weather station around the time of observations on the 4th. Shortly afterwards on the same day the observer noted the first swifts of the season. Distant thunder was heard on the afternoon of the 21st, while further swifts, swallows, and a pair of goldfinches were seen around the time of observations on the 24th.
There were no gales, nor hail showers or rainbows this month. However, a strong and colourful display of the aurora borealis or northern lights occurred on the 10th and was widely seen. The phenomenon lasted from approximately 22:30 to midnight (GMT) with a resurgence a couple of hours later.
Those with an interest in Irish literature may know that the playwright, poet and author, William Carleton (1794–1869), gave a remarkably accurate description of an aurora in his story “The Priest’s Funeral”. It seems probable that the description was based on his own observations of the Great Aurora of September 1833, which was seen worldwide including at Armagh around the middle of that month. Astronomers name craters on the planet Mercury after deceased artists, musicians, painters and authors who have made outstanding or fundamental contributions to their field, and both Seamus Heaney and
William Carleton have been so commemorated.
May 2024 saw approximately 136.8 hours of strong sunshine, slightly more than three quarters of the average for May at . This was the dullest May at Armagh for five years, that is, since May 2019 (126.8 hours of strong sunshine). The sunniest day was the 10th with 13.3 hours of strong sunshine, followed by 11.4 hours on the 11th, and 10.1 hours on the 19th.
Taking the three months March, April and May together the meteorological spring was the warmest spring on record at Armagh, shared with spring 2017. It was also wetter and much duller than average. The average spring temperature was a very warm 10.2 C (nearly 50.4 F), approximately 2.1 C warmer than the 225-year (1796–2020) long-term spring average at Armagh (8.1 C) and 1.2 C warmer than the most recent (1991–2020) 30-year average (9.0 C). The four warmest springs at Armagh are now 2024 and 2017 (both 10.2 C), and 2011 and 1945 (both 10.0 C). Total spring precipitation was 198.0 mm including 12 trace values. Despite the drier than average May, spring 2024 was approximately 15% wetter at Armagh than average.
March, April and May were all duller than average, together producing 357.2 hours of strong sunshine, which is approximately 83% of the spring average at Armagh. This was the 18th dullest spring on record and the dullest at Armagh for five years, that is, since the exceptionally dull spring 2019 (312.7 hours).
These data refer to observations at Armagh Observatory, which has been recording the weather at Armagh since 1795.
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