WETTEST MAY FOR 18 YEARS, COLDEST FOR SIX YEARS, SUNNIER THAN AVERAGE.  SPRING 2021 SUNNIER AND COOLER THAN AVERAGE WITH ABOUT AVERAGE RAINFALL

Armagh Observatory reports that May 2021 was wetter, colder and sunnier than average.  Taking March, April and May together, the meteorological spring was sunnier and cooler than average, with a little less than average rainfall.

Total May precipitation was 107.15 mm (4.22 inches), including 3 trace values (i.e., 107.0 mm if trace values are ignored).  This is approximately 183% of the 183-year long-term (1838-2020) average May precipitation at Armagh (58.55 mm) and 184% of the most recent (1991-2020) 30-year average (58.20 mm).  The wettest day was the 20th with 30.7 mm (1.21 inches) of rainfall, followed by the 12th with 14.3 mm.

In contrast to May 2020 (which was the 13th driest May on record at  Armagh), this May was the 12th wettest May in a series of daily rainfall  measurements that began at Armagh in 1838.  It was the wettest May for 18 years, that is, since slightly more precipitation (108.7 mm) was recorded in May 2003.

This inclement May sometimes felt as if April showers had arrived a  month late.  Large hailstones were recorded on the 4th and 5th, with  sleet also on the 5th, and small hail on the 23rd.  Thunder was  reported on the 10th, 12th, 18th and 23rd.  The damp, cool and changeable weather pattern switched near the end of the month to provide a spell of warm, dry weather for the Spring bank holiday.

Swallows were seen on the 10th, swifts on the 11th, and gulls on the  morning of the 18th and 21st. 

The mean temperature was 10.22 degrees Celsius (50.40 Fahrenheit),  approximately 0.4 C cooler than the 225-year long-term (1796-2020) average May temperature at Armagh (10.62 C) and 1.2 C cooler than the most recent (1991-2020) 30-year average (11.42 C).   This was the coolest May at Armagh for six years, that is, since May 2015 (mean temperature 9.85 C). 

The warmest day (highest maximum air temperature) was 22.9 C on the 30th, followed by 21.1 C on the 31st and preceded by 20.8 C on the 29th.  These days at the end of the month were the first this year during which the maximum temperature at Armagh exceeded 20 C, the 30th being the warmest day of the year so far.  The coolest two days (lowest maximum air temperature) were 9.5 C on the 5th preceded by 10.6 C on the 4th.

The highest minimum air temperature (usually the warmest night) was 11.3 C on the 29th, preceded by 11.0 C on the 28th.  The lowest minimum air temperature, or coolest night, was -1.2 C on the 6th, preceded by 0.3 C during the early morning of the 2nd.  The morning of the 6th was the coolest May night at Armagh for six years, that is, since -1.5 C was recorded on 1st May 2015.

There were 20 nights with ground frost, that is, during which the minimum grass temperature fell to less than or equal to zero, the three hardest of which were -7.7 C on the 2nd, -7.4 C on the 1st and -7.3 C on the 6th.  There was only one air frost, namely -1.2 C recorded on the morning of the 6th. 

Despite the low average temperature, May 2021 was a very sunny month, presenting a total of 208.7 hours of strong sunshine.  This was nearly 19% more than the 140-year long-term (1881-2020) average May sunshine at Armagh (175.6 hours) and almost 17% more than the most recent (1991-2020) 30-year average (178.8 hours).  The sunniest day was the 30th with 14.7 hours of strong sunshine, followed by 13.1 hours on the 31st.

Taking the meteorological spring March, April and May together, spring 2021 was sunnier and cooler than average, but with about average rainfall.  The very wet May effectively compensated for a dry March and the exceptionally dry April.

Total spring precipitation was 164.25 mm (6.47 inches) including 9 trace values (that is, 163.80 mm if trace values are ignored).  This is approximately 98.4% of the 183-year long-term (1838-2020) spring precipitation at Armagh (167.0 mm) and 94.5% of the most-recent (1991-2020) 30-year average (173.8 mm).  

The mean spring temperature was 8.48 C (47.3 F), approximately 0.36 C warmer than the 225-year long-term (1796-2020) average spring temperature at Armagh (8.12 C) and 0.52 C cooler than the most recent (1991-2020) 30-year spring average (8.99 C).  This was the coolest spring at Armagh for six years, that is, since spring 2015 (mean temperature 8.0 C). 

The dull March, the exceptionally sunny April and sunnier than average May led to spring 2021 being rather sunnier than average.  There were 491.8 hours of strong sunshine during the three months, which is approximately 16% more than the 140-year long-term (1881-2020) spring average at Armagh (422.9 hours) and nearly 14% more than the most recent (1991-2020) 30-year average (431.6 hours).

These data refer to observations at Armagh Observatory, which has been recording the weather at Armagh since 1795.


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