Temperatures (all temperatures in degrees Celsius; all averages 1991-2020)
Mean daily maximum: 14.3 Difference from average: -1.1
Mean daily minimum: 5.0 Difference from average: -1.5
Mean air temperature: 9.6 Difference from average: -1.4
Absolute maximum: 20.4 (29th)
Absolute minimum: -1.0 (6th)
Mean grass minimum: 3.1 Difference from average: -1.5
Absolute grass minimum: -2.8 (6th)
Mean concrete minimum: 5.5
Absolute concrete minimum: -0.9 (6th)
Number of ground frosts: 7 Difference from average: -3
Number of air frosts: 2
Mean soil temperature at 300 mm depth: 11.7
Mean soil temperature at 1000 mm depth: 10.2
Rainfall (all totals in millimetres; all averages 1981-2010)
Total for the month: 86.8 Difference from average: +42.3
Percentage of the average: 195%
Wettest day: 16 (3rd)
Number of rain days (>0.1 mm): 19
Number of wet days (=>1.0 mm): 17
3-month total rainfall to 31st May 2021: 125.2 Difference from average: -46.8
6-month total rainfall to 31st May 2021: 453.2 Difference from average: +107.0
12-month total rainfall to 31st May 2021: 884.8 Difference from average: +235.8
Sunshine (all totals in hours; all averages 1981-2010)
Total for the month: 158 Difference from average: -29
Percentage of the average: 85%
Mean daily sunshine: 5.1 Difference from average: -0.9
Sunniest day: 10.4 (30th)
Number of days with no recorded sunshine: 0
Cumulative sunshine since 1st January: 641 Difference from average: +32
Note that the monthly sunshine total is now calculated from the UKMO E&NE regional total. Daily sunshine totals quoted here and in the data summary are uncorrected.
Wind (kph; all data from the West Building roof, Lower Mountjoy campus)
Average wind speed: 4.5
Maximum gust: 45.1 (4th, 12:15, N)
Comments on May 2021
May was a cool, wet month. Mean air temperature was equal 72nd lowest since 1844 (n = 178), the coldest since 1996. The mean maximum was also the coldest since 1996, equal 82nd lowest since 1844. There was the lowest mean minimum since 1996, the equal 61st coolest since 1844. There was the highest number of ground frosts in May since 2013. In the end, there were three days at 20 °C or above; just over a quarter of Mays (51) have more days above 20 °C than this, so in the end a total of three is not that unusual. However, there were no days with a maximum above 21 °C; 98 Mays (just over half) have had at least one day above 21 °C since 1844.
It was the 21st wettest May on record since 1850, the wettest May since 2013. The 12-month total is the 47th highest for any 12 months on record (n=2046), just outside the top 2% of totals. It has been a very wet ‘year’ therefore, notwithstanding a dry March and April.
The sunshine total is 53rd lowest since 1882, the lowest May total since 2007. Even so, there was 85% of normal, perhaps surprising given the high rainfall.
Spring had a mean air temperature of 7.8 °C (0.7 °C below average), equal 120th warmest since 1844, the coolest spring since 2013. The mean grass minimum was also the lowest (-0.4 °C) since 2013, equal 59th lowest since 1874. There were 28 ground frosts but this is only the equal 89th highest spring total since 1874. After the driest spring on record in 2020, this spring was close to average (125 mm): a combination of very dry March and April and a very wet May. There were 501 hours of bright sunshine; the estimated total is surprisingly high, ranking 13th highest since 1882, no doubt helped by an exceptional April.
Emeritus Professor Tim Burt
Department of Geography