May 2021

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 (29^th)

Absolute minimum: -1.0 (6^th)

Mean grass minimum: 3.1 Difference from average: -1.5

Absolute grass minimum: -2.8 (6^th)

Mean concrete minimum: 5.5

Absolute concrete minimum: -0.9 (6^th)

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 (3^rd)

Number of rain days (>0.1 mm): 19

Number of wet days (=>1.0 mm): 17

3-month total rainfall to 31^st May 2021: 125.2 Difference from average: -46.8

6-month total rainfall to 31^st May 2021: 453.2 Difference from average: +107.0

12-month total rainfall to 31^st 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 (30^th)

Number of days with no recorded sunshine: 0

Cumulative sunshine since 1^st 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 (4^th, 12:15, N)

Comments on May 2021

May was a cool, wet month. Mean air temperature was equal 72^nd lowest since 1844 (n = 178), the coldest since 1996. The mean maximum was also the coldest since 1996, equal 82^nd lowest since 1844. There was the lowest mean minimum since 1996, the equal 61^st 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 21^st wettest May on record since 1850, the wettest May since 2013. The 12-month total is the 47^th 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 53^rd 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 120^th warmest since 1844, the coolest spring since 2013. The mean grass minimum was also the lowest (-0.4 °C) since 2013, equal 59^th lowest since 1874. There were 28 ground frosts but this is only the equal 89^th 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 13^th highest since 1882, no doubt helped by an exceptional April.

Emeritus Professor Tim Burt

Department of Geography

Durham University