Sunshine, cracks, fly-tipping, aphids: Phew, what a May!

By: Roger Brugge

Those spending more time in their gardens and in local parks and woods in and around Reading due to the current lockdown situation will have noticed how the ground surface had been resembling the look and feel of a badly-laid concrete patio – grey in colour, very hard, somewhat uneven and with some rather large cracks. It has very definitely dried out since I was sliding around in the mud in my local woods while walking my dogs this past winter.

Cracks in the ground in the Maidenhead Thicket woods.

Indeed, in my home location of Boyn Hill in west Maidenhead, May 2020 was the second driest May since before 1859.

Monthly rainfall totals during May for the Boyn Hill area of Maidenhead, 1859-2020.

Compared to an average May rainfall of 53 mm, just 2.7 mm fell this year on two days. In fact, the rainfall spells lasted barely 40 minutes in total. If we go back to 1990, we find our driest May when 2.1 mm fell – while in 1979 we had our wettest May with 126 mm of precipitation.

In Reading, May 2020 has been even drier, with just 1 mm of rainfall – the driest May in a record back to 1901, with other dry Mays having occurred in 1990 (3.1 mm), 1956 (5.1 mm) and 1919 (5.9 mm). However, go into Oxfordshire and the weather station at RAF Benson recorded just a ‘trace’ – i.e. no measurable rainfall – during May 2020. This lack of rain to wash away pests has led to an abundance of black fly on my broad beans and aphids in the garden!

Locals may remember how the River Pang (which flows through nearby Pangbourne) ran dry as the summer months of 1990 brought a summer heatwave. In Windsor, people were encouraged to share their bath water (after use) with trees in the Castle gardens. Hosepipe bans, tarmac turning to treacle and poisonous blue-green algae at Dinton Pastures Country Park were other features of the summer along with a maximum temperature of 35.5 °C on 3 August – still the fourth highest maximum temperature of all time in the Reading record. One wonders what lies ahead for this summer.

But, be warned. The summer of 1956, in contrast, was rather poor – in Reading only six days in July and one in September reached 25 °C. In 1919, 30 °C was reached on a total of three days at the University (but as late as August and on 11 September – one of the warmest September days in the Reading record). Looking back further, the very dry May of 1895 was followed by a wet second half to the year in Berkshire, with similar features to the rainfall distribution occurring in 1896 after another very dry May.

In Reading, one of the measurements made at 0900 UTC each day by our weather observers is that of the state of the ground, using the code shown in this table (assuming that there is no lying snow):Codes used to describe the state of ground (without lying snow) in the UK.

During late spring and summer in Reading the state of the ground at 0900 UTC is usually described as dry or moist; it might be wet following a heavy downpour within the previous 24 hours or be described as being ‘dry with cracks’ after a warm, sunny, dry spell lasting (say) 7-10 days.

May 2020 has seen our observers noting that the soil was moist (code 5/1, on the 1st due mainly to some rainfall at the end of April), dry (code 5/0, on the 2nd-4th) but extremely dry with cracks thereafter – so 27 days with cracked soil). A typical year has about 12 days when a code of 5/9 is recorded – and we had 25 such mornings in April 2020 also!

Following the wet spell from late September 2019 to early March 2020, such a ground state is quite surprising. But then spring 2020 has also been the sunniest on record in Reading (in a record back to 1956) and across the wider area of Central Southern and South-East England since before 1929:

Spring sunshine totals in Cent S and SE England (black, 1929-2019, data courtesy of the Met Office) and at the University of Reading (red, 1958-2020).

Monthly sunshine totals this spring (as measured by a Campbell-Stokes sunshine recorder) in Reading have been as follows:

  • March 2020 – 170.2 h (seventh sunniest March on record; the sunniest was 178.7 h in 2007)
  • April 2020 – 250.9 h (the sunniest April on record; previous record was 234.0 h in 1984)
  • May 2020 – 345.1 h (the sunniest month of any name on record; previous record 305.6 h in June 1975)

This makes a spring total of 766.5 hours – the sunniest spring on record at the University, while only three summers have been sunnier than spring 2020 in Reading.

The University’s Campbell-Stokes sunshine recorder. A glass sphere focuses the sun’s rays on to a graduated card and the length of the burn trace on the card corresponds to the duration of sunshine.

Daily mean sea level pressure at 0900 UTC during spring 2020 at the University of Reading.

These sunny conditions are not unrelated to the generally high pressure we have had for much of spring. At 0900 UTC the mean sea level pressure in Reading has often been in the range 1020-1040 hPa, except for two notable cyclonic spells in early March and at the end of April/beginning of May. Winds have also often been between the drier north-east and south-east, rather than the more usual south-westerly quadrant of the compass. High pressure, associated with generally descending air, often helps to reduce cloud cover in the spring/summer, leading to a reduction in precipitation and, hence, sunnier conditions.

Daily wind direction at 0900 UTC during spring 2020 at the University of Reading.

In May 2020, the average pressure at 0900 UTC was 1022.1 hPa – the third highest May value (after May 1991 and May 1944) in the Reading record since 1908.

Clear skies can sometimes lead to cool nights as well as warm days. In fact, during a cool northerly flow of air in mid-May the grass minimum temperature in Reading fell to -9.3 °C on the 15th, thereby doing severe damage to delicate crops such as potatoes. My potatoes suffered too – although meteorological foresight did allow me to carry out what looked like a bad case of fly-tipping on my allotments in a successful attempt to minimise the damage by covering most of the early crop:

Anti-frost measures on the author’s allotments in Maidenhead, mid-May 2020.

This spring in Reading the mean daily maximum temperature has been 2.5°C above average, while nights have been slightly cooler than normal overall, with the mean daily minimum temperature 0.2°C below average, with 50 ground frosts (compared to the 38 we would normally expect in spring). Overall, it was the warmest spring since 2017.

In summary, provided that we have been willing to water our gardens and allotments, the weather conditions have generally been quite conducive to enduring the prevailing lockdown conditions – indeed, my dogs have often picked up the whiff of evening barbeques!

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