The month started well with a ridge of high pressure bringing three dry days and welcome sunshine.
However, the barometric pressure then started to fall as an unsettled period began to assert itself with modest falls of rain with 6.9mm later on the 4th. On the 5th a rash of showers covered the south of England with brief intervals of sunshine.
An unwelcome depression arrived in the early hours of the 6th and progressed up the English Channel bringing persistent rain and cool temperatures for the Coronation Day. However, the thick cloud meant it was the warmest start to a day since November 11th. The daily rainfall total for the 8th, of 16.4mm, was the wettest day for a month.
Thankfully a drier period then arrived with four continuous dry days from the 10th to the 13th but the wind predominantly from the northeast was cool, depressing temperatures. This period did allow some gardening to be attempted, particularly as the soil temperature at a depth of 5cm averaged around 12C for several days.
Sunday the 14th gave us many hours of warm sunshine that lifted the temperature to 20.7C that made it the warmest day since 29th October.
Mid-month saw an anticyclone centre itself just to the west of the UK that brought more settled weather with numerous hours of welcome sunshine. However, the breeze from the north was cool and the nights were chilly with a minimum of 2.3C in the early hours of the 17th, which was 4.5C below the average and the coldest night since 25th April.
The Jet Stream had for the latter half of May repositioned itself to flow north of the UK rather than south, that had brought the unsettled weather from the Atlantic earlier in the month.
The extensive Azores High developed from the 16th, positioned to the northwest of the UK and centred over the Eastern Atlantic. This produced a run of mainly north-easterly winds as they circulated in a clockwise direction. The airstream had travelled over the North Sea, which is cool at this time of year, and picked up moisture thus many mornings arrived with cloud cover that began to thin and disappear towards midday as the sun got to work.
There were seventeen continuous dry days from the 15th with temperatures by day around the average with the warmest day of the month occurring on the 28th with a maximum of 21.9C. May brought us 25 days without measurable rainfall, but in 2020 we enjoyed a total of 28 dry days. This period also brought high UV levels and on the 31st rose to very high.
The long run of dry days, under the strong breeze and numerous hours of sunshine, produced an increased rate of evaporation from ground sources and plant. For many days during the last two weeks more than 4mm of equivalent rainfall was lost to the atmosphere. The loss for the complete month was 89.1mm.
The rainfall for May amounted to 52.7mm being 7.8mm below the 39-year average.
The daily highs varied between 13.9C on the 6th and 21.9C on the 28th. The overnight minima, that occurred in the early hours, ranged from 2.3C on the 17th to 11.7C on the 9th. The mean temperature of 12.83C was 0.7C above the 39-year average. The trend for the May mean has been continuous since 1996 with a rise of 1.0C.
Thanks to the persistent anticyclone the average barometric pressure has been 7mb above the long-term average with a peak of 1032.4mb on the 26th.
Earlier this month there were headlines about a possible heatwave from one forecaster. A study of the Jetstream, projected up to two weeks ahead, did not support the suggestion of a Spanish Plume arriving. The Meteorological Office define a heatwave as ‘an extended period of hot weather, typically lasting at least three days, with daily maximum temperatures meeting or exceeding the heatwave temperature threshold’. The temperature threshold for Wiltshire is 25C.
We would have liked some real heat in May rather than the persistent, cool north-easterly of the past fortnight. However, the following article might be useful if during the summer a very hot period does arrive.
A new system of colour-coded heat warnings is being launched to warn the public and the NHS about dangerously high summer temperatures.
The Heat Health Alerting (HHA) system will focus on the impact that warm weather could have on the health of the population in England, and where the NHS is likely to face the biggest pressures.
It will monitor weather forecasts and identify where hot spells are likely.
Alerts will be coded green, yellow and red to correspond with the current weather warning system used by the Meteorological Office. The alert system has been devised jointly by the UK Health Security Agency (UKHSA).
A similar system is expected to be launched in November, focusing on the impact of cold weather on public health.
Red, the highest level of warning, will signal a “significant risk to life”, even among people who are otherwise healthy, and will be treated as an emergency.
An amber alert indicates that impacts are likely to be felt across the whole health service. This may require a more co-ordinated response with non-health sectors.
Yellow will cover a range of situations, and alerts maybe issued if the weather is likely to affect particularly vulnerable people. Green will indicate normal temperatures, with no level of warning necessary, because conditions are unlikely to affect health.
St Katherine’s schoolchildren raise £400 to help Brendoncare Froxfield’s garden project
