Sunniest day of the month on Sunday – May & Spring Review

The month ended on Sunday with 13.06 hours of sunshine although the peak temperature of 24.0C was slightly down on Saturday but still 6.9C above average. The wind edged back from east into north of east and was stronger with a maximum gust of 21mph.

Another dry day with the UV around midday into the ‘Very High’ category again, the eighth successive day.

The new month of June and the start of Meteorological Summer arrived with sunshine again after dawn that lifted the minimum of 8.4C (+1.4C) to 14.7C at 08.00.

May and Spring Review

The month began with April showers producing 5.6mm of rainfall on the 1st and an intense shower of hail late afternoon with another 1.9mm on the 3rd as winds from the west brought average temperatures.

By the 4th a battle began to build with high pressure to the northwest and low pressure to the southwest producing strong north-easterly winds gusting to 24mph and a drier period with 19 consecutive days without measurable rainfall. As the wind changed orientation on the 6th, to come from the south, we enjoyed three very warm days with the thermometer soaring to 23.4C on the 8th.

A dramatic change to this weather pattern crossed the country on the 10th as an intense anticyclone between the UK and Iceland drove Arctic air across the country during the morning with a drop of 12C from the morning high during daylight hours. The north-easterly wind built in strength during the morning gusting to 35mph producing a wind chill that outside it felt 3C below the temperature indicated on a thermometer.

On the 11th the thermometer stubbornly refused to rise above 11.1C, which was 6C below the 36-year average and the coldest day in May. The strong north-easterly wind persisted throughout this period producing a wind chill that meant it felt 2 – 3C below that registered on a thermometer.

This period of May was reminiscent of April, when after a little rain at the beginning of the month we had a long period of consecutive dry days.

During the past two months The World Meteorological Organisation [WMO] has been concerned about the increasing impact of the COVID-19 pandemic on the quantity and quality of weather observations and forecasts as well as atmospheric monitoring. Metrological measurements taken from aircraft plummeted 75-80% compared to normal in the northern hemisphere. The Aircraft Metrological Data Relay observing system normally produces 800,000 high-quality observations per day of air temperature, wind speed and direction along with humidity and turbulence measurements.

In addition there has also been a substantial reduction in the availability of surface-based observations in parts of the world, that were carried out manually, due to illness and absence of staff. Much equipment is now partially or automatically operated but there was the possibility that some might not be fully maintained. There has also been a reduction in sea traffic when scientists take rides on container ships to gather data.

Some countries launched extra radiosondes [weather balloons that transmit metrological data] to partly mitigate the loss of aircraft data, especially in Europe. The WMO stated that “The overall impact of the missing observations probably will not be fully assessed and understood until well after the virus outbreak is over”.

On the 16th the wind began to change its orientation from the persistent north easterlies to northwest and then west for the following three days as the anticyclone slipped south down the eastern Atlantic. This brought milder air and as a result temperatures began to rise after being below average for five days. By the 20th the wind had backed further into the south bringing a maximum of 25.9C resulting from the Continental air mass.

The first named tropical storm in the Atlantic developed east of Florida on the 17th May called Arthur. The storm period officially starts on 1st June, however, it is not unusual for a storm to develop before this date but what is unusual is that storms have developed before this date for the past six years. Global average surface temperatures are running at near record levels so it is likely that this has again influenced the recent earlier starts to the storm season that injects more energy into the atmosphere.

A storm is named when maximum sustained wind speeds are above 40mph. By the 19th the sustained wind speeds had reached 60mph before the storm fizzled out as it moved northwards and eastwards into cooler water. Warmer than average sea surface temperatures across large parts of the globe in May and June are forecast to result in above-normal land temperatures, particularly at tropical latitudes and much of the northern hemisphere.

From the 24th an anticyclone developed in the eastern Atlantic, slowly migrated south around the UK to settle over Scandinavia. This brought eight consecutive dry days, numerous hours of sunshine and the wind changing from northwest to east at the end of the month. On some days there was wall-to-wall sunshine as on the 31st with 13.06 hours of strong sunshine.

The month was quite exceptional for the lack of rain and the 28 dry days. There were just three days with measurable rainfall, the wettest produced 5.6mm on the 1st. The rainfall total of 8.2mm was not a record as in 1990 just 6.7mm mm was recorded. The numerous days with sunshine, drying winds and above average temperatures meant that during the course of many days evaporation from ground sources and plant life amounted to 4 to 5mm of equivalent rainfall daily lost to the atmosphere that in total amounted to the equivalent loss of 125.4mm of rainfall for the month.

There were 318.8 hours of strong sunshine during May so it was not surprising to find that the mean temperature was 1.2C above the 36-year average. The cool nights meant that the minimum was 0.7C below average but the hot, sunny days produced an average maximum that was 3C above the long-term average. The maximum temperature logged every day bar one in the period from the 17th to 31st was above average with the hottest day occurring on the 20th with 25.9C.

Climate change is frequently in the news and my data going back to 1984 shows that this is a fact. The average mean temperature for the three months of Meteorological Spring show an almost continuous rise in the graph, following the cold years of 1984-1987, with the odd blip such as the very cold Springs in 1996 and 2013. The Spring of 2020 was 1.1C above the 36-year average with the mean maximum the highest I have recorded.

There have been 64 dry days during this period, when the average is 48, with rainfall total amounting to 149.7mm, which is 29mm below the average. As a comparison, the driest Spring was in 2011 that produced just 65mm but the contrast was the very wet season in 2000 with 279mm compared to the 37-year average of 179mm.