Last day of Meteorological Spring brought more warmth

Although the fog, then mist, took a while to clear on Monday, by mid-morning the sun began to shine strongly pushing the thermometer to a peak of 23.2C late in the afternoon at 16.17, which was 5.9C above the average. It was another dry day, only the eighth in May, with the UV again rising to ‘Very High’ for an hour between 12.30 and 13.30, midday in BST.

The thermometer dropped just below the early June average with a minimum of 9.4C overnight.

Tuesday arrived with no fog or mist to clear so the sun shone strongly after dawn lifting the thermometer to 16.1C at 08.00.

The barometric pressure has been falling for the past two days signalling a modest change in our weather with the wind currently coming from the southeast.

May 2021 Review

A month of contrasts

The pool of cool air continued to affect the temperatures at the beginning of the month with maxima still significantly below average and air frosts occurring.

A deep Atlantic low pressure arrived and crossed the country on the 3rd with very strong winds gusting to 41mph. This was the strongest gust of wind I had recorded in May over 37 years; the previous record was 38mph in 2015.

By the 5th the wind had veered from the west with a more northerly component. During the afternoon as the temperature eased upwards, shower activity increased. At 14.30 a heavy hail shower arrived with another at 14.35 and a third at 14.55. During the first shower a thunderclap was heard at 14.34. During this brief half hour the temperature dropped from 11.5C to 5.9C.

The 7th brought us the wettest day since 27th January with a daily total of 16.5mm. A depression in the Atlantic began to influence our weather from that date and continued for two weeks. This was due to a blocking high-pressure system over Scandinavia blocking its eastwards movement. As a result we had several days of sunshine and showers, some quite heavy.

In fact the unstable weather continued until the 26th as a succession of depressions arrived from the Atlantic producing frequent showers, some heavy.

The Jet Stream for some time had been positioned south of the UK so the cooler air to the north prevailed for several weeks.

During the 20th & 21st an intense depression arrived from the Atlantic that brought not only more rain but strong winds with a maximum gust of 42mph on the 21st.

In the western Atlantic the first Atlantic Sub-Tropical Storm of the season developed and was named Ana.

At last, on the 25th the weather decided it was late spring not late winter when an anticyclone developed to the east and brought warm and dry weather after 20 consecutive wet days. The first day in May that saw the maximum temperature rise above average occurred on the 27th with a peak of 20.8C. That was followed by the warmest day of the month on the 31st when the thermometer reached 23.2C.

This dry and warm weather persisted until the end of the May that allowed the ground to dry out and plant life to develop. This much needed warmth resulted in the soil temperature at a depth of 5cm begin to rise into double figures with a reading of 13.9C at 08.00 on the 29th being as low as 4.8C on the 6th and 9C on the 23rd. Gardeners will know that plant life struggled to get moving until late May as the soil temperature at a depth of 5cm, measured at 08.00 daily, recorded a mean temperature 2.2C below the long-term average.

We all know that May felt cold with the statistics showing that the mean temperature of 10.5C was 1.5C below the 37-year average being the sixth coldest since this station started in 1984. The truly exceptional months occurred in 2017 with a mean of 13.7C and the very cold May of 1996 when a mean of 9.16 was recorded.

This exceptional month produced only 8 dry days after 20 consecutive wet days with total rainfall of 101.2mm. The total was 41.6mm above the 37-year average and the wettest May since 2014. The contrasting years for rainfall occurred in 1990 with 6.7mm and in 2007 with 149.5mm.

After such a wet month it is perhaps appropriate to consider the history associated with the collection of rainfall data.

The first known rainfall records were kept by the ancient Greeks in 500BC, also in India at a similar time period. Many different types of rain gauge have been designed and used over the years but comparison of statistics can only be made when using an identical gauge and site. History records that King Sejong the Great in Korea invented the first standardized rain gauge in the fourth month of 1441. This was two hundred years before Christopher Wren invented the tipping bucket gauge in 1662 in collaboration with Robert Hooke.

My rainfall data is taken daily using a Meteorological Office standard copper gauge, which has a sharp brass rim to split any rain drops falling on it, with a diameter of 12.7centimetres, previously known as the 5 inch gauge. This gauge is set into a grassed area, solid surfaces close by could cause splashing into the gauge when intense rainfall occurs, it is sited 30 centimetres above ground level. The entrance to the gauge through the funnel tends to be narrow to avoid debris clogging the gauge and undesirable evaporation in hot weather with the precipitation falling into an enclosed, removable container. The international time for taking the daily rainfall total is at 0900 when it is poured into a glass gauge that has a calibrated scale inscribed on its side accurate to 0.1mm, which is in exact relation to the gauge diameter.

Many unmanned weather stations use a tipping-bucket gauge. These are engineered so that each time it collects a set quantity of rain the bucket empties automatically and sends an electronic pulse to the monitoring station that is usually set to record each time a rainfall increment of 0.2mm has been detected. When one bucket empties the opposite bucket is raised to continue recording. I find this gauge, which is set at 45 centimetres above the surrounding ground level so useful in recording not only the actual time of rainfall but also its intensity.