Fifth below average rainfall month this year

A minimal 1.6mm of rain fell on Thursday as showers were frequent but not this area. The rainfall total for June stands at 27.2mm being just 50% of the 38-year average of 27mm below.

The thermometer rose to 18.9C being 1.3C below average.

The minimum overnight temperature was 10.4C, just above average.

June 2022 Review

The month began with a cool north-easterly for three days, temperatures were depressed by day. Modest rain showers fell on three days, the wettest brought 8.7mm on the 5th, the highest daily rainfall in three weeks.

E-tropical Storm Alexa brought a glancing blow to our area, passing well to the north, but produced windy days from the 8th to the 10th with a peak gust of 24mph on the 8th.

By the 11th an Azores high began to influence our weather with many hours of sunshine and temperatures by day above average.

The media made much of a possible heatwave starting on the 14th. In fact the thermometer did rise very high but not quite sufficient to be classed as a heatwave for this area. At the end of this review is an article from the Meteorological Office of June 2022 on heatwaves and some research I have done using the Marlborough data from recent years showing very interesting results.

The plume of hot air arrived on the 15th and built over the next three days peaking at 31.3C on the 17th making it the hottest day since June 2020 but just below the record of 32.2C on 21st June 2017.

It was all change on the 18th as a cold front arrived and an associated change in wind direction from south to north. The change brought much colder air that meant the thermometer hovered around 13C most of that day, almost 20C below the previous day’s peak.

As the colder air bumped against the hot, moist air from the south heavy rain showers built with 6.8mm during the late evening of the 17th. Much more heavy and prolonged rain fell to the northwest of this area so my garden didn’t receive as much as I would have liked.

Another hot spell arrived on the 21st with a peak of 26.9C on the 22nd as winds from the northeast were light with high solar energy and the UV level peaking at Very High around midday.

The last week saw minimal rainfall as a blocking high to the east of the UK kept the majority of the rain bands to the west of Marlborough or weakened any showers that got through. On numerous previous occasions I have watched the 5-minute rainfall radar display and seen as the rain showers approached the cloud thinned or often diverted to the west or east of this area. There were three days when heavy downpours were reported in the south of the country but we received just 1.0mm, 1.3mm and 1.6mm on the 27th, 28th and 30th respectively.

The rainfall for June totalled 27.2mm, just 50% of the 38-year average or minus 27mm. The trend of drier months this year has continued since January as only February was above average. The wettest June was in 1998 when 143.2mm was recorded and the driest occurred in 2018 with just 5.3mm. The wettest day occurred on the 5th with 8.7mm.

Not only was June relatively dry but also the loss of equivalent rainfall through evaporation from ground sources and plant life was 95.3mm. Therefore there was a total loss of 68.1mm of equivalent rainfall.

Another record has been established for this station. The total rainfall for the six-month period from January to June was 246.9mm. This made it the driest since my records began in 1984 being 140.7mm below the average.

The mean temperature for June was 0.3C above the 38-year average. In fact the trend for warmer Junes can be seen in a rising trend since 1996.

What is a heatwave?
A heatwave is an extended period of hot weather relative to the expected conditions of the area at that time of year, which may be accompanied by high humidity.

What is the definition for a UK heatwave?
A UK heatwave threshold is met when a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold. The threshold varies by UK county in the UK.

The Met Office heatwave threshold has been updated ahead of summer 2022. The initial heatwave thresholds were calculated based on the 1981-2010 climatology of daily maximum temperature at the mid-point of the meteorological summer (15 July). The revised thresholds will use the 1991-2020 average period introduced in January 2022. The geographical differences reflect the differences in climate across the UK.

Why do heatwaves happen?
Heatwaves are most common in summer when high pressure develops across an area. High pressure systems are slow moving and can persist over an area for a prolonged period of time, such as days or weeks. They can occur in the UK due to the location of the jet stream, which is usually to the north of the UK in the summer. This can allow high pressure to develop over the UK resulting in persistent dry and settled weather.

Are heatwaves linked to climate change?
Summer 2018 was the equal-second warmest summer in a UK series from 1884 for mean daily maximum temperature (shared with 1995) with summer 1976 hottest.
Heatwaves are extreme weather events, but research shows that climate change is making these events more likely. A scientific study by the Met Office into the Summer 2018 heatwave in the UK showed that the likelihood of the UK experiencing a summer as hot or hotter than 2018 is a little over 1 in 10. It is 30 times more likely to occur now than before the industrial revolution because of the higher concentration of carbon dioxide (a greenhouse gas) in the atmosphere. As greenhouse gas concentrations increase heatwaves of similar intensity are projected to become even more frequent, perhaps occurring as regularly as every other year by the 2050s. The Earth’s surface temperature has risen by 1 °C since the pre-industrial period (1850-1900) and UK temperatures have risen by a similar amount.

Heatwave impacts
The UK experiences occasional heatwaves but of a lesser frequency and intensity of those seen elsewhere globally. The summer of 2018 was the equal-warmest summer for mean temperatures in the UK along with 2006, 2003 and 1976. The hottest day of the summer was on 27 July, with 35.6 °C recorded at Felsham, Suffolk.

The weather and climate body said the three-day temperature threshold for a heatwave would increase from 27C to 28C for six counties: Surrey, Berkshire, Buckinghamshire, Bedfordshire, Hertfordshire, Cambridgeshire.

“As greenhouse gas concentrations increase, heatwaves of similar intensity are projected to become even more frequent, perhaps occurring as regularly as every other year.”

I analysed my data for the past twelve years going back to 2010 to see if any patterns emerged. The heatwave threshold for Marlborough was lifted to 27C in the 1981-2010 and the more recent 1991-2020 data set. The two graphs illustrate quiet clearly that changes are evident. Do remember that a heatwave happens when a location records at least three days in a row with daily maximum temperatures meeting or exceeding a certain threshold.
The data illustrates that the number of days during which a heatwave occurs show a rising trend. The limited data period indicates that the number of days has risen from three to five or six. In addition the average temperature in a heat wave also shows a rising trend with +1C over the last 20 years.