Author: Eric Gilbert

Thursday 13th August

Amber Warning of Extreme Heat issued at 10.01

N.B. The peak of 33.9C on Wednesday was exceed at 11.38 today, Thursday. The previous record of 34.1C set in 2020 was exceeded at 12.01.

The intense heat just built and built on Wednesday that resulted in a peak of 33.9C at 16.39 being a significant 12.3C above my long-term August average and 0.2C below my record for August set in 2020. The heat was slow to dissipate overnight that eventually produced a minimum of 16.9C at 05.37 being 5.1C above average, just before sunrise in Marlborough at 05.49. It was the hottest day since 9th July and the warmest might since 24th June.

As the partial eclipse reached its peak on Wednesday evening I observed that the pigeons, who frequent our roofs and who had fallen silent a little earlier, suddenly moved off together just after 19.00 and just before the peak of the partial eclipse. Previous to the start of the partial eclipse my records show that the solar radiation was falling steadily at a rate of about 30W/m2 every fifteen minutes. As soon as the eclipse started the solar radiation fell significantly at the rate of around 70W/m2 until about 19.00 when it began to tail off reaching a low at 19.30 before increasing again as the sun began to emerge from behind the earth’s shadow.

Thursday arrived with yet more strong sunshine that had lifted the temperature to 21.5C by 08.00, making it the warmest start to a day at that time since 11th July.

The soil temperature at a depth of 5cm, read daily at 08.00, already very warm after so many hot days, has continued to absorb more warmth during this heatwave with 20.9C, 21.6C and 24.2C recorded respectively over the past three days. I checked last night at 19.30, just as the extreme heat of the day was beginning to reduce, when the thermometer read 24.2C.

Often in the news it is commented when the temperature reaches 30C, well above the heatwave threshold for Wiltshire of 27C. Spending time indoors during the current heatwave I analysed my data to cover three five year periods for occasions when I recorded 30C or above. During the period of 2011-2015 there were just 5 days, from 2016-2020 there were 19 days and during the last five year period from 2021-2025 a significant increase to 28 occasions. This year there have already been 19 days when 30C or above has been recorded.

There were 3 days in September 2023 when 30C or above was recorded, a first for my station for September. Will that occur this year? A very clear picture of how the climate is heating up.

On Wednesday 30C was reached at 11.33, today, Thursday, 30C was reached at 10.14.

There are early signs that the heatwave will begin to ease after Friday and drop below the threshold by Saturday, when a cold front is forecast to slowly cross the country indicating a significant change into next week. Behind the cold front the Azores high will attempt to push a ridge back across the country although still dry on Saturday and Sunday. The wind will then come from a northwesterly direction, a cooler direction, that will result in lower temperatures by day. There has been a divergence of opinion over the last two days for the forecast from Monday, between the BBC and Met Office. As a gardener I hope the BBC is correct as they suggest we might receive light showers on Monday, although they only rate the possibility at 66% and even lower at 33% on Wednesday.

Wednesday 12th August
The fifth heatwave of 2026 did officially arrive on Tuesday, as we are certain to have three consecutive days with maxima above or equal to 27C, that saw the thermometer reach 28.3C at 16.06, being 6.7C above average. The modest breeze from the southeast did help make it a little more comfortable. The past night was mild, in fact the warmest since the 5th, with a minimum of 14.4C logged at 06.09 being 2.6C above average. Sunrise in Marlborough was at 05.47.

Today will see the heat build significantly as the light breeze from the southeast wafts hot air from the Continent. By 08.00 the thermometer had reached 18.4C with the likelihood the maximum will exceed 30C by the end of the day. The centre of the large area of high pressure is now over Germany and Scandinavia that will carry the hot, dry air around its western periphery towards the UK. This will be repeated on Thursday that after a very warm night tonight will see the thermometer climb even higher.

By Friday there will be a significant change as the current high pressure eases further to the east and the Azores high begins to build to the southwest that will push across to the UK. This will be the dominant feature for a day or two as the breeze will then veer into the northwest, which will bring a cooler air stream off the Atlantic with maxima falling considerably to just a few degrees above the August average.

I remember the summer of 1976 very well, unfortunately my weather station did not start up until 1984 so I do not have the statistics for that significant summer.

Why 1976 remains significant

The summer of 1976 remains one of the most well-remembered in UK weather history.
Heatwave spells were recorded in May, June, July and August, including a 23-day spell from 21 June to 13 July and a further 22-day spell from 6 to 27 August.

Its place in public memory is partly due to the persistence of the heat and dry weather. The records show that 1976 was not just a single hot spell, but a summer with repeated heatwave conditions over several months. That helps explain why it remains a benchmark against which many later summers are compared.

The 2022 milestone
While 1976 is remembered for duration, 2022 is remembered for intensity. On 19 July 2022, the UK recorded 40.3°C at Coningsby, Lincolnshire, the first time the UK had recorded a maximum temperature above 40°C. The Met Office described the event as a milestone in UK climate history, with the previous UK record exceeded by 1.6°C.

Tuesday 11th August
It was such relief to find that Monday was a pleasant day without the recent intense heat, all due to the cooler air that flowed behind the cold weather front, which traversed the UK during the day. Eventually the thermometer peaked at 25.4C, logged at 15.31, being 3.8C above my long-term average. Unusually the temperature, after falling to a low of 12.3C at midnight began to edge upwards again reaching 15.2C for over an hour between 02.00 and 3.00. The back track of the cloud radar indicated that small banks of cloud temporarily drifted down from the north, before receding. It then began to fall again to plateau at 13.1C between 05.48 to 06.23 before beginning to rise again, in response to the sunrise at 05.46, to reach 17.3C by 08.00. The low was 0.5C above the average.

Sunshine greeted us again at the start to a new day on Tuesday. There has been a significant change in wind direction today, due to the high pressure easing eastwards towards Denmark today and eastern Europe extending over the next few days. This subtle shift in position will result in the breeze today and up to Thursday, coming in from an east-southeast or southeast direction. The airstream, having travelled over a very hot Continent, will waft very hot air across the UK heralding the start of a short lived, thankfully, fifth heatwave that could see maxima reach 34C on Wednesday and Thursday.

By Friday the forecast pressure charts indicate that an Azores high will begin to push across the country from the southwest. However, although it will be a significant anticyclone, the air will be rotating around it clockwise, as they do around high pressure systems, which will mean the wind will have a westerly or northwesterly component having travelled around its western and northern flank, across the mid-Atlantic, thus be much cooler.

The heat over recent months has been exacerbated due to the very dry ground. If we had received any precipitation in recent weeks, the rise in temperature for a day or two would have been muted slightly as evaporation of the moisture in th ground would have taken with it some of the stored warmth.

The longest UK heatwaves

Looking across the Met Office’s UK heatwave dataset from 1900 to 2026, the longest recorded UK heatwave spell listed (where at least one station in the UK met or exceeded its heatwave threshold) is from 18 July to 25 August 1995, lasting 39 days. The second longest is the recent 2026 spell from 30 June to 5 August, lasting 37 days, followed by 26-day spells in 1947 and 1990.

Other notable long-running events include 1916, with a 24-day heatwave, and several 23-day spells, including 1976, 1983, 2013 and 2018. These figures show that prolonged heat is not new to the UK, but recent years continue to feature prominently in the records.

Monday 10th August
After a cool start to Sunday with variable cloud at first, the heat built steadily to exceed the heatwave threshold with a maximum of 30.6C at 16.02, being a significant 9C above my long-term August average. The past night was mild, compared to the previous chilly nights, with a minimum of 14.2C at 05.59, just after sunrise in Marlborough at 05.44. This low was 2.4C above average.

Monday began with thin high cloud that initially limited the rise in temperature, reaching 18.6C by 08.00. The high pressure centre is currently over the north of the UK but will begin to make a subtle move in position over the next day or two that will result in a significant increase in the temperature from Tuesday onwards.

Today a cold weather front is passing over the country containing a little, variable cloud. The air behind it is cooler, combined with less strong sunshine this morning, will result in the peak temperature today lower than recent days, not sure if it will reach the heatwave threshold. The air stream will veer from north-northwest to north as the day progresses, and north-northeast later this the evening as the high pressure eases towards the North Sea.

The forecast pressure charts indicate that the anticyclone will continue to very slowly migrate eastwards that will result in the wind veering into the southeast or east-southeast for the next few days. The air coming from this direction will carry heat from the Continent with the result temperatures are going to push well above the heatwave threshold, likely into the low 30s by Wednesday.

The original heatwave thresholds were based on the reference climate period of 1981-2010 and were intended to be flexible and revised in the light of climate change, therefore the heatwave thresholds were revised in 2022, using the most recent 1991-2020 climate averaging period, with the local threshold temperature rising from 26C to 27C. The review saw six counties move from 27C to 28C, one from 26C to 27C and one from 25C to 26C, the areas changed were in the south of the UK and the Midlands.

Sunday 9th August
The heat continued to build on Saturday, eventually peaking at 27.2C, which was just above the heatwave threshold. The maximum was logged early in the afternoon at 14.14 as just before 14.00 cloud began to drift across then thicken as the afternoon progressed thus limiting any further rise in temperature. The past night was not quite so chilly as the previous two nights, dropping to a minimum of 8.6C at 06.08, which was 3.2C below average. The sun’s strength was very limited after sunrise, due to considerable cloud cover, hence the much later time when the thermometer reached the minimum.

Due to the cloud cover the thermometer had only reached a temperature of 14.0C by 08.00 on Sunday, making it the second coolest start to a new day at that time, Friday read 13.7C. The back track of the cloud radar showed bands of cloud arriving from the east just before 06.00. The cloud will be variable with sunny periods this morning, that are likely to intensify and lengthen after around 13.00, which will likely push the temperature very high again late afternoon.

The depression between Iceland and Norway will attempt to push a cold front down across the country overnight, sadly no rainfall within it, that will slowly peter out and leave us with perhaps a cloudy start to Monday. The air behind the weather front, as the name indicates, will mean a less hot day on Monday. By Tuesday the high-pressure will be a little further to the east that will once again push hot Continental air across the country, which will mean temperatures will be well above the heatwave threshold, likely 30+C.

I regularly monitor the Thames Water website for date on the Groundwater situation and on local rivers. All the local Groundwater boreholes say the State of all eight are ‘Normal’ with the close by Draycot borehole, near Chiseldon, stating the Trend is ‘Falling’, all the other seven show the trend is ‘Steady’. However, I was concerned yesterday when looking at the river data that the depth gauge for the River Og at Poulton Farm registered ‘Height’ 0m, ‘Trend’ ‘Steady’ and ‘State’ ‘Low’ and that it had registered the height of 0m at 03.30 on August 4th. I wandered down to the river by Poulton Bridge fearing the site of a dry river bed, but thankfully found that in fact it had not dried up, however the average depth must have been around 2/3cm and it was flowing so very slowly.