Category: Commentary

Although we had wall to wall sunshine on Saturday the peak of the heatwave was on Friday. There was very high thin cloud for most of the day that meant both the UV light and solar activity were slightly down on the previous day, resulting in a maximum of 31.0C at 16.27, down almost 2C on the heatwave high of Friday, but still 8.3C above my long-term average. The wind came from the east, a slightly cooler direction also brisker than recently with a maximum gust of 18mph. However, yet again we had the opposite of wind chill in the winter with the heat index calculating that at 14.00, although the thermometer registered 30.6C outside it felt more like 33.9C, taking into account the four factors of air temperature, humidity, wind strength and direction plus solar activity.

The past night was the coolest for four days with a minimum of 10.8C at 05.25, 20 minutes after sunrise in Marlborough, being 1C below average.

The sun was in evidence after sunrise that, following a cooler start, lifted the temperature to 20.4C by 08.00. The ground has been absorbing the extreme heat over the past few days with the soil temperature at a depth of 5cm reading 25.2C at 08.00.

The high pressure is still the dominant weather feature, although it is slowly losing its grip it will give us a fine day on Sunday. The barometric pressure has fallen another 4mb since yesterday with a reading of 1014.0mb at 08.00. The depression to the northwest has associated weather fronts that are edging closer to the UK so that temperatures for the next three days will slowly fall away before recovering from Wednesday onwards, but not so extreme as the past heatwave days. By this evening the wind veer from the east to southwest, a direction not seen since June, as the weather systems relocate, heralding the arrival of an Atlantic influence.

Still Heatwave conditions but less extreme temperatures and slower to rise. Changes in weather pattern already underway.

Update:
27C reached at 13.29
28C reached at 14.14
29C reached at 15.33
Peak of 29.5C reached at 17.30

Hedge Brown butterflies on Ragwort in my wild flower garden.

The heat was relentless on Friday that saw the thermometer rise through the heatwave threshold of 27C at 10.05 then shortly after midday reach 30C. Light, fluffy clouds began to form after that time, which were fair weather clouds, that slowly began to limit the rise in temperature, however, the heat continued to build reaching a maximum of 32.9C at 16.18 being a significant 10.2C above average. With so much heat absorbed by the ground, even under clear skies, the past night was very mild with a minimum of 13.0C at 05.33, just after sunrise at 05.04 in Marlborough. The sun was initially weaker through thin high cloud, however, by 06.00 it was in full strength that had lifted the temperature to 22.6C by 08.00.

The new day on Saturday was a repeat of the previous two days with continuous sunshine. Due to a slight relocation of the high pressure, any breeze will come from an easterly quadrant today. We have now had three consecutive days when the heatwave threshold of 27C has been equalled or exceeded thus it has officially been classed as a heatwave, the third this year.

Today, as on Friday, fluffy white clouds are likely to appear, referred to as fair weather clouds or cumulus clouds. These clouds are due to the sun heating the ground and causing warm air to rise, which cools as it gains in height and any moisture condenses into clouds.

The anticyclone has been edging away, ridging over Scandinavia, that has meant the low pressure to the northwest has begun to have a slight influence over our weather, resulting in a very modest fall in the barometric pressure, being down 6mb over two days. This slight readjustment in our weather will see the peak temperatures begin to fall away a few degrees tomorrow and then lower at the beginning of next week. At the moment the anticyclone is fending off any weather fronts associated with the depression. From Monday onwards we will come under an Atlantic influence with the wind coming from the west.

Update:
27C reached at 09.21
28C reached at 09.62
29C reached at 11.11
30C reached at 13.17
31C reached at 16.27

Heating up quicker than Friday initially then slower rise from mid-morning with 30C reached an hour later.

The image is of a Small Tortoiseshell butterfly.

The heatwave threshold for Wiltshire is 27C, which was reached on Thursday at 11.54, before the relentless sunshine continued to raise the temperature reaching a maximum of 31.2C late in the afternoon at 17.17, being a significant 8.5C above my long-term average. It was the hottest day since 14th August 2022 when 33.2C was recorded. The heat was very slow to ebb away into the very warm atmosphere overnight with the minimum of 13.4C logged at 05.03 at the time the sun rose over Marlborough. The humidity level, as expected thanks to the long, dry, hot period, was very low registering a minimum of 35.3% at 18.22.

The start to Friday was a repeat of Thursday with strong sunshine after sunrise that saw the thermometer reach 22.0C by 08.00 under a cloudless sky.

Not surprisingly, the rate of evaporation into the atmosphere from ground sources and plant life is considerable each day. On Thursday we lost the equivalent rainfall of 5.7mm with the total loss for July todate of 43.5mm when we have received just 6.2mm of actual rainfall.

The centre of the high pressure has relocated a little further east. As a result the very light breeze will vary in direction between south-southwest and southeast on Friday, a warmer direction than the recent northwesterly, that will mean the peak temperature is likely to exceed that of Thursday.

By Sunday there are signs that the anticyclone will have relocated further east and dropped in pressure over the UK allowing a low pressure system to the northwest to edge a little closer, reducing maxima with possibly some cloud cover.

Over the next few days I will include a variety of butterfly images that have been abundant this week. Today is a Peacock butterfly.

Update:
27C reached at 10.05
28C reached at 10.29
29C reached at 11.01
30C reached at 12.08
31C reached at 13.28
32C reached at 14.28
32.9C maximum reached at 16.18

The heat continued to build on Wednesday, peaking at 27.8C late in the afternoon at 17.08, being a significant 5.1C above my long-term average. This is the first day that the temperature has equalled or exceeded the Heatwave threshold temperature, which is likely to be confirmed on Saturday after three consecutive days exceeding the threshold, although the next five days will also be above 27C. The past night has been very mild with the thermometer falling to 13.7C at 05.23, just after sunrise in Marlborough at 05.02, being 1.8C above average.

The sun shone strongly just after sunrise that had lifted the temperature to 21.8C by 08.00 making it the warmest start to a day at that time since 25th June 2023. The anticyclone continues to build with a pressure reading of 1024.2mb at 08.00.

The centre of the high pressure is easing eastwards, currently over the southern Celtic Sea, that will see the breeze continue from the northwest today and be very light. However, as it continues that migration, in the early hours of Friday the direction of the breeze will change significantly to come from the southeast on Friday allowing the heat to continue to build.

There is a Buck Moon tonight at 21.37, when the moon will sit unusually low in the sky. Historically, July’s full moon is called the Buck Moon because it aligns with the time of year when the antlers of male deer, or bucks, are quickly growing, named by Indigenous North American Algonquin people.

Tuesday was the third consecutive day when maxima were steadily rising with a peak of 24.8C late in the afternoon at 17.42 being 2.1C above my long-term average. There was a rather surprising low temperature overnight with a minimum of 8.5C at 05.02 early Wednesday, just as the sun rose over Marlborough at 05.01, which was 3.4C below average.

Wednesday began with glorious sunshine that was quite comfortable first thing until the sun got to work just after 07.30 which lifted the thermometer to 17.5C by 08.00. However, minimal, high cloud was noted drifting down on the light northwesterly breeze that coil limit sunshine this afternoon.

We might just reach heatwave conditions today as the maximum temperature indicated by both major weather forecasters is for a maximum of 27C. The high pressure continues to build that has seen an increase of 11mb since Monday with a reading of 1022.5mb at 08.00.

We are still under the Azores High that is not quite centred over the UK hence the airstream running clockwise around its western flank, then turning east and south over the UK, will continue the northwesterly breeze for today and one more day on Thursday. By Friday the centre of the anticyclone is forecast to build as it shifts further east over the UK that will see the wind change to come from a southeasterly quadrant for the following few days when the heat will really build.