Author: Eric Gilbert

Wednesday 24th September
Although there was less full sunshine on Wednesday the wind from the northeast being a little less strong allowed the temperature to recover just a little with a maximum of 15.7C, almost a degree up on Tuesday, however, this high was still 3.4C below average. The past night was very different from the two previous nights as variable cloud began drift across around 21.00, that minimised the loss of warmth into the atmosphere, which resulted in a minimum of 7.8C at 02.57 when thicker cloud then arrived stabilising the temperature around 8C. The low was just 0.9C below average, quite a contrast to the ground frost that developed during the previous early morning.

Wednesday revealed a cloudy sky after dawn that at times was so low that it obscured the high ground but by 08.00 this was beginning to thin allowing weak sunshine to break through. Variable cloud will limit sunshine during the day as the breeze is picking up moisture from the North Sea as the high pressure slowly relocates towards Scandinavia, a subtle change in direction.

The anticyclone will continue to dominate our weather until Friday, even as it eases further over Scandinavia leaving a ridge of high pressure over the UK that will block any advance of weather fronts arriving from the Atlantic. Low pressure systems will be present in the eastern Atlantic but make no further movement toward the UK until the weekend. We should still be able to enjoy the dry, fine weather with sunny intervals, even if my garden would like some rain after a week with little rainfall. The air stream will continue from a northeasterly quadrant that will become lighter allowing temperatures by day and night to recover closer to the average for September.

The track of Hurricane Gabrielle is now becoming clear, which will see it head towards the Iberian peninsula by the weekend. At the moment it does not seem if it will have any great impact on the UK weather.

Throughout Westonbirt Arboretum each specimen is labelled, either on the trunk or low-hanging branch. Blue labels indicate Westonbirt’s “champion trees”, the tallest or largest of their kind in Britain.

Tuesday 23rd September
The very cold start to the morning on Monday resulted in the temperature taking much longer to recover and in fact the maximum was below the Sunday peak with a high of just 14.9C, which was 4.1C below my long-term average even though we enjoyed many hours of sunshine. The culprit for the depressed temperatures was the continuing stream of cool air on a brisk northeasterly breeze, gusting to 20mph, originating around Iceland.

The past night was colder again with a minimum of 0.4C logged at 05.26 early Tuesday morning, which was the first ground frost of the autumn season but not quite an air frost . The two factors producing this low, which was a significant 8.3C below average, were that we are still under the pool of cool but also that the anemometer became becalmed at 19.21. Thus there was no air movement to stir up the atmosphere and minimise the loss of warmth into the atmosphere with at the time of writing this report at 08.20, still no movement of air. The centre of the anticyclone has edged further over the UK thus there is reduced pressure gradient that will result in much lighter winds today. The anemometer awoke at 08.35. The barometric pressure has risen a little higher with a reading of 1029.2mb at 08.00, up 2mb since Monday.

Tuesday arrived as Monday with glorious sunshine that was muted at first due to thin high cloud over the eastern horizon, which meant the thermometer had only recovered to 3.9C by 08.00.

The high pressure will dominate our weather for the rest of the week although with variable amounts of cloud on Wednesday. The centre of the high pressure will slowly extend eastwards over Scandinavia leaving a ridge over the UK that will fend off any approaching weather fronts until Saturday at least. However, the air movement will continue from the northeast until Friday. At that time the high pressure will have lost most of its influence that will result in the wind direction veering into the southeast. This significant change in wind direction will mean that the nights will become less cold as the flow of cool air from the north will have been cut off.

Storm Gabrielle spun up into a hurricane on Sunday with maximum sustained winds of at least 75mph. The latest forecast for the track of Gabrielle is for it to change direction into an easterly movement, well south of the UK, heading for a region around the Azores but by that time having weakened as it travels over cooler ocean water.

Westonbirt Arboretum. Silk Wood is a different experience from the Old Arboretum I mentioned yesterday as it contains some exotic plantings, at its centre is a traditional working woodland, dating back to the 13th century.

Monday 22nd September
The effects of the change in wind direction were felt on Sunday as the air stream, brought down from a northeasterly direction, meant a chilly day and cold night. The thermometer reached a peak of 15.0C by 15.13 Sunday afternoon in pleasant sunshine, however, it was the coolest day since 8th May being a significant 4.0C below my long-term September average. Under the combination of clear skies overnight and still air conditions, any residual warmth quickly dissolved into the atmosphere that resulted in the coldest night since 3rd May with a minimum of just 2.8C logged at 04.53, once again a significant difference from the average of 5.9C. A detailed look at the temperature graph shows an almost steady decline overnight, producing a straight line fall after 18.00, with a slight wobble and minor recovery around 20.00.

Monday greeted us with sunshine after sunrise from a clear sky, but this did little to raise the low temperature as the thermometer had only risen to 5.3C by 08.00 making it the coldest start to a day at that time since 6th May.

The soil temperature at a depth of 5cm very clearly shows the arrival of autumn with temperatures of 16.6C, 15.7C, 15.2C, 10.1C and 7.0C respectively over the past five days, read at 08.00.

The anticyclone is now dominating our weather, and will do so up to Thursday at least with fine, mainly sunny weather but depressed temperatures due to the flow of cool air from a northerly quadrant. The barometric pressure has risen another 11mb over the past twenty-four hours with a reading of 1027.6mb at 08.00, with the centre of the high over Southern Ireland. It is the highest barometric pressure the month.

Storm Gabrielle that became a hurricane on Sunday is expected to pass east of Bermuda as its track now indicates it veering into a northeasterly direction back into the mid-Atlantic. It remains to be seen what effect, if any, it has on our weather next week, that depends if it eventually gets caught up in the Gulf Stream heading our way but by that time it will have decreased in strength significantly.

Westonbirt Arboretum. There are two main areas in the arboretum. The Old Arboretum is a carefully designed landscape with several avenues and a number of rare and exotic trees from across the globe dating back to the 1850s.

Sunday 21st September
Saturday was the last day for a week when the southwesterly air flow bringing warm air from the Azores gave us a fine dry and sunny day that produced a maximum of 18.9C. This high was almost exactly average for September at -0.1C, logged at 13.20 before the cloud increased and the wind picked up gusting to 29mph. The change in the weather brought a light shower starting with drizzle at 15.15 and heavier at 15.30 totalling just 0.4mm. During the day large areas of heavy rain passed to the west and east of our area, that which arrived from the south of Marlborough was more broken and petered out on its northwards track.

The change in our weather was first observed Saturday evening when the wind began to slowly change direction. By 21.15 the wind had veered into the west and as the evening progressed into the north-northwest. This change in wind direction, resulting in a cooler air stream, combined with clearing skies, saw the thermometer drop away to reach a minimum of 6.8C at 05.12, which was 1.9C below average.

There was a glorious start to Sunday with the sun shining brightly after sunrise that had lifted the temperature to 8.4C by 08.00, although this was the second coolest start to a day this month. The light breeze has settled in a northerly direction this morning, not recorded for a month. I observed a bank of cloud just to the northwest of Marlborough that over the past two hours has appeared to be static and not getting any closer.

The recent depression has now migrated further over Scandinavia that has allowed the high pressure, just to the west, to influence our weather and for most of the rest of the week. The anticyclone has been building and edging over the UK, the barometric pressure having rose 6mb since Saturday with a reading of 1016.6mb at 08.00. The high pressure will be with us until Thursday at least that will bring dryer and brighter weather although the down side will be maxima and minima temperatures below average due to the air originating somewhere around Iceland before it arrives here on a northerly or northeasterly breeze.

Some of the recent warmth in the ground has begun to dissipate under cooler conditions. The soil temperature at a depth of 5cm read 10.1C at 08.00, which was the lowest since 6th May.

Westonbirt Arboretum forms part of a site which is listed Grade 1 on the Register of Historic Parks and Gardens of Special Historic Interest.

Saturday 20th September
The drift of very warm, moist air from around the Azores region, on a brisk southwesterly breeze on Friday, gave us a very pleasant autumn day that saw the thermometer rise very high for early September reaching a peak of 25.0C at 14.57. This was a signifiant 6.0C above my long-term average and the warmest day since 25th August (27.5C). It was also a dry day that gave splendid sunshine in the afternoon during which time the UV level rose at its peak to the top end of ‘Moderate”.

The past night was also warm with a minimum of 13.1C just after midnight at 00.45, but instead of continuing to fall, as is usual in the early hours, it lifted to 13.8C for a couple of hours then rose to 14.4C by 08.00.

Saturday after dawn revealed thin cloud in the eastern horizon with the sun trying to break through whilst the thicker cloud continued to drift across from the west bringing misty conditions ahead of the next weather front that is likely to produce rain before midday.

The low pressure system currently sitting over the UK will slowly ease away towards Scandinavia on Saturday, which will allow the very large area of high pressure sitting just off the coast of Ireland to edge closer. This will mean the recent drift of warm, moist air from the Azores will be cut off resulting in temperatures dropping significantly on Sunday as the wind veers first into the west overnight then north on Sunday morning. The maximum in the very cool air, that originates around Iceland, will see the maximum perhaps 5C below the long-term average.

Storm Gabrielle is now forecast to strengthen and become a hurricane on Sunday, to the north of the Caribbean and east coast of America, however, it is forecast that swells generated by Gabrielle are likely cause life threatening surf conditions. The forecast track will see it slowly rotate clockwise into a northeasterly direction into the mid-atlantic.

Westonbirt Arboretum was planted in the heyday of Victorian plant hunting in the mid-19th century, as part of the Westonbirt House estate.