20th February 2018
Established 1872. Online since 1996.

Drier and sunnier than average, but Fair Isle managed to be wettest too

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AUGUST was very wet and exceptionally dull across most of the UK, but drier and sunnier than average across Orkney and Shetland. Fair Isle, with a monthly total of 168.3 hours, was the sunniest place in the UK. However, with Fair Isle recording a 24-hour rainfall total of 101.2 mm on the 10th – an exceptional event likely to be equalled only once in 200 years – the Isle also experienced its wettest August on record!

The first two weeks of the month saw depressions tracking east across mainland Britain, with the majority of fronts kept south of the Pentland Firth by high pressure to the north.

During the third week of the month Shetland was one of the few places to see anything approaching a settled spell, as an anticyclone – wandering over Scandinavia and the Nor­wegian Sea – extended its influence to include Shetland.

The last week of the month brought unsettled conditions as pressure built to the south of the British Isles and depressions – taking a more northerly course east through the Iceland region – trailed fronts across northern Scotland.

The month began with a complex area of low pressure over the British Isles. After a mostly dry day on the 1st, outbreaks of rain – accompanied by mist, hill fog and a strengthening easterly wind – reached most areas by evening as a cold front moved north over Shetland. As rain cleared overnight and winds fell light, the 2nd was mainly dry, though with lingering low cloud. A small low centre, tracking north-east to the south of Fair Isle, brought some early rain and hill fog on the 3rd, with a drier and brighter conclusion as light and variable winds settled northerly in direction. The 4th and 5th were cloudier with some patchy drizzle, as the main area of low pressure transferred across the North Sea into Scandinavia and north-westerly winds freshened for a time.

The 6th and 7th remained dry and sunny, though northerly winds again freshened across Shetland as another complex area of low pressure developed over the British Isles. Winds eased and backed north-westerly on the 8th – another dry, sunny day – as once more low pressure moved away into southern Scandinavia and pressure built to the west. During these three days Lerwick saw a total of over 30 hours sunshine.

A ridge declined east early on the 9th, with freshening south-east to easterly winds bringing rain east late in the day, as a deepening Atlantic low moved towards Faroe. The 10th was a dull, misty day with heavy rain, extensive low cloud and light cyclonic winds, as a small centre – developing on the trailing cold front – moved north close to Fair Isle. During its passage this brought some 24 hours of “monsoon-like” rain to the Isle. Rain cleared overnight and the 11th was a drier, brighter day with moderate westerly winds, as a complex trough of low pressure over the Northern Isles slowly filled.

Pressure then built over the Northern Isles, bringing a dry, bright day on the 12th with light east or north-easterly winds over Shetland, as yet another area of low pressure drifted east across the UK. The 13th was mostly dry and bright with fresh or strong north to north-westerly winds until later in the day, when a front – moving south across Shetland – brought outbreaks of rain or drizzle. The 14th was mostly cloudy with patchy light drizzle and moderate north-westerly winds, as a small anticyclone built across the Northern Isles and southern Norwegian Sea.

From the 14th through to the 18th Shetland experienced mainly dry conditions, as an anticyclone meandered backwards and forwards between Scandinavia and the Norwegian Sea. While quite warm by day with sunny periods, under clear skies and with mainly light south-east to easterly winds nights tended to be cool. This effectively prevented fronts, associated with a low moving north-east across the UK, from getting further north than the Scottish mainland.

A developing northerly airflow kept fronts to the south of Shetland on the 19th, as the filling low continued north-east over the North Sea. Though low pressure moved into Scandinavia on the 20th as high pressure to the north declined, fronts were kept clear of Shetland as pressure began to build strongly over the north-east Atlantic. A ridge of high pressure persisted over Shetland on the 21st and 22nd, maintaining fine, settled conditions. This declined east on the 23rd, with fronts finally approaching Shetland from the Atlantic later in the day, bringing an end to the dry spell as outbreaks of rain accompanied by strengthening southerly winds spread east overnight.

On the 24th a deep depression moved east to the south of Iceland and strong south-easterly winds – gusting to 40kt on Fair Isle – developed ahead of fronts approaching Shetland. These brought rain – heavy at times – early on the 25th. Brighter conditions followed as winds eased and veered south-westerly. With the slow-moving depression gradually filling, the 26th was mostly dry and bright, though showers followed on the 27th as moderate south-westerly winds veered fresh westerly.

A small low centre moved east close to Shetland on the 28th, bringing some patchy rain or drizzle and misty conditions, with fresh westerly winds veering northerly later. More rain followed on the 29th as a warm front moved east. Increasing pressure held the following cold front to the west. This left Shetland in a broad warm sector for the last two days of the month, with south to south-easterly winds bringing a mixture of misty conditions with outbreaks of rain and drier, brighter interludes. The cold front eventually cleared east, bringing heavy overnight rain as the month ended.

Initial Statistical details for Scotland as a whole (1961-1990 average) for August: Mean Temperature – 13.5°C (1.0°C above average).

Rainfall – 167.6 mm (144 per cent of average).

Sunshine – 96.9 hours (72 per cent of average). Dullest August since 1978, when 93.4 hours was recorded.

Data from the UK Met Office (Unfortunately the Lerwick Met Office are no longer able to provide on a regular basis the monthly maximum gust and highest hourly wind speed. However, when it is noteworthy – for example during gales – I will either watch the “live” Lerwick data or make enquiries so that I can include the gust and hourly mean within the text.)

AUGUST STATISTICS RECORDED AT LERWICK OBSERVATORY

August 2008  :  Averages 1971-2000
Mean maximum temperature 15.4°C   :  14.2°C
Mean minimum temperature 11.1°C  :  8.8°C
Daily mean temperature 13.2°C  :  11.8°C
Mean sea-level pressure 1006.0 hPa (mb)  :  1012.0 hPa (mb)
Total rainfall 77.1mm  :  78.3 mm
Wet days => 1 mm 9 days  :  12.5 days
Sunshine (electronic sensor) 153.8 hours  : 124.6 hours
Air frosts 0 days  :  0.0 days
Ground frosts 0 days  :  0.4 days
Snow/sleet 0 days  :  0.0 days
Days with gale 0 days  :  0.4 days
Maximum daily mean temp. 15.8°C on 1st
Minimum daily mean temp. 11.3°C on 22nd
Highest maximum 18.6°C on 1st
Lowest day maximum 12.2°C on 28th
Highest night minimum 13.5°C on 1st
Lowest minimum 6.7°C on 15th
Lowest grass minimum 0.1°C on 15th
Wettest day 21.4mm on 10th
Sunniest day 13.3hours on 6th

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