24th April 2018
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Coldest February for eight years, but sunshine was only slightly diminished

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In Shetland, in common with the rest of the UK, the first half of the month was cold, with snow and severe frosts. Initial south-easterly winds backed north-east to northerly as high pressure to the east moved away, to be replaced by low pressure over Scandinavia.

From mid-month high pressure to the south of the British Isles became the dominant synoptic feature. As a result the second half was much milder, particularly so for Shetland on the northern extremity of the high, and under the influence of an unsettled south-west to westerly Atlantic-type airflow.

Across much of Scotland the mean temperature for the month was slightly above normal. Sunshine duration was below normal and it was the dullest February since 1998. Rainfall was also below normal, particularly over western Scotland, where it was the driest February since 1986.

Shetland experienced the coolest February since 2001, with mean temperatures quite close to average. However, here we were not as dull as the rest of Scotland, with the sunshine total for the month only slightly reduced. We also differed markedly from the rest of Scotland in having above-normal rainfall for the month.

Not surprisingly, with the pre­dom­inantly anticyclonic conditions, there were fewer gales than normal; with Lerwick reporting just the 25th having a 10-minute average in excess of 33kt. Fair Isle also reported fewer gales than normal, with just two days of gales. Statistically Feb­ruary is the quietest, most anti­cyclonic month of winter.

With high pressure across north-west Russia and Scandinavia, the month opened with Siberian easterly winds over much of the British Isles. Across Shetland, where it was grey and cloudy though mostly dry, winds were fresh to strong south-easterly in direction on the 1st. Having a longer track across the North Sea, winds here were somewhat less cold, but still raw and penetrating.

Through the 2nd and 3rd winds tended more easterly, with a few wintry showers affecting Shetland as high pressure pulled away east­wards and low pressure developed across southern England.

By the 4th a complex area of low pressure had formed over the UK. A cold front moving south brought a period of snow, with winds increas­ing strong to gale-force north-easterly across the south of Shetland and gusts of over 50mph reported from Fair Isle. By evening most places had a covering of several centimetres of snow, with tempera­tures falling to freezing or just below.

Winds eased and became cyclonic variable on the 5th, as low pressure extended north to encompass the Northern Isles. A small low, develop­ing on the edge of the southward flow of cold Arctic air to the west of Shetland, brought a mixture of wintry showers or longer periods of sleet or snow for a time.

Between the 6th and the 8th, as low pressure drifted away east, cold north to north-easterly winds edged in from the west, bringing Arctic air down over Shetland. Fresh or strong northerly winds eased and snow showers all-but died out by the 8th.

During the next couple of days, relatively clear skies and light winds resulted in low temperatures and hard frosts. Even by the shore, cooling air sinking down off the surrounding hills and “ponding” in the locality, saw the temperature at Baltasound falling to -9.4° Celsius early on the 9th. However, on the hill (the cold air rolled down into Lerwick) the minimum was only -4.3° Celsius at the observatory. On mainland Scotland an overnight minimum of -18.4°C was recorded at Aviemore.

This was the coldest February night here for 23 years. As a ridge crossed from the west a fine, cold day followed, with daytime tempera­tures hardly rising above freezing. However, with just light north-east to northerly winds and 7.4 hours of bright sunshine in Lerwick – the sunniest place in the UK – it was a reasonably pleasant winter’s day.

Another severe overnight frost followed, with temperatures down to -8° Celsius at Baltasound, -5° Celsius in Lerwick and -3° Celsius at Sumburgh and in Fair Isle. Snow-surface temperatures fell to -11° in Lerwick and -8° Celsius in Fair Isle. On the 10th, while an intense depression moved east over southern Britain, pressure remained relatively high in the north, giving Shetland another fine, cold day with light north or north-westerly winds.

On the 11th, as the depression in the south moved away eastwards and the Greenland high ridged south, Arctic winds brought snow showers to the Northern Isles, the lying snow drifting in the strengthening north to north-westerly winds. Ler­wick report­ed a maximum daytime temperature of 0.9° Celsius, the lowest in the UK.

Winds eased as a weak ridge built across the area, but further wintry showers or longer periods of snow continued, as the ridge was replaced by a shallow trough.

Further heavy snow across parts of Shetland brought significant accumulations early on the 12th, with Lerwick reporting 20cm of lying snow at 9am. It was a cold bright day with scattered showers, light winds and temperatures barely rising above freezing.

As another narrow ridge made a brief appear­ance, the 13th was mostly dry and also cold, bright with little or no wind.

The ridge declined early on the 14th, with milder south-westerly winds bringing rain or drizzle accompanied by misty conditions, as fronts moved off the Atlantic to affect Shetland. With high pressure persisting to the south of the UK, mild and fresh to strong south-west to westerly winds followed between the 15th and 21st.

While there were a few brighter moments, generally weak fronts – brushing around the northern edge of the high – brought outbreaks of rain or drizzle and some misty con­di­tions to Shetland from time to time The 22nd was cooler with strong west to north-westerly winds bring­ing showers, as a depression tracked east to the north of Shetland into Scandinavia. However, the potential for a return to the cooler conditions of the first half of the month were quickly curtailed as a ridge swung east on the 23rd, and a fine day end­ed with cloud and patchy drizzle.

Milder, misty conditions with patchy rain or drizzle accompanied by fresh south-westerly winds re­turn­ed on the 24th. It was brighter on the 25th with showers, these merging into rain later in the day as a deep depression, developing south of Iceland, drove fronts east, and south-west to westerly winds increased gale or severe gale-force, with Fair Isle reporting gusts to 65mph.

These cleared overnight as the depression continued into Scandin­avia and strong westerly winds brought showers across Shetland on the 26th. These died out early on the 27th, as another transient ridge swung east across the area.

As this declined and high pressure – which had been persistent close to southern Britain during the second half of February – slipped further south, south-easterly winds freshen­ed and heavy rain spread from the Atlantic later in the day.

A wet night followed, with the 28th continuing cloudy with rain at times and mild south to south-westerly winds, as further fronts moved east over Shetland.

FEBRUARY STATISTICS RECORDED AT LERWICK OBSERVATORY

February 2009 : Averages 1971-2000

Mean maximum temperature 4.8 °C : 5.4°C
Mean minimum temperature 1.9 °C : 1.4°C

Daily mean temperature 3.3 °C : 3.1°C

Mean sea-level pressure 1010.8 hPa (mb) : 1010.2 hPa (mb)

Total rainfall,135.5mm 107.8 mm

Wet days => 1 mm 20 days : 17.8 days

Sunshine (electronic sensor) 47.8 hours : 52.3hours

Air frosts 10 days : 7.7 days

Ground frosts 14 days : 14.9 days

Snow/sleet 12 days : 12.2 days

Days with gale 1 day : 6.1 days

Maximum daily mean temp 7.7 °C on 24th

Minimum daily mean temp -2.0 °C on 9th

Highest maximum 8.7°C on 24th

Lowest day maximum 0.2 °C on 9th

Highest night minimum 7.2°C on 21st

Lowest minimum -4.6 °C on 10th

Lowest grass minimum -11.1°C on 10th

Wettest day 30.8 mm on 27th

Sunniest day 4 hours on 9th

Highest mean hourly wind 35 knots on 25th

Highest gust 51 knots on 25th