by DAVE WHEELER
The month began with high pressure over the UK. However, this quickly declined and cyclonic conditions prevailed across Shetland during the first two weeks of April, as depressions tracked north to the west of Scotland.
The 11th to the13th was notably sunny as pressure built across the area. The third week of the month saw mainly settled conditions as an anticyclone meandered down from Iceland, across the UK, and then drifted away into Scandinavia. This left the last week again potentially unsettled, as Atlantic low pressure systems tracked northeast between Scotland and Iceland. However, benefiting from the shelter of the Scottish mainland and closer to high pressure, Shetland saw little if any rainfall.
While pressure was well below normal over western Britain, it was near-average over the Northern Isles, resulting in an anomalous south-easterly airflow over the British Isles. As with the rest of the country, Shetland was much warmer than average.
For the UK as a whole, this was the third warmest April since 1914 (April 2007 was the warmest). While rainfall was close to normal in many parts, it was dry in the east – Shetland included.
Sunshine was also close to normal, but above normal in the north-west. Shetland was duller than average. Sunshine percentages ranged from 141 per cent at Wattisham (Suffolk) to 71 per cent at Fair Isle.
On the 1st mild south-westerly winds, blowing round the northern edge of an anticyclone centred over the British Isles, brought cloudy but mainly dry conditions to Shetland. While it remained dry and initially quite sunny on the 2nd, as the high moved over southern Scandinavia, winds backed south-easterly and began blowing over a cold North Sea, resulting in mist, low cloud and sea-fog becoming quite extensive by the 3rd.
A depression – tracking east over Shetland – brought misty conditions with low cloud, patchy fog and outbreaks of rain and drizzle for the 4th. These cleared on the 5th as the low moved away and cyclonic winds became strong to gale westerly for a time.
Winds eased overnight, and the 6th dawned chilly with a slight ground frost in places. It was a mainly dry day with sunny or clear spells, the wind backing southerly again as a complex area of low pressure developed to the west of the British Isles. Southerly winds strengthened on the 7th, with some early rain or drizzle as a front moved north across the isles.
With high pressure intensifying over Scandinavia and low pressure deepening to the west, south or south-easterly winds strengthened on the 8th, but eased later as a slack, complex area of low pressure developed between Scotland and Iceland and outbreaks of rain spread from the west.
South-easterly winds had strengthened again by the 10th as a low – deepening off northwest Scotland – pushed fronts across the Northern Isles, bringing more rain later in the day. Fronts cleared overnight as the low drifted north over Faeroe, leaving fresh southerly winds and a mainly dry, bright day on the 11th.
Winds eased during the 12th as the low moved away past Iceland. A ridge, building across the area, gave a mainly dry day with plenty of sunshine. The following night was chilly, with many places seeing a ground frost.
Pressure then remained relatively high across the area, giving another fine, dry and sunny day with light winds on the 13th. By the 14th – a dry but mainly cloudy day – south-easterly winds were again freshening as the next Atlantic depression moved east towards the British Isles.
However, with pressure remaining high to the north and east, fronts made little headway towards northeast Scotland.
As the low slipped southwards into Biscay and high pressure built to the north, a cool easterly breeze blew across the Northern Isles during the next two days. It was dry, with sunny spells by day and – with clear periods – chilly nights, with a touch of ground frost early on the 16th.
Easterly winds backed north-easterly and freshened on the 17th as an anticyclone developed to the east of Iceland, a few showers affecting parts of Shetland later in the day. The 18th was mostly dry though rather cloudy, with lighter north-easterly winds as the high ridged down across northern Scotland.
With an anticyclone, developing near Faeroe, moving south over the Northern Isles and expanding eventually to cover much of the UK, North Sea and southern Scandinavia, the next two days remained dry. The 19th was rather cloudy and – later on the 20th – as light and variable winds became south-easterly, patchy fog and mist affected eastern districts.
As the high pressure relaxed eastwards, a waving front moved towards Shetland on the 21st, bringing outbreaks of rain. High pressure, building over Biscay, then ridged north across Scotland, bringing a fine, dry and sunny day on the 22nd. However, this ridge collapsed overnight and south or south-easterly winds strengthened ahead of fronts pushed east.
The fronts – eventually amalgamating into one slow-moving front over the Northern Isles – produced some rain on the 23rd. As the front weakened further, the rain died out overnight and – after a misty start with local hill fog – the 24th was mostly dry, though rather cloudy.
The 25th was also mainly dry but brighter, with hazy sunshine and light south-easterly winds as high pressure to the east, building across Shetland, pushed the front back into the Atlantic.
The mainly dry, though generally cloudy weather with moderate or fresh south-easterly winds, continued through the 26th and 27th. As pressure built to the north of Shetland, the 28th was brighter with sunny or clear spells and lighter easterly winds.
A high, developing over the Norwegian Sea and ridging south, brought another dry though generally cloudy day with a freshening north-easterly breeze to Shetland on the 29th. The 30th remained dry, though winds veered south-easterly ahead of fronts pushed north towards the Northern Isles, as the high weakened and the next depression moved east towards Scotland.
Initial Statistical details for Scotland as a whole (1961-1990 average) for April: Mean Temperature: 7.8°C (2.1°C above average) Rainfall: 83.6 mm (97 per cent of average) Sunshine: 151.4 hours (116 per cent of average).
APRIL STATISTICS RECORDED AT LERWICK OBSERVATORY
April 2009 : Averages 1971-2000
Mean maximum temperature 9.5°C : 7.7°C
Mean minimum temperature 5.9°C : 2.9°C
Daily mean temperature 7.5°C : 5.3°C
Mean sea-level pressure 1012.7 hPa (mb) : 1012.7 hPa (mb)
Total rainfall 45.0mm : 74.2mm
Wet days =>1mm 8 days : 14.4 days
Sunshine (electronic sensor) 120.0 hours : 129.9 hours
Air frosts 0 days : 4.3 days
Ground frosts 4 days : 11 days
Snow/sleet 0 days : 7.5 days
Days with gale 1 days : 1.9 days
Maximum daily mean temp 9.4°C on the 1st
Minimum daily mean temp 6.1°C on the 16th
Highest maximum 12.8°C on the 28th
Lowest day maximum 6.7°C on the 3rd and 4th
Highest night minimum 7.9°C on the 10th
Lowest minimum 2.5°C on the 13th
Lowest grass minimum -3.6°C on the 13th
Wettest day 8.2mm on the 10th
Sunniest day 12.9 hours on the 22nd
Highest mean hourly wind 34 knots on the 5th
Highest gust 48 knots on the 5th