High pressure, high temperatures, high rainfall – remarkable month
Apart from a couple of days during the first half of the month and shortly after mid-month – when shallow lows passed close to Shetland – June was a notably anticyclonic month with mean pressure well above average.
With the main centre of high pressure between Greenland and Scandinavia, and low pressure to the west of Biscay, the normal pressure gradient over the Atlantic was reversed (negative North Atlantic oscillation). As a result, there was an anomalous easterly airflow across Shetland.
June temperatures were on the whole near or even a little below average. However, during the last five days, very warm air – originating over North Africa and drawn over a warm Europe – was carried west over the Northern Isles from Scandinavia. Because of this, the Northern Isles experienced some unseasonably hot weather as the month ended and July began.
June 2009 was the hottest, driest and sunniest month in the UK since July 2006. Average maximum temperatures ranged from 22.3C at Heathrow to 12.9C in Fair Isle.
It was also very sunny, with both Lerwick and Fair Isle having their highest June totals on record. While Lerwick’s 239.8 hours just beat the previous record of 239.7 hours (set in 1935), Fair Isle’s total of 299.7 hours easily beat the previous high of 232.8 hours (June 1992), and was not far short of the all-time record 304.7 hours (May 2008).
|JUNE STATISTICS RECORDED|
AT LERWICK OBSERVATORY
|Mean maximum temperature||13.6°C||12.2°C|
|Mean minimum temperature||8.0°C||7.4°C|
|Daily mean temperature||10.9°C||9.8°C|
|Mean sea-level pressure||1018.7 hPa (mb)||1013.6 hPa (mb)|
|Total rainfall||39.8 mm||58.6 mm|
|Wet days => 1 mm||12 days||11.3 days|
|Sunshine (electronic sensor)||239.8 hours||148.2 hours|
|Air frosts||0 days||0.0 days|
|Ground frosts||0 days||1.2 days|
|Snow/sleet||0 days||0.1 days|
|Days with gale||0 days||0.5 days|
|Maximum daily mean temp||18.3°C on 29th|
|Minimum daily mean temp||7.0°C on 5th|
|Highest maximum||21°C on 29th & 30th|
|Lowest day maximum||9.6°C on 9th|
|Highest night minimum||14.9°C on 29th|
|Lowest minimum||4.2°C on 4th & 5th|
|Lowest grass minimum||0.2°C on 14th|
|Wettest day||19.0 mm on 17th|
|Sunniest day||16.3 hours on 30th|
|Highest mean hourly wind||28 knots on 5th|
|Highest gust||42 knots on 5th|
June began with a large anticyclone extending from south of Iceland across the Northern Isles. A weakening front remained slow-moving close to Shetland on the 1st, bringing generally cloudy – though mainly dry – weather.
With light south-easterly winds carrying warm, moist air across the cooler waters of the Fair Isle Channel, fog formed at times in this area. There were a few showers early on the 2nd as the front edged south, but following light north-westerly winds brought much clearer conditions.
During the 3rd and 4th, though mostly dry and bright, north-westerly winds freshened as a depression slipped south-east into Scandinavia.
The 5th and 6th were cool, showery days with fresh or strong north to north-easterly winds, as the low moved south over the Baltic. The 7th was drier, with sunny spells and easing northerly winds, as the Baltic low filled and the East Greenland high extended south-eastwards towards Shetland, There were some mainly light showers on the 8th, but the 9th and 10th were mainly dry and sunny. Light northerly winds backed north-westerly as a shallow low developed over the Norwegian Sea.
There were more frequent showers and winds freshened for a time on the 11th, as the low moved south past Shetland.
Between the 12th and 16th Shetland enjoyed mostly dry, bright weather with light and variable winds, as a small anticyclone drifted south-east over the Northern Isles. Under mostly clear skies, nights were cool.
After a fine, dry start, occasionally heavy rain spread from the south-west later on the 17th, as south-easterly winds strengthened ahead of an occluding front, pushed north-east across Scotland by an Atlantic depression.
With the low slow-moving to the west, early patchy rain on the 18th turned to bright spells and scattered showers, as winds veered fresh south-south-westerly behind the front.
On the 19th there was some showery rain – occasionally heavy and thundery – more especially over Orkney, where lightning strikes damaged digital equipment on Sanday.
Winds, falling light cyclonic, later freshened west to north-westerly as the filling low drifted east over Orkney. Scattered showers and light north-westerly winds followed for the 20th as the low continued east.
A ridge, crossing the area from the west, brought a fine day on the 21st. West to north-westerly winds backed south-westerly as the ridge declined, bringing misty conditions with patchy drizzle and hill fog for the 22nd, as weak fronts trailed across Shetland. Patchy drizzle and fog cleared early on the 23rd, as an anticyclone over southern England ridged north.
From the 24th, as the anticyclone transferred north-east over the British Isles into Scandinavia, winds settled easterly, bringing a marked change to very warm, dry and mostly sunny conditions. As high pressure persisted to the north-east or north of Shetland, no further rain fell during the remaining days of June.
While most places experienced high temperatures, patchy low cloud and sea fog affected some hills and coasts, mostly – though not exclusively – in the east. However, because of the dry air of the original airmass, fog was not the usual problem when very warm air is carried across the North Sea to the Northern Isles.
On the 29th a maximum temperature of 21.7C was recorded at Lerwick. Scatsta – on the more sheltered west side – reported a temperature of 23C for much of the day. Between 6-7pm, Fair Isle reached 19.4C, the highest temperature of the year there. On the 30th the Observatory again reported a maximum of 21.7, a June record for Lerwick.
Initial statistical details for Scotland as a whole (1961-1990 average) for June 2009: Mean Temperature – 12.0C (1.0C above average). Rainfall – 73.8 mm (87 per cent average). Sunshine – 211.7 hours (136 per cent of average).
Data from the UK Met Office.