The first half of the summer this year, from approximately mid-May to mid-July, was extraordinary for its warmth and dryness, but since then there has been a marked drop in sunshine with corresponding increase in cloud and rainfall.
From a climatological perspective, this is often the case across the Hebrides, and can be summed up by saying “as the summer matures, so does the humidity”.
The principal reason for the deterioration in the weather from mid-July onwards is the return of our prevailing south-westerly winds, and their moisture laden trajectory from across the Atlantic Ocean.
This well-known return of the ‘westerlies’ is associated with what is known as the “European Monsoon”, a distinct climatic feature of the northern hemisphere.
It is in effect, the same phenomenon which gives rise to the Indian monsoon – a fall of air pressure over the Eurasian continent, and a rise of pressure to the south as the Earth’s thermal equator shifts north during the summer season.
This year, however, the high pressure zone to the south has positioned itself much further north than normal, lying close to Ireland and southern Britain.
Furthermore, the moist southwesterly current of air around its northern perimeter has not been particularly deep (about 1.5km in thickness), meaning that the even-modest sized mountains of the West Highlands of Scotland have been able to block the flow of air eastwards on occasion.
This has allowed sheltered north-eastern districts such as Aberdeen and Inverness to keep the heatwave conditions at times, in direct contrast to the frequent drizzly and murky conditions further out west.
The long term statistics for Stornoway clearly show that the European monsoon occurs most years, with some 200 hours of sunshine expected during an average May at Stornoway Airport, falling to 165 hours in June, but only a mere 130 hours in July (despite the longer amount of daylight during the latter two months).
This coincides with a corresponding increase in rainfall, especially during August. Further east and south across Scotland, this sequence of weather is less pronounced, and it is even reversed over England.
For example, for more than 80% of the time, Stornoway’s weather is pposite to that of London’s, in terms of its deviation from normal of air temperature, sunshine and precipitation.
This means that if it’s warmer, sunnier and drier than normal in London, there is a very strong chance that it’ll be cooler, wetter and duller in Stornoway.
This stark contrast in weather (which fools many a southern holidaymaker when visiting the Isles) does not happen by chance – it is largely predictable and the statistics show it quite clearly.
But coming back to 2018, what a start to the summer it was! Here are a few of the most memorable statistics:
The longest absolute drought on record in Stornoway since at least 1930, with 23 consecutive days without any measureable rain from 22nd May to 13th June.
Measureable rain means anything that can be measured in a rain gauge i.e. the smallest measureable amount is typically about 0.1mm
The sunniest nine consecutive days since 2012, with an average of 13.2 hours of daily sunshine from 22nd May to 31st May inclusive (more than twice what was received in Tenerife at the same time)
The heat reached its peak intensity on 4 July, with a high of +25.3C (78F), the highest airtemperature since July 2014, and the 5th highest air temperature ever recorded in the Stornoway region
All three summer months to date, namely May, June and July recorded well below average rainfall.