Has 2015 really been as windy as you think?
Being a relatively small collection of islands at the eastern end of the Atlantic Ocean, the UK is no stranger to windy weather, exposed to areas of low pressure that rattle in from the west.
However, one of the most frequent questions that I have been asked this year is: has it been a windy year?
One of the groups of people to ask this question most frequently have been members of the cycling community, who notice the wind more acutely than the rest of us, for obvious reasons.
And the answer? In a nutshell, yes, it has been a windy year so far – not so much in the strength of the wind, but the lack of calm days.
Fewest number of calm days in 20 years
The Met Office released some interesting statistics a few days ago, which looked at the number of calm days across the UK – i.e. those which have not been windy.
A calm day is considered to be when at least 20 weather stations have recorded a maximum wind gust of 11mph or less – equivalent to a gentle breeze.
So far this year, the UK has only had eight such days, which makes it the fewest number of calm days in the last 20 years.
Why has it been a windy year?
Wind is driven by contrasts between high and low pressure. The greater the difference in air pressure over a given distance, the faster the wind blows.
In the northern hemisphere, this difference in air pressure over the Atlantic Ocean between Iceland and the Azores is called the North Atlantic Oscillation (NAO).
The graphic below from January to June this year shows that pressure has been lower than normal north of the UK and higher than normal to the south.
This has led to a bigger difference in pressure, resulting in a weather pattern that is more favourable for low pressure to affect the UK and thus give a greater number of windy days.
In effect, we’ve had low pressure dominating, with a distinct lack of high pressure, which would bring calmer days with little wind.
Wind patterns determine rainfall
The wind patterns this year have also heavily influenced rainfall amounts across the UK.
A frequent occurrence of low pressure and winds from a westerly direction has meant that moist air of the Atlantic Ocean has been piling in across the north west of the UK.
However, south eastern parts of the UK aren’t as exposed to a westerly wind, so have only seen the remnants of weather fronts moving eastwards, giving less rain.
As a result, Achnagart in the western Highlands of Scotland has had 2082mm of rain between 1 January and 22 July, compared to just 237mm at St James’s Park in London during the same time period – a staggering nine times as much.
With low pressure set to influence our weather during the next week or so, the chances of getting any calm days soon are looking low.
Graphics: Met Office, NOAA/ESRL