Following a welcome taste of summer a week or so ago, the weather has now reverted to something more typical of early April.
This has been due to the return of an ‘Atlantic’ weather pattern, with west or south-westerly winds spreading plenty of wind, cloud and rain across the UK, especially to the west.
At least this has meant an improvement in pollution levels, though, with no further Saharan dust on our cars.
A few weeks ago I discussed why, on average, north-west Britain is much wetter than the south and east, describing the importance of the Polar jet stream’s path across the North Atlantic. Another major factor is the UK’s topography, i.e. the position of the country’s hills and mountains.
The Welsh mountains, Pennines and Highlands all act to concentrate the UK’s rainfall towards the west.
The meteorological term for this effect is ‘orographic enhancement’ and occurs when the prevailing moisture-laden west to south-westerly winds hit these mountains, forcing the air upwards and creating thicker cloud and heavier rain on their western up-slopes.
As the air then descends on the eastern side of this higher ground, it dries out and so rainfall is much reduced.
For example, Newcastle, east of the Pennines, receives, on average, less than one third of the annual rainfall which falls in the Lake District (600mm and 2,000mm respectively).
Further south, Sennybridge in the Brecon Beacons, south Wales, typically receives three times the annual rainfall of east Essex (1,500mm and 500mm respectively), climatologically the driest area of the UK. By way of comparison, the Bucks average annual rainfall is around 600-700mm.
The effect of moist air flowing over mountains not only influences rainfall distributions, but the areas downstream of the higher ground can also become very warm in certain conditions.
Back in Aylesbury Vale a fairly unsettled, westerly flow looks set to continue for the rest of the week and into the weekend. Mainly dry but often rather cloudy for the rest of today, tomorrow and Friday with just the odd shower, although more persistent rain looks possible on Saturday. Perhaps a little brighter by Sunday, but breezy and chilly.