Britain basks over the weekend

12.05.08 15:46

' "The sun is out, the sky is blue. There’s not a cloud to spoil the view..." Or at least, there wasn’t a few minutes ago and now it’s as black as night and pouring with rain.'
 
Our first spell of summer weather in the UK has brought warm, sunny days across many areas. As most people enjoyed the sizzling conditions outdoors in their droves across a large part of England, Wales and Scotland, a few were sent scurrying for cover as the atmosphere snapped like a coiled spring.
 
The fine weather across the UK over the weekend was thanks to south-easterly winds dragging very warm air northwards from France, and even further afield. These winds were all part of a  very typical summer scenario, as frontal systems sitting to the west of the UK allowed the warm continental air the their east to feed northwards.

Although the air is warm and, for the most part, stable, it does have its weaknesses. As air at the earth’s surface warms it starts to rise, but as it rises higher through the lower atmosphere it will gradually cool again. This cooling air then condenses, forming cumulus clouds. These clouds are common on lazy summer afternoons and are usually considered to be fair-weather clouds, rarely threatening more than a landscape-hugging shadow. These clouds are usually kept in check by a ‘capping inversion’ in the atmosphere; a layer of dry, stable air thousands of feet above the cloud that acts as an invisible lid, stunting the cloud’s growth. However, during the heat of a sultry summer afternoon, as temperature and humidity both increase almost relentlessly, the cumulus clouds grow more quickly and vigorously. There comes a point where these normally placid clouds are able to break right through the capping inversion, exploding skywards at spectacular speed. When this happens, the lazy summer afternoons are often brought to an abrupt halt.
 
On Saturday 10th May, much of central southern and south-east England was bathed in warm sunshine with temperatures reaching the mid twenties Celsius. Apart from a few wisps of high cloud turning the sunshine hazy, there was little sign of what was to come. At around four o’clock in the afternoon an area of cumulus cloud over west London began to take a more sinister turn, as a combination of high surface temperatures and a weakness in the upper atmosphere started to fuel their vertical growth. At around half past four, the clouds suddenly erupted skywards in an almost volcanic fashion, with the landscape of west London darkening as an ominous shadow descended on the summer afternoon.

By five o’clock sporadic raindrops were falling from the towering cumulonimbus cloud and within minutes this had become a deluge. The darkened skies were suddenly lit by blinding bolts of lightning and deafening thunder, and cold gusts of wind came crashing downwards out of the cloud. The cumulonimbus grew to form a mountainous thunderstorm that could be seen from as far south as Kent and as far north as the Midlands. The storm rolled slowly northwards, soaking parts of Buckinghamshire, Hertfordshire and Northamptonshire, before running out of steam a couple of hours after it exploded into life.
 
Whilst this thunderstorm cell was as magnificent in its isolation as in its development, it wasn’t the only storm to form before the day was out. Smaller thunderstorms developed in Berkshire during the evening, followed by even more spectacular thunderstorms across the Bristol area and into South Wales on Saturday night. These night-time storms brought flooding and hailstones the size of marbles to parts of South Wales.

On Sunday 11th May it was the turn of Scotland and northern England to witness the snapping of the atmospheric elastic band, with widespread torrential downpours and thunderstorms. At the same time, southeast England reached the dizzy heights of 28 degrees Celsius.
 
The new week promises yet more sunny and warm weather but with little chance of nature’s fireworks returning. The air will become slightly cooler and more stable as the week progresses, although temperatures will still remain respectably above average in many places. However, North Sea coasts of England and Scotland will be plagued by low cloud, mist and haar, keeping temperatures lower here.

By: Brendan Jones & Laura Gilchrist