By Philip Eden

Severe thunderstorms broke out widely last Tuesday and Wednesday (July 3 and 4), with southern England, Wales, and southwestern Scotland worst affected. A torrential downpour on Tuesday evening led to flooding in Lanarkshire and the southern outskirts of Glasgow and floods were also reported between Douglas and Castleton on the Isle of Man. On Wednesday morning a localised cloudburst over Aran Benllyn and Aran Fawddwy in the Cambrian Mountains of mid-Wales produced raging torrents of water in the valleys feeding into Lake Bala; one of these torrents wrecked a caravan site. Serious flooding also hit Cardiff, there were mudslides in the Welsh Valleys, many homes were struck by lightning in south Wales, Somerset and Gloucestershire, and a church in Havant, H ampshire, was seriously damaged by fire following a direct strike.

The highest rainfall total to hand for Wednesday is Cardiff's 67mm - over a month's worth in less than six hours. However, a close examination of the rainfall-radar information indicates that much more than this fell over a small area of mid-Wales - perha ps in excess of 125mm. Unfortunately it is not possible to calibrate the radar sufficiently precisely to provide us with an accurate site-specific rainfall figure. Further storms hit southern England and south Wales on Thursday, and they broke out more wi dely on Friday and Saturday across England and Wales causing extensive disruption to Wimbledon and to the Edgbaston Test. Some 70mm fell at Cheltenham on Saturday.

There is a popular misconception that a spell of excessive heat and humidity will always be broken by a thunderstorm. It does happen sometimes, for example when a cold front advances from the west and air of tropical origin is replaced by much cooler air from the Atlantic. But last week provided an excellent example of the other sort, when the frontal systems move northwards from France or northwestwards from central Europe. On these occasions the change of air is imperceptible, so it remains every bit as humid after the storms have cleared.