I was visiting Shropshire, UK, when I woke abruptly in the early morning. Outside it was hot and humid. As I was sitting facing the window I saw a bright single-point flash of vivid blue light through the thin curtains, just as if I had looked at a photographer’s flash going off. Then there was a single heavy rumble of thunder followed by a sudden and intense downpour of rain, lasting only a few minutes. Then all was quiet. Later, looking through the same window, I realised the location of the flash was a lightning conductor on the roof of a school. Talking to others I discovered the whole event had been very localised, not even extending across the village. I thought thunderstorms worked on a larger scale. How had there just been the single flash at the lightning conductor and such a small-scale storm, contained within a hundred metres of the school?
• In many areas in Africa one may sit on a hilltop overlooking expanses of veldt enjoying a panorama of well-separated thunderheads, each occupying a cell a few hundred metres across, precipitating bolts and showers on all beneath.
In South Africa’s drought-prone they call such a cell “jaloersreën”, which translates from the Afrikaans as “envy rain” – each farmer thinks the shower is over his neighbour’s property and envies the other.
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This type of thunderstorm often occurs when conditions favour strong, localised updraughts without much lateral wind. However, on its own such an updraught might bring no rain. It is heavy ionisation from a lightning bolt that is to blame. This disrupts the uniform electric charges on droplets in the thundercloud, causing many of them to attract other droplets with different charges. When droplets grow large enough their aerodynamic behaviour changes and they collide with smaller neighbours, growing increasingly bigger until there is a bout of heavy rain. If the updraught is strong enough, it might cause a hailstorm.
The Shropshire school storm sounds like an unusually small cell of such a type, triggered by the lightning conductor.
Jon Richfield, Somerset West, South Africa