Weather Wise

THE authorities in Singapore this week announced a new public campaign to reduce the number of people hit by lightning. Last year, six people were killed by lightning, and nine died in the previous year - which is roughly the same as the number of deaths from lightning in Britain, where the population is 20 times the size.

Singapore, in fact, has one of the highest rates of lightning-strikes in the world, with 20 per square kilometre per year. Because of that, the present plans include the construction of lightning shelters in parks and on beaches, the installation of a 15,000 Singapore dollars (about pounds 6,000) thunderstorm early warning system in every school, anti-lightning netting over school entrances and bus shelters, setting up a lightning safety hot line, and a programme of education about the hazards of lightning. Presumably this would include such sensible advice as not to use the lightning hotline when thunderstorms are about.

A complete scientific explanation of lightning has still not been found. At a high level in the atmosphere, the ionosphere carries a positive electric charge thanks to the action of the Sun's ultraviolet radiation. The Earth, however, is negatively charged during fine weather, so cloud droplets may acquire an induced positive charge on their Earth-facing sides, and a negative charge on their upper sides. The breakup of raindrops and splintering of ice crystals, however, both increase the separation of electric charges of opposite sign, but the mathematical models of electrical behaviour within clouds does not quite tally with measurements taken. Generally, clouds tend to be positively charged at the top, and negatively at the bottom, but some anomalous patches of electrical activity occur within clouds that have so far defied explanation.

Lightning occurs when a high negative charge at the bottom of a cloud induces a positive charge on the ground beneath it. When these electrical forces are large enough, they will overcome the resistance of the air and a flash will come down from the cloud to bridge the gap in the circuit. It will be met by a return stroke from the ground, but since both travel at about a third of the speed of light, the two flashes appear as one. Further strokes flash back and forth until the cloud has drained its negative charge. The temperature of the lightning flash is around 30,000C, which creates a rapid expansion of the air around its channel, resulting in the shock waves of sound that we hear as thunder.

According to a paper in the Journal of Meteorology in 1991 ("Lightning Deaths and Sex", by Paul R Brown), statistics show that men are six times as likely as women to be killed by lightning. This, he suggests, is simply because men are more likely than women to be outside in exposed areas. Or, to put it simply, men play golf.

If you are hit by lightning, you may die from the burns, or more likely from the jolt of electricity that can cause heart failure. Nevertheless, records show that nine out of 10 people struck by lightning survive. Holding a golf club, however, greatly shortens the odds.