PROF. B. F. J. SCHONLAND, of the University of Cape Town, has recently given an interesting account of recent advances in our knowledge of thunder storms (Science Service of June 19). The first noticeable point is that the quantity of electricity stored’ up in the average thunderstorm is surprisingly small. It is only about twenty coulombs, that is, the quantity of electricity that flows through an electric glow lamp in a minute. The thundercloud generates this quantity in five seconds, and after maintaining it at this value for some time it is forced to let it disappear as a lightning flash at a pressure of about 5,000 million volts. It is this enormous pressure that makes the discharge so spectacular and so dangerous. The thundercloud machine is continuously generating electricity at this high pressure. The author estimates that a single cloud can develop three million kilowatts of power. The motive power behind this great electrical machine is the wind, which blows up from below the cloud with tremendous force, like a gale up a chimney. It is this upward current of air which supports the cloud which may contain 300,000 tons of water, and sometimes hailstones of considerable size are suspended by it. The photographs taken of flashes in South Africa by slow-speed photography show that at first a little tongue of light stretches earthwards about 50 yards from the cloud. The light then pauses and fades out for the ten thousandth part of a second. It then reappears and stretches another 50 yards and so on until the ground is reached. Branching tongues may come from it, but the instant the leader touches the ground the main part of the stroke begins. A brilliant flame sweeps upward from the ground towards the cloud retracing the path blazed by the leader. This second stroke is much quicker, lasting only about fifty millionths of a second. The full explanation of the mechanism of this phenomenon is not yet understood.
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