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CHAPTER 2: LIGHTNING PROTECTION

Lightning phenomena. The planet earth is similar to a huge battery continuously losing electrons to the atmosphere. These electrons could be lost in less than an hour unless the supply is continually replenished. It is widely agreed among physicists and scientists that thunderstorms occurring thousands of times daily around the earth return electrons to earth to maintain normal magnitude of electrons at or near the surface of- the earth. | TM 5-811-3 AFM 88-9 Chap. 3 CHAPTER 2 LIGHTNING PROTECTION 2-1. Discussion a. Lightning phenomena. The planet earth is similar to a huge battery continuously losing electrons to the atmosphere. These electrons could be lost in less than an hour unless the supply is continually replenished. It is widely agreed among physicists and scientists that thunderstorms occurring thousands of times daily around the earth return electrons to earth to maintain normal magnitude of electrons at or near the surface of- the earth. The rate of electron loss from earth called the air-earth ionic current has been calculated to be 9 microampere for every square mile of earth s surface. Thunderstorms supply electrons back to earth by an opposite electron potential gradient of perhaps 10 kilovolts per meter within a thundercloud. This feedback forms a potential difference of from 10 to 100 megavolts in a single discharge between the center of a cloud and earth. These lightning discharges carry currents varying from 10 to 345 kiloamperes to earth at an average rate of 100 times per second with duration of less than second per flash. Each flash consists of up to 40 separate strokes Each stroke of lightning lasting for this brief instant releases about 250 kilowatt-hours of energy-enough to operate a 100-watt light bulb continuously for more than three months at the rated voltage of the lamp. Lightning discharges do not always bring electrons to earth because so-called positive ground-to-cloud strokes consist of low power energy transmissions from earth to small negative charge pockets in a thunder cloud. However magnitudes of discharge voltages and currents are approximately the same from cloud to earth and all occur within the same discharge timeframes. Just before the lightning flash the ground within a radius of several miles below the cloud becomes deficient in electrons. Repelled by the army of electrons in the cloud base many of the free electrons on the ground are pushed away. The .