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ice grainhailstone4Emergence of thundercloudThere are a variety of theories how thunderclouds emerge, but generally it is explained as below.When atmosphere near the earth surface or ocean surface containing a large amount of water vapor warms up by the heat from the sun and other sources, it expands and ascends. When it reaches a high altitude, it is cooled down, resulting the vapor in the atmosphere becomes water droplets and then a cloud. When it ascends even higher, water droplets in the cloud becomes ice grains and some of them concentrate and grow to hailstones. At that time, these ice grains and hail-stones are decomposed by electric current. Then, ice grains are charged positively and hailstones are charged negatively. Ice grains ascend even higher by riding up-draft and hailstones grow bigger and fall by the gravity. Charge separation continues by the coulomb force. Before long, top of the cloud is positively charged and the bot-tom negatively. A cloud accumulates elec-trical energy, which is when a thundercloud emerges.What is lightning surge?When electric charges are built up in thunderclouds to such level that could break atmospheric insula-tion, an electric discharge eventually occurs between these clouds or between the clouds and the ground. Electric current reaches 20-150 kA. An abnormally high voltage generated by direct lightning discharge applied to electric power cables or communication cables at that instance is called ʻdirect lightning surgeʼ.Correspondingly, such voltage induced by electrostatic or electromagnetic induction on those cables lo-cated close to the point where a direct lightning hits, is called ʻinduced lightning surgeʼ.Also, when lightning strikes a lightning rod and the ground potential rises, instrumentsʼ grounding poten-tial becomes also high. This causes an abnormally great potential difference between the cables and the ground, which is called ʻlightning surge caused by increased ground potentialʼ.Direct lightning surge energy is enormous. A surge protector alone cannot protect the instruments. It is necessary to share the job by lightning rods and overhead grounding wires to absorb most of the ener-gy, and by surge protectors to absorb only the rest of the energy. Here, we explain the mechanism how lightning surges occur, except for the direct lightning surge.Figure 1-1

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