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How far must you be to prevent getting struck by lightning?
Lightning is a form of visible electric discharge between rain clouds or between a rain cloud and the Earth. The discharge is seen as a brilliant arc and produces a sound wave that is heard as thunder. Most rain clouds are negatively charges at the base and positively charged at the top. When the electrical potential between two clouds or between a cloud and the Earth reaches a sufficiently high value, the air becomes ionised along a narrow path and lightning flash results. Lightning that strikes the Earth consists of one or more electrical discharges called strokes. The bright light that we see in a flash of lightning is called a return stroke. A typical lightning flash involves a potential difference between cloud and ground of several hundred million volts, with peak currents on the order of 20,000 amperes. The flash of cloud-to-ground lightning is initiated by the neutralization of the small net-positive charge in the lowest region of the cloud. A cloud-to-ground flash comprises at least two strokes: a leader stroke and a return stroke. A leader stroke carrying a negative charge passes from cloud to ground. Occasionally, however, the leader stroke is from ground to cloud--especially with very high structures such as church steeples, multi-story buildings, or tall trees. The leader stroke is not very bright and is often stepped and has many branches extending out from the main channel. As it nears the ground, it induces an opposite charge, concentrated at the point to be struck, and a return stroke carrying a positive charge from ground to cloud is generated through the channel. The two strokes generally meet about 50 m above the ground. At this juncture, the cloud is short-circuited to the ground and a highly luminous return stroke of high current passes through the channel to the cloud. The entire process is very rapid; the leader stroke reaches the juncture point or the ground in about 20 milliseconds, and the return stroke reaches the cloud in about 70 microseconds. Lightning usually strikes the nearest object. The general precaution against lightning is to avoid hilltops, beaches, small structures in open areas and open water. When you swim in open water, you head above water naturally becomes the closest object for lightning to strike. Lightning protection systems control electrical discharges by directing them through a low-resistance path to the ground, avoiding passage through parts of a structure and reducing risk of fire or other damage. Air terminals (rods) fastened to the building intercept electric discharges, directing them through conductors to a grounding system and thence into the ground. It is a common misconception that lightning rods silently discharge the electric charge in a thundercloud and thereby prevent lightning. Actually lightning only serve to route the lightning harmlessly to ground. In doing so they divert lightning when it is 10 to 100 metres away. A house protected by a single lightning rod has an assumed 45 degree-angle cone of protection. If the height of the rod were H metres, the base area of the effective cone would have a radius of H metres. Objects within this cone are assumed to be safe from a lightning strike. | 
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发表于 2004-9-8 15:43:00
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