闪电

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闪电(lightning)

当大气一部分需要足够的电荷以克服空气阻力时所见到的放电现象。在一个雷暴中，云与云间、云内、云和大气间或云到地面间，都可能发生闪电。闪电经常发生在积雨云（雷雨云）中，但也发生在雨层云、雪暴和尘暴中，有时还出现在活火山喷出的气体中。在一次典型的闪电中，云和地面之间的电位差可达几亿伏特。闪道温度为30,000K（50,000F）。从云到地面的闪电至少包括两次闪电：打到地面的闪光较微弱的先导闪电，以及回闪到云的强大亮光。先导闪电约在20毫秒内到达地面，回闪则约在70微秒内到达云中。伴随闪电的雷声是因为在整个闪道上空气被迅速加热产生的，被加热的空气以超音速向外膨胀，但在一两公尺内冲击波衰减为声波，受到空气介质和地形的影响，因此产生一系列隆隆的雷声。亦请参阅thunderstorm。

English version：

lightning

Visible discharge of electricity when part of the atmosphere acquires enough electrical charge to overcome the resistance of the air. During a thunderstorm, lightning flashes can occur within clouds, between clouds, between clouds and air, or from clouds to the ground. Lightning is usually associated with cumulonimbus clouds (thunderclouds) but also occurs in nimbostratus clouds, in snowstorms and dust storms, and sometimes in the dust and gases emitted by a volcano. A typical lightning flash involves a potential difference between cloud and ground of several hundred million volts. Temperatures in the lightning channel are on the order of 30,000K (50,000°F). A cloud-to-ground flash comprises at least two strokes: a pale leader stroke that strikes the ground and a highly luminous return stroke. The leader stroke reaches the ground in about 20 milliseconds; the return stroke reaches the cloud in about 70 microseconds. The thunder associated with lightning is caused by rapid heating of air along the length of the lightning channel. The heated air expands at supersonic speeds. The shock wave decays within a meter or two into a sound wave which, modified by the intervening air and topography, produces a series of rumbles and claps. See also thunderstorm.

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