什麼是閃電

當大氣一部分需要足夠的電荷以克服空氣阻力時所見到的放電現象。在一個雷暴中，雲與雲間、雲內、雲和大氣間或雲到地面間，都可能發生閃電。閃電經常發生在積雨雲（雷雨雲）中，但也發生在雨層雲、雪暴和塵暴中，有時還出現在活火山噴出的氣體中。在一次典型的閃電中，雲和地面之間的電位差可達幾億伏特。閃道溫度為30,000K（50,000F）。從雲到地面的閃電至少包括兩次閃電：打到地面的閃光較微弱的先導閃電，以及回閃到雲的強大亮光。先導閃電約在20毫秒內到達地面，回閃則約在70微秒內到達雲中。伴隨閃電的雷聲是因為在整個閃道上空氣被迅速加熱產生的，被加熱的空氣以超音速向外膨脹，但在一兩公尺內衝擊波衰減為聲波，受到空氣介質和地形的影響，因此產生一系列隆隆的雷聲。亦請參閱thunderstorm。

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.