Lightning

lightning, electrical discharge accompanied by thunder, commonly occurring during a thunderstorm. The discharge may take place between one part of a cloud and another part (intracloud), between one cloud and another (intercloud), between a cloud and the earth, or earth and cloud. Lightning may appear as a jagged streak (forked lightning), as a vast flash in the sky (sheet lightning), or, rarely, as a brilliant ball (ball lightning). Illumination from lightning flashes occurring near the horizon, often with clear skies and the accompanying thunder too distant to be audible, is referred to as heat lightning. There are numerous theories on why charges accumulate in the atmosphere. It is thought that temperature and water vapor pressure in thunderstorm clouds are associated with the positive and negative ions that cause lightning. Long-lasting lightning flashes with lower current are more damaging to nature and humans than shorter flashes with higher currents. Benjamin Franklin, in his kite experiment (1752), proved that lightning and electricity are identical (see lightning rod).

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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 30,000 K (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 metre 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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Lightning

See also Thunder.

Agni

god of fire and lightning. [Hindu Myth.: Benét, 15]

double ax

variation of Jupiter’s thunderbolt. [Rom. Myth.: Jobes, 163]

Elicius

epithet of Jupiter as god of lightning. [Rom. Myth.: Kravitz, 87]

Franklin, Benjamin

(1706–1790) flew kite in thunderstorm to prove electricity existed in lightning. [Am. Hist.: NCE, 1000]

Jupiter Fulgurator

Jupiter as controller of weather and sender of lightning. [Rom. Myth.: Howe, 147]

Thor

bravest of gods; protected man from lightning. [Norse Myth.: Brewer Handbook, 1099]