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Lightning

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Lightning

Lightning is one of nature’s most common and dangerous phenomena. At any one time,

approximately 2,000 thunderstorms are in progress around the globe, with lightning striking

the earth over 100 times per second. According to IEEE C.62, during a single year in the

United States lightning strikes an average of 52 times per square mile, resulting in 100

deaths, 250 injuries, and over 100 million dollars in damage to equipment property.

The Lightning Phenomenon

Lightning is caused by the complex interaction of rain, ice, up drafts, and down drafts that

occur during a typical thunderstorm. The movement of rain droplets and ice within the cloud

results in a large build up of electrical charges at the top and bottom of the thunder cloud.

Normally, positive charges are concentrated at the top of the thunderhead while negative

charges accumulate near the bottom. Lightning itself does not occur until the potential

difference between two charges is great enough to overcome the insulating resistance of air

between them.

Formation of Lightning

Cloud-to-ground lightning begins forming as the level of negative charge contained in the

lower cloud levels begins to increase and attract the positive charge located at Ground.

When the formation of negative charge reaches its peak level, a surge of electrons called a

stepped leader begins to head towards the earth. Moving in 50-meter increments, the

stepped leader initiates the electrical path (channel) for the lightning strike. As the stepped

leader moves closer to the ground, the mutual attraction between positive and negative

charges results in a positive stream of electrons being pulled up from the ground to the

stepped leader. The positively charged stream is known as a streamer. When the streamer

and stepped leader make contact, it completes the electrical circuit between the cloud and

ground. At that instant, an explosive flow of electrons travels to ground at half the speed of

light and completes the formation of the lightning bolt.

Lightning Bolt

The initial flash of a lightning bolt results when the stepped leader and the streamer make

connection resulting in the conduction of current to Ground. Subsequent strokes (3-4) occur

as large amounts of negative charge move farther up the stepped leader. Known as return

strokes, these subsequent bolts heat the air to temperatures in excess of 50,000 °F and

cause the flickering flash that is associated with lightning. The total duration of most

lightning bolts lasts between 500 ms and one second.

During a lightning strike, the associated voltages range from 20,000 V to 1,000,000 V while

currents average around 35,000 A. However, maximum currents associated with lightning

have been measured as high as 300,000 A.

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