Lightning
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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