Lightning
SIDACtor
Data Book
5 - 20
Teccor Electronics
(972) 580-7777
,*
Overview
Lightning is one of nature
’
s most common and dangerous phenomena. At any one
time, there are approximately 2,000 thunderstorms in progress around the globe with
lightning striking the earth over 100 times per second. During a single year in United
States, lightning will strike 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
The formation of 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.
The Formation of Lightning
Cloud to ground lightning will begin 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 will begin 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 and can rise up out of a tree, roof of a house,
or even the top of your head. 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.
The 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 500ms and 1 second.
During a lightning strike, the associated voltages range from 20,000V to 1,000,000V
while currents average around 35,000A. However, it should be noted that maximum
currents associated with lightning have been measured as high as 300,000A.