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Lightning Lightning
the visible discharge of atmospheric electricity that occurs when a region
of the atmosphere acquires an electrical charge, or potential difference,
sufficient to overcome the resistance of the air.
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 from erupting volcanoes. During a thunderstorm, a
lightning flash can occur within a cloud, between clouds, between a cloud
and air, or from cloud to ground.
Lightning occurs
because regions of net charge are generated by charge-separation processes
that produce an electric dipole structure in a cloud. The charges within a
thundercloud are distributed between a large net-positive charge in the
upper region of the cloud, a large net-negative charge in the lower
region, and a small net-positive charge in the lowest part of the cloud.
Charges reside on water drops, ice particles, or both. If the surrounding
air has a net charge, an air discharge from the cloud may occur.
The flash of
cloud-to-ground lightning is initiated by the neutralization of the small
net-positive charge in the lowest region of the cloud. A cloud-to-ground
flash comprises at least two strokes: a leader stroke and a return stroke.
A leader stroke carrying a negative charge passes from cloud to ground.
(Occasionally, however, the leader stroke is from ground to
cloud--especially with very high structures such as church steeples,
multistory buildings, or tall trees.) The leader stroke is not very bright
and is often stepped and has many branches extending out from the main
channel. As it nears the ground, it induces an opposite charge,
concentrated at the point to be struck, and a return stroke carrying a
positive charge from ground to cloud is generated through the channel. The
two strokes generally meet about 50 m (164 feet) above the ground. At this
juncture, the cloud is short-circuited to the ground and a highly luminous
return stroke of high current passes through the channel to the cloud.
A typical lightning
flash involves a potential difference between cloud and ground of several
hundred million volts, with peak currents on the order of 20,000 amperes.
Temperatures in the channel are on the order of 30,000 K (50,000° F). The
entire process is very rapid; the leader stroke reaches the juncture point
or the ground in about 20 milliseconds, and the return stroke reaches the
cloud in about 70 microseconds.
The thunder
associated with lightning is caused by the rapid heating of air to high
temperatures along the whole length of the lightning channel. The air thus
heated expands at supersonic speeds, but within a metre or two the shock
wave decays into a sound wave, which is then modified by the intervening
medium of air and topography. The result is a series of claps and rumbles.
Although lightning
strikes are dangerous because of their high-voltage discharges, the
tendency of strikes to occur at high points enables lightning rods of
conductive metal to draw the strikes and conduct them harmlessly into the
ground. By moving indoors or by sheltering in a low, depressed area, such
as a ditch, exposed persons can avoid being struck.
Lightning is a transient, high-current electric discharge whose path
length is measured in kilometers. The most common source of lightning is
the electric charges separated in ordinary thunderstorm clouds
(cumulonimbus). Well over half of all lightning discharges occur within
the thunderstorm cloud and are called intracloud discharges. The usual
cloud-to-ground lightning, sometimes called streaked or forked lightning,
has been studied more extensively than other lightning forms because of
its practical interest, as the cause of injuries and death, disturbances
in power and communicating systems, and the ignition of forest fires, and
because lightning channels below cloud level are more easily photographed
and studied with optical instruments. Cloud-to-cloud and cloud-to-air
discharges are less common than intracloud or cloud-to-ground lightning.
All discharges other than cloud-to-ground are often lumped together and
called cloud discharges.
Lightning is a
self-propagating and electrodeless atmospheric discharge that transfers
through the induction process the electrical energy of an electrified
cloud into electrical charges and current in its ionized and thus
conducting channel. Positive and negative leaders are essential components
of the lightning. Only when a leader reaches the ground, the ground
potential wave (return stroke) affects the lightning process. Natural
lightning starts as a bidirectional leader although at different stages of
the process unidirectional leader development can occur. Artificially
triggered lightning starts on a tall structure or from a rocket with a
trailing wire. Most of the lightning energy goes into heat, with smaller
amounts transformed into sonic energy (thunder), radiation, and light.
Lightning, in its various forms, is known by many names such as the common
streak lightning, forked lightning, sheet lightning, heat lightning and
the less common air discharge; also, the rare and mysterious ball
lightning and rocket lightning. For some detailed explanation of lightning
processes, see lightning discharge and related terms.
An abrupt,
discontinuous natural electric discharge in the atmosphere. Lightning
produces a sound wave that is heard as thunder.
An important effect
of world-wide lightning activity is the net transfer of negative charge
from the atmosphere to the earth. This fact is of great importance in one
problem of atmospheric electricity, the question of the source of the
supply current. Existing evidence suggests that lightning discharges
occurring sporadically at all times in various parts of the earth, perhaps
100 per second, may be the principal source of negative charge that
maintains the earth-ionosphere potential difference of several hundred
thousand volts in spite of the steady transfer of charge produced by the
air-earth current. However, there also is evidence that point discharge
currents may contribute to this more significantly than lightning.
How does
Lightning form?
Lightning, a
flow of electrical current between the earth and storm clouds, occurs as
varying charges of positive and negative polarity build up in the
atmosphere during a storm. The result is a discharge or current sent
rushing toward the earth. As this downward force nears the earth's
surface, positive charges rise up to meet it. As the negatively charged
stepped leader thrusts toward the ground readying to discharge its energy,
its path is erratic. Nearing the earth, positive charges are attracted by
it and strain up from roof edges, lighting poles, antennas, etc. When the
two opposing charge systems meet, they create a closed circuit. As the
path to the ground is completed, a flash is created.
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Facts About Lightning!
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Packs between 35,000
to 40,000 amperes of current.
Can generate temperatures as high as 50,000
degrees Celsius.
Falls somewhere on the earth every second.
Travels as far as 40 miles.
Kills nearly 100 people each year in the
U.S. and injures hundreds of others.
Can, and does strike the same place twice.
Causes billions of dollars in property
damage each year, many times resulting in fire and total
property loss.
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