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|Q1. Funny thing about lightning is a) it never strikes the same place twice b) if it strikes a place once, it may easily strike there dozens of times.|
|Q2. Is it possible to use lightning as an energy source?|
A1. The most common type
of lightning discharge between a thundercloud and
Earth's surface, which accounts for 90% of all lightning discharges
involving ground, begins in the cloud. Before the brilliant lightning
channel bridges the gap, a lightning process called a leader takes
place. This process creates a downward branched, low-luminosity
channel, not observable with a naked eye or with an ordinary
photographic camera. The leader channel extends from the cloud in
search for a termination point on the ground. If we consider a
terrain that is essentially flat and geologically uniform, the
lightning termination point on ground can be viewed as random. In
this case, the saying "lightning never strikes the same place twice"
would be essentially true, as illustrated next. A small area of 1
square meter in an open field in Florida is struck by lightning on
average once every 100 millennia. Thus, if you saw a lightning
strike to that 1 m2 area, another one could be expected in 1000 human
generations or so, which is for all practical purposes equivalent to
never. It is worth noting that each lightning flash is typically
composed of 3 to 5 component strokes that usually retrace the same
channel to ground. However, these component strokes occur within a
second or less and can be detected by a naked eye only as the
flickering in luminosity of the lightning channel.
In reality, ground is not homogeneous and, as a result, the descending
lightning leader will be attracted to some terrain features more than
to others. Grounded metallic objects dominating the area are more
likely to be struck by lightning than the surrounding ground or lower
structures nearby (this is how lightning rods work). In general, the
taller the object, the more often it is struck. For example, a 30-m
tower located in Florida is struck by lightning, on average, once in 3
to 4 years, while a 60-m tower is struck on average once every year.
As the height of an object increases beyond 100 m or so, a different
type of lightning discharge to this object can occur, in addition to
the type described above. This additional type of lightning also
involves a leader process, but the leader channel originates on the
object and extends toward the cloud. Clearly, in this case the strike
point is predetermined, in contrast with the case of lightning
initiated by a descending leader. If the object is very tall, such
object-initiated lightning may occur literally dozens of times every
thunderstorm season. For example, the Ostankino TV tower in Moscow,
Russia, which is 540 m high, experiences about 30 lightning strikes
per year. If this tower were relocated to the Tampa Bay area,
Florida, where lightning activity is 3 to 4 times more intense, it
would experience 90 to 120 strikes per year.
In summary, from the lightning standpoint, the saying "lightning never
strikes the same place twice" is certainly not true. It does strike
its preferential targets such as tall towers over and over again.
However, from the standpoint of a small area in a large open field the
saying is essentially true.
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A2. It appears to be impractical to utilize lightning energy, as illustrated in this section. Each cloud-to-ground lightning flash involves an energy of the order of 109 J. This is approximately equal to the energy required to operate five 100-W light bulbs continuously for one month:
5 x 100 W x 3600 s x 24 x 30 = 1.3 x 109 J
or about 360 kilowatt-hours (1 kW-hr = 3.6 x 106 J), probably comparable to the monthly energy consumption of an average household. Even if it were possible to capture all flash's energy (the bulk of this energy is not delivered to the strike point since it is lost to heating the air and producing thunder, light, and radio waves), one would need to attract 12 flashes to the energy storage facility in order to operate these five light bulbs for one year. The probability of lightning strike to a given point on ground is very low. For example, a 1 m2 area in Florida is struck by lightning on average, once in 105 years. A grounded structure protruding above earth's surface is more likely to be struck by lightning. A 60-m tower located in Florida is expected to be struck by lightning once every other year. Thus, one needs 24 such towers covering a large area of 1 km2 or so to operate five 100-W light bulbs, which appears rather impractical. Most of the U.S. experiences a factor of 2 to 3 lower lightning activity than in Florida. As a result, the number of lightning capturing towers needed to operate only five 100-W bulbs in areas of moderate lightning activity would be 48 to 72. Thus the two main problems with the utilization of lightning energy can be formulated as follows:
(1) The power associated with a lightning flash is very high, but it is released in pulses of short duration (of the order of 10-4-10-5 s). As a result, lightning energy, the integral of high power over a short period of time, is rather moderate, comparable to the energy consumption by a typical household (the integral of relatively low power over a long period of time). This energy is equivalent to that released in the burning of 20 to 30 kg of oil.
(2) The capturing of lightning strikes would require the use of a large number of tall towers, which is rather impractical.
Additionally, as noted above, not all the lightning energy is delivered to the strike point. Using a typical measured value of energy per unit resistance (action integral) for negative lightning of 105 A s2 and an assumed range of effective resistances at the strike point of 10 to 100 Ohm, we estimate a range of the lightning energy delivered to the strike point to be from 106 to 107 J, which is only 10-2-10-3 of the total energy.
Answers to other frequently asked questions about lightning
can be found at other Web Sites.
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