Lightning

From Wikipedia, the free encyclopedia

For other uses, see Lightning (disambiguation).

Lightning is  an atmospheric discharge of electricity, usually, but not always, during a rain storm.

Contents 1 Early research 2 Modern research 3 How it  is formed 3.1 Polarization mechanism theory 3.2 Electrostatic induction theory 3.3 The discharge 3.3.1 Gamma rays and  the runaway breakdown theory 3.4 Positive lightning 4 Types of lightning 4.1 Intracloud lightning, sheet lightning, anvil crawlers 4.2 Cloud-to-ground lightning, anvil-to-ground lightning 4.3 Bead lightning, ribbon lightning, staccato lightning 4.4 Cloud-to-cloud lightning 4.5 Ground-to-cloud lightning 4.6 Heat lightning or  summer lightning 4.7 Ball lightning 4.8 Sprites, elves, jets, and  other upper atmospheric lightning 4.9 Streak lightning 4.10 Triggered lightning 4.11 Lightning during volcanic eruptions 4.12 Rocket Lightning 4.13 Lightning throughout the Solar System 5 Lightning safety 6 Facts and  trivia 7 See also 8 References 9 External links 9.1 Jets, sprites & elves

[edit] Early research

During early investigations into electricity via Leyden jars and  other instruments, a number of people (D. William Wall (1708), Stephen Gray (1735), and  Abbé Nollet) proposed that small-scale sparks shared some similarity with lightning.^[1]

Benjamin Franklin also invented the lightning rod, endeavouring to test this theory using a spire which was being erected in Philadelphia. While he was waiting for the spire completion, some others (Thomas-François Dalibard and  De Lors) conducted at Marly in France what became known as the Philadelphia Experiment that Franklin had suggested in his book.

Franklin usually gets the credit, as he was the first to suggest this experiment. The Franklin experiment is  as follows:

Whilste waiting for completion of the spire, he got the idea of using a flying object, such as a kite, instead. During the next thunderstorm, which was in June 1752, he raised a kite, accompanied by his son as an  assistant. On his end of the string he attached a key and  tied it  to a post with a silk thread. As time passed, Franklin noticed the loose fibers on the string stretching out; he then brought his hand close to the key and  a spark jumped the gap. The rain which had fallen during the storm had soaked the line and  made it  conductive.

However, in his autobiography (written 1771-1788, first published 1790), Franklin clearly states that he performed this experiment after those in France, which occurred weeks before his own experiment, without his prior knowledge as of 1752.^[2]

As news of the experiment and  its particulars spread, the experiment was met with attempts at replication. However, experiments involving lightning are  always risky and  frequently fatal. The most well-known death during the spate of Franklin imitators was that of Professor Georg Richmann, of Saint Petersburg, Russia. He had created a set-up similar to Franklin's, and  was attending a meeting of the Academy of Sciences when he heard thunder. He ran home with his engraver to capture the event for posterity. While the experiment was under way, ball lightning appeared, collided with Richmann's head and  killed him, leaving a red spot. His shoes were blown open, parts of his clothes singed, the engraver knocked out, the doorframe of the room split, and  the door itself torn off its hinges.^[3]

[edit] Modern research

Although experiments from the time of Franklin showed that lightning was a discharge of static electricity, there was little improvement in theoretical understanding of lightning (in particular how it  was generated) for more  than 150 years. The impetus for new research came from the field of power engineering: as power transmission lines came into service, engineers needed to know much more  about lightning in order to adequately protect lines and  equipment.

An initial bipolar discharge, or  path of ionized air, starts from a negatively charged region in the thundercloud. The discharge ionized channels are  called leaders. The negative charged leaders, called a "stepped leader", proceed generally downward in a large number of quick jumps, each up to 50 metres long. Along the way, the stepped leader may branch into a number of paths as it  continues to descend. The progression of stepped leaders takes a comparatively long time (hundreds of milliseconds) to approach the ground. This initial phase involves a relatively small electric current (tens or  hundreds of amperes), and  the leader is  almost invisible compared to the subsequent lightning channel. When the downward leader is  quite close to the ground, one or  more smaller discharges (called positive streamers) arise from nearby, usually tall, grounded objects due to the intense electric field created by the approaching leaders.

As one of the rising streamers meets a stepped leader, the circuit is  closed, and  the main lightning stroke (often referred to as the return stroke) follows with much higher current. The main stroke travels at about 0.1 c (30 million meters/second or  100 million feet/second) and  the peak current lasts for tens of microseconds or  so. After the peak, the current typically decays over tens or  hundreds of microseconds.

In addition, negative lightning usually contains a number of restrikes along the same channel. Each restrike is  separated by a much larger amount of time, typically 30 milliseconds or  so. Additional return strokes are  punctuated by intermediate dart leader strokes akin to, but weaker than, the initial stepped leader. This rapid restrike effect was probably known in antiquity, and  the "strobe light" effect is  often quite noticeable.

Positive lightning (a rarer form of lightning that originates from positively charged regions of a thundercloud) does not generally fit the above pattern.

NASA scientists have  also found that the radio waves created by lightning clear a safe zone in the radiation belt surrounding the earth. This zone, known as the Van Allen Belt slot, can potentially be a safe haven for satellites, offering them protection from the Sun's radiation.

[ it  is formed">edit] How it  is formed

The first process in the generation of lightning is  still a matter of debate:^[4] one common idea from scientists is  that lightning forms from the ejection of charged particles from the sun, which reach Earth through the solar wind.^[5] These charged particles cause the Earth to acquire an  electric charge in its outer atmospheric layers, especially the ionosphere. Large quantities of ice in the clouds are  suspected to enhance lightning development.^[6] This charge will neutralize itself through any available path. This may assist in the forcible separation of positive and  negative charge carriers within a cloud or  air, and  thus help in the formation of lightning.

[edit] Polarization mechanism theory

The mechanism by which charge separation happens is  still the subject of research, but one theory is  the polarization mechanism, which has two components:

1. Falling droplets of ice and  rain become electrically polarized as they fall through the atmosphere's natural electric field;
2. Colliding ice particles become charged by electrostatic induction.

Source: science.nasa.gov: Electric Ice, 2006-09-13

[edit] Electrostatic induction theory

Another theory is  that opposite charges are  driven apart by the above mechanism and  energy is  stored in the electric fields between them. Cloud electrification appears to require strong updrafts which carry water droplets upward, supercooling them to -10 to -20 C. These collide with ice crystals to form a soft ice-water mixture called graupel. The collisions result in a slight positive charge being transferred to ice crystals, and  a slight negative charge to the graupel. Updrafts drive lighter ice crystals upwards, causing the cloud top to accumulate increasing positive charge. The heavier negatively charged graupel falls towards the middle and  lower portions of the cloud, building up an  increasing negative charge. Charge separation and  accumulation continue until the electrical potential becomes sufficient to initiate lightning discharges.^{[citation needed]}

[edit] The discharge

When sufficient negative and  positive charges gather, and  when the electric field becomes sufficiently strong, an  electrical discharge (the bolt of lightning) occurs within clouds or  between clouds and  the ground. During the strike, successive portions of air become conductive as the electrons and  positive ions of air molecules are  pulled away from each other and  forced to flow in opposite directions.

A theory proposed by Alex Gurevich of the Lebedev Physical Institute in 1992 suggests that lightning strikes are  triggered by cosmic rays which ionize atoms, releasing electrons that are  accelerated by the electric fields, ionizing other air molecules and  making the air conductive by a runaway breakdown, then starting a lightning strike.^{[citation needed]}

As the cloud progresses over the Earth's surface, an  equal but opposite charge is  induced in the Earth below, and  the induced ground charge follows the movement of the cloud. When a step leader approaches the ground, the presence of opposite charges on the ground enhances the electric field. The electric field is  highest on trees and  tall buildings. If the electric field is  strong enough, a conductive discharge (called a positive streamer) can develop from these points. This was first theorized by Heinz Kasemir. As the field increases, the positive streamer may evolve into a hotter, higher current leader which eventually connects to the descending stepped leader from the cloud. it  is also possible for many streamers to develop from many different objects simultaneously, with only one connecting with the leader and  forming the main discharge path. Photographs have  been taken on which non-connected streamers are  clearly visible. When the two leaders meet, the electric current greatly increases. The region of high current propagates back up the positive stepped leader into the cloud with a "return stroke" that is  the most luminous part of the lightning discharge. Lightning can also occur within the ash clouds from volcanic eruptions,^[7][8] or  can be caused by violent forest fires which generate sufficient dust to create a static charge.

It has been seen using "stop action" movies of lightning strikes that most lightning strikes consist of several (up to 12) separate discharges of different intensities, causing the "flickering" effect commonly seen during a lightning discharge. Each successive stroke re-uses the heated path taken by the previous stroke. The electrical discharge rapidly superheats the leader channel, causing the air to expand rapidly and  produce a shock wave heard as thunder. The rolling and  gradually dissipating rumble is  caused by the heating and  cooling of the discharge channel, by successive lightning strokes, and  the time delay of sound coming from different portions of a long stroke. The variations in successive discharges are  the result of smaller regions of charge within the cloud being depleted by successive strokes.^{[citation needed]}

An average bolt of negative lightning carries a current of 30-to-50 kiloamperes (kA), although some bolts can be up to 120kA, and  transfers a charge of 5 coulombs and  500 megajoules (enough to light a 100 watt light bulb for 2 months). However, it  has been observed from experiments that different locations in the US have  different potentials (voltages) and  currents, in an  average lightning strike for that area. For example, Florida, with the largest number of recorded strikes in a given period, has a very sandy ground saturated with salt water, and  is surrounded by water. California, on the other hand, has fewer lightning strikes (being dryer). Arizona, which has very dry, sandy soil and  a very dry air, has cloud bases as high as 6,000-7,000 feet above ground level, and  gets very long, thin, purplish discharges, which crackle; while Oklahoma, with cloud bases about 1,500-2,000 feet above ground level and  fairly soft, clay-rich soil, has big, blue-white explosive lightning strikes, that are  very hot (high current) and  cause sudden, explosive noise when the discharge comes. Potentially, the difference in each case may consist of differences in voltage levels between clouds and  ground. Research on this is  still ongoing.^{[citation needed]}

[ and  the runaway breakdown theory">edit] Gamma rays and  the runaway breakdown theory

It has been discovered in the past 15 years that among the processes of lightning is  some mechanism capable of generating gamma rays, which escape the atmosphere and  are observed by orbiting spacecraft. Brought to light by NASA's Gerald Fishman in 1994 in an  article in Nature in 1994, these so-called Terrestrial Gamma-Ray Flashes (TGFs) were observed by accident, while he was documenting instances of extraterrestrial gamma ray bursts observed by the Compton Gamma Ray Observatory (CGRO). TGFs are  much much shorter in duration, however, lasting only ~1ms.

A 1996 study in the journal Geophysical Research Letters by Professor Umran Inan of Stanford University linked a TGF to an  individual lightning stroke occurring within 1.5 ms of the TGF event, proving for the first time that the TGF was of atmospheric origin and  associated with lightning strikes.

CGRO recorded only about 77 events in 10 years, however more  recently, the RHESSI spacecraft, as reported by David Smith of UC Santa Cruz, has been observing TGFs at a much higher rate, indicating that these occur ~50 times per day globally (still a very small fraction of the total lightning on the planet). The energy levels recorded exceed 20 MeV, implying that they came from particles (likely electrons) at speeds extremely close to the speed of light.

Scientists from Duke University have  also been studying the link between certain lightning events and  the mysterious gamma ray emissions that emanate from the Earth's own atmosphere, in light of newer observations of TGFs made by RHESSI. Their study suggests that this gamma radiation fountains upward from starting points at surprisingly low altitudes in thunderclouds.

Steven Cummer, from Duke University's Pratt School of Engineering, said, "These are  higher energy gamma rays than come from the sun. and  yet here they are  coming from the kind of terrestrial thunderstorm that we see here all the time."

Early theories of this pointed to lightning generating high electric fields at altitudes well above the cloud, where the thin atmosphere allows gamma rays to easily escape into space, known as "relativisitic runaway breakdown", similar to the way sprites are  generated. Subsequent evidence has cast doubt, though, and  suggested instead that TGFs may be produced at the tops of high thunderclouds. Though hindered by atmospheric absorption of the escaping gamma rays, these theories do not require the exceptionally high electric fields that high altitude theories of TGF generation rely on.

The role of TGFs and  their relationship to lightning remains a subject of ongoing scientific study.

[edit] Positive lightning

Positive lightning makes up less  than 5% of all lightning. it  occurs when the leader forms at the positively charged cloud tops, with the consequence that a negatively charged streamer issues from the ground. The overall effect is  a discharge of positive charges to the ground. Research carried out after the discovery of positive lightning in the 1970s showed that positive lightning bolts are  typically six to ten times more  powerful than negative bolts, last around ten times longer, and  can strike tens of kilometres/miles from the clouds. The voltage difference for positive lightning must be considerably higher, due to the tens of thousands of additional metres/feet the strike must travel. During a positive lightning strike, huge quantities of ELF and  VLF radio waves are  generated.

As a result of their greater power, positive lightning strikes are  considerably more  dangerous. At the present time, aircraft are  not designed to withstand such strikes, since their existence was unknown at the time standards were set, and  the dangers unappreciated until the destruction of a glider in 1999.^[9]

Positive lightning is  also now believed to have  been responsible for the 1963 in-flight explosion and  subsequent crash of  am  Flight 214">Pan am  Flight 214, a Boeing 707. Subsequently, aircraft operating in U.S. airspace have  been required to have  lightning discharge wicks to reduce the chances of a similar occurrence.

Positive lightning has also been shown to trigger the occurrence of upper atmosphere lightning. it  tends to occur more  frequently in winter storms and  at the end of a thunderstorm.

An average bolt of positive lightning carries a current of up to 300 kiloamperes (about ten times as much current as a bolt of negative lightning), transfers a charge of up to 300 coulombs, has a potential difference up to 1 gigavolt (a billion volts), and  lasts for hundreds of milliseconds, with a discharge energy of up to 3x10¹¹joule.

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[edit] Types of lightning

Some lightning strikes take on particular characteristics; scientists and  the public have  given names to these various types of lightning.

[edit] Intracloud lightning, sheet lightning, anvil crawlers

Intracloud lightning is  the most common type of lightning, and  occurs completely inside one cumulonimbus cloud; it  is termed sheet lightning because the bolt is  not seen, instead one sees the whole cloud light up from inside.^[10] Lightning that appears to travels extensively along the cloud anvil or  its base is  commonly called a crawler, or  sometimes 'spider lightning'. Discharges of electricity in anvil crawlers travel up the sides of the cumulonimbus cloud branching out at the anvil top.

[edit] Cloud-to-ground lightning, anvil-to-ground lightning

Cloud-to-ground lightning is  a great lightning discharge between a cumulonimbus cloud and  the ground initiated by the downward-moving leader stroke. This is  the second most common type of lightning. One special type of cloud-to-ground lightning is  anvil-to-ground lightning, a form of positive lightning, since it  emanates from the anvil top of a cumulonimbus cloud where the ice crystals are  positively charged. In anvil-to-ground lightning, the leader stroke issues forth in a nearly horizontal direction until it  veers toward the ground. These usually occur miles ahead of the main storm and  will strike without warning on a sunny day. They are  signs of an  approaching storm and  are known colloquially as "bolts out of the blue".

[edit] Bead lightning, ribbon lightning, staccato lightning

Another special type of cloud-to-ground lightning is  bead lightning. This is  a regular cloud-to-ground stroke that contains a higher intensity of luminosity. When the discharge fades it  leaves behind a string of beads effect for a brief moment in the leader channel. A third special type of cloud-to-ground lightning is  ribbon lightning. These occur in thunderstorms where there are  high cross winds and  multiple return strokes. The winds will blow each successive return stroke slightly to one side of the previous return stroke, causing a ribbon effect. The last special type of cloud-to-ground lightning is  staccato lightning, which is  nothing more  than a leader stroke with only one return stroke.

Rare image of bead lightning by Marty Pouwelse

[edit] Cloud-to-cloud lightning

Cloud-to-cloud or  intercloud lightning is  a somewhat rare type of discharge lightning between two or  more completely separate cumulonimbus clouds.

[edit] Ground-to-cloud lightning

Ground-to-cloud lightning is  a lightning discharge between the ground and  a cumulonimbus cloud from an  upward-moving leader stroke. These thunderstorm clouds are  formed wherever there is  enough upward motion, instability in the vertical, and  moisture to produce a deep cloud that reaches up to levels somewhat colder than freezing. These conditions are  most often met in summer. Lightning occurs less  frequently in the winter because there is  not as much instability and  moisture in the atmosphere as there is  in the summer. These two ingredients work together to make convective storms that can produce lightning. Without instability and  moisture, strong thunderstorms are  unlikely. Lightning originates around 15,000 to 25,000 feet above sea level when raindrops are  carried upward until some of them convert to ice. For reasons that are  not widely agreed upon, a cloud-to-ground lightning flash originates in this mixed water and  ice region. The charge then moves downward in 50-yard sections called step leaders. it  keeps moving toward the ground in these steps and  produces a channel along which charge is  deposited. Eventually it  encounters something on the ground that is  a good connection. The circuit is  complete at that time, and  the charge is  lowered from cloud-to-ground. The return stroke is  a flow of charge (current) which produces luminosity much brighter than the part that came down. This entire event usually takes less  than half a second.

However, it  has been proven by movies taken of typical lightning strikes and  then, using single-frame examination (looking at each frame of a sequence), that a typical lightning strike is  made up of anywhere from 8 to 12 or  more individual discharges, with each successive discharge being less  intense and  farther apart in time. This is  easily explained by a process known in the electronics industry as damped oscillation, which is  sustained by the magnetic field that is  built up in the surrounding air during current flow in each discharge, and  then that magnetic field starts collapsing when current flow starts decreasing at the end of the current flow. This causes induced current that continues in the same direction, sustaining current flow beyond the point where the original charge voltage would have  been depleted, and  possibly reversing the charge voltage polarity, bringing on the next successive discharge, as long as sufficient charge is  available to sustain another discharge. (This is  almost exactly the type of current-flow used in alternating-current circuits to drive motors, lamps, etc.).

[ or  summer lightning">edit] Heat lightning or  summer lightning

Heat lightning (or, in the UK, "summer lightning") is  nothing more  than the faint flashes of lightning on the horizon or  other clouds from distant thunderstorms. Heat lightning was named because it  often occurs on hot summer nights. Heat lightning can be an  early warning sign that thunderstorms are  approaching. In Florida, heat lightning is  often seen out over the water at night, the remnants of storms that formed during the day along a seabreeze front coming in from the opposite coast. In some cases, the Thunderstorm may be too distant to hear the associated thunder from the lightning discharge.

Some cases of "heat lightning" can be explained by the refraction of light or  sound by bodies of air with different densities. an  observer may see nearby lightning, but the sound from the discharge is  refracted over his head by a change in the temperature, and  therefore the density, of the air around him. As a result, the lightning discharge seems to be silent.^[11]

[edit] Ball lightning

Main article: Ball lightning

Ball lightning is  described as a floating, illuminated ball that occurs during thunderstorms. They can be fast moving, slow moving or  nearly stationary. Some make hissing or  crackling noises or  no noise at all. Some have  been known to pass through windows and  even dissipate with a bang. Ball lightning has been described by eyewitnesses but rarely, if ever, recorded by meteorologists.

The engineer Nikola Tesla wrote, "I have  succeeded in determining the mode of their formation and  producing them artificially".^[12] There is  some speculation that electrical breakdown and  arcing of cotton and  gutta-percha wire insulation used by Tesla may have  been a contributing factor, since some theories of ball lightning require the involvement of carbonaceous materials. Some later experimenters have  been able to briefly produce small luminous balls by igniting carbon-containing materials atop sparking Tesla Coils.

Several theories have  been advanced to describe ball lightning, with none being universally accepted. Any complete theory of ball lightning must be able to describe the wide range of reported properties, such as those described in Singer's book "The Nature of Ball Lightning" and  also more  contemporary research. Japanese research shows that ball lightning has been seen several times without any connection to stormy weather or  lightning.

Ball lightning field properties are  more extensive than realized by many scientists not working in this field. The typical diameter is  usually standardized as 20–30 cm (8-12 inches), but ball lightning several meters in diameter has been reported (Singer). A recent photograph by a Queensland ranger, Brett Porter, showed a fireball that was estimated to be 100 meters (330 feet) in diameter. The photograph has appeared in the scientific journal Transactions of the Royal Society. The object was a glowing globular zone (possibly the breakdown zone) with a long, twisting, rope-like projection (possibly the funnel).

Ball lightning has been seen in tornadoes, and  has also been seen to split apart into two or  more separate balls and  recombine. Ball lightning has carved trenches in the peat swamps in Ireland. Vertically linked fireballs have  been reported. One theory that may account for this wider spectrum of observational evidence is  the idea of combustion inside the low-velocity region of axisymmetric (spherical) vortex breakdown of a natural vortex (e.g., the 'Hill's spherical vortex'). The scientist Coleman was the first to propose this theory in 1993 in Weather, a publication of the Royal Meteorological Society.

Another very strong possibility is  that ball lightning may be caused by plasma.

St Elmo's fire was correctly identified by Benjamin Franklin as electrical in nature. it  is not the same as ball lightning.

In 2000, students at Highland Park High School in Dallas Texas were sent home after ball lightning reportedly traveled through a school corridor.

[ and  other upper atmospheric lightning">edit] Sprites, elves, jets, and  other upper atmospheric lightning

Reports by scientists of strange lightning phenomena above storms date back to at least 1886. However, it  is only in recent years that fuller investigations have  been made. This has sometimes been called megalightning.

Sprites are  now well-documented electrical discharges that occur high above the cumulonimbus cloud of an  active thunderstorm. They appear as luminous reddish-orange, neon-like flashes, last longer than normal lower stratospheric discharges (typically around 17 milliseconds), and  cause the discharges of positive lightning between the cloud and  the ground. Sprites can occur up to 50 km (30 miles) from the location of the lightning strike, and  with a time delay of up to 100 milliseconds. Sprites usually occur in clusters of two or  more simultaneous vertical discharges, typically extending from 65 to 75 km (40 to 47 miles) above the earth, with or  without less  intense filaments reaching above and  below. Sprites are  preceded by a sprite halo that forms because of heating and  ionization less  than 1 millisecond before the sprite. Sprites were first photographed on July 6, 1989, by scientists from the University of Minnesota and  named after the mischievous sprites in the plays of Shakespeare. These Sprites may be the result of the neutralization of accumulated charge from the Earth sweeping up particles from the Solar Wind, as described at the beginning of this article.

Recent research^[13] carried out at the University of Houston in 2002 indicates that some normal (negative) lightning discharges produce a sprite halo, the precursor of a sprite, and  that every lightning bolt between cloud and  ground attempts to produce a sprite or  a sprite halo. Research in 2004 by scientists from Tohoku University found that very low frequency emissions occur at the same time as the sprite, indicating that a discharge within the cloud may generate the sprites.^[14] more  probably, as said before, they may be generated from interaction with the upper atmosphere's neutralizing a charge derived from the Earth's movement through the Solar Wind.

Blue jets differ from sprites in that they project from the top of the cumulonimbus above a thunderstorm, typically in a narrow cone, to the lowest levels of the ionosphere 40 to 50 km (25 to 30 miles) above the earth. They are  also brighter than sprites and, as implied by their name, are  blue in color. They were first recorded on October 21, 1989, on a video taken from the space shuttle as it  passed over Australia. Again, this could be currents being generated from potential differences in the upper atmosphere caused by the same derivation of charge from the Solar Wind.

Elves often appear as a dim, flattened, expanding glow around 400 km (250 miles) in diameter that lasts for, typically, just one millisecond.^[15] They occur in the ionosphere 100 km (60 miles) above the ground over thunderstorms. Their color was a puzzle for some time, but is  now believed to be a red hue. Elves were first recorded on another shuttle mission, this time recorded off French Guiana on October 7, 1990. Elves is  a frivolous acronym for Emissions of Light and  Very Low Frequency Perturbations From Electromagnetic Pulse Sources. This refers to the process by which the light is  generated; the excitation of nitrogen molecules due to electron collisions (the electrons possibly having been energized by the electromagnetic pulse caused by a discharge from the Ionosphere).

On September 14, 2001, scientists at the Arecibo Observatory photographed a huge jet double the height of those previously observed, reaching around 80 km (50 miles) into the atmosphere. The jet was located above a thunderstorm over the ocean, and  lasted under a second. Lightning was initially observed traveling up at around 50,000 m/s in a similar way to a typical blue jet, but then divided in two and  sped at 250,000 m/s to the ionosphere, where they spread out in a bright burst of light.

On July 22, 2002, five gigantic jets between 60 and  70 km (35 to 45 miles) in length were observed over the South China Sea from Taiwan, reported in Nature.^[16] The jets lasted under a second, with shapes likened by the researchers to giant trees and  carrots.

Researchers have  speculated that such forms of upper atmospheric lightning may play a role in the formation of the ozone layer. Alternatively, they may be due to differences in potential that result in current from the ozone layer.

[edit] Streak lightning

Most lightning is  streak lightning. This is  nothing more  than the return stroke, the visible part of the lightning stroke. Because most of these strokes occur inside a cloud, we do not see many of the individual return strokes in a thunderstorm.

[edit] Triggered lightning

Lightning has been triggered directly by human activity in several instances. Lightning struck the Apollo 12 soon after takeoff, and  has struck soon after thermonuclear explosions. it  has also been triggered by launching rockets carrying spools of wire into thunderstorms. The wire unwinds as the rocket climbs, making a convenient path for the lightning to use. These bolts are  typically very straight, due to the path created by the wire.^[17]

[edit] Lightning during volcanic eruptions

Extremely large volcanic eruptions, which eject gases and  solid material high into the atmosphere can trigger lightning, and  this phenomenon was documented by Pliny The Elder during the AD79 eruption of Vesuvius in which he perished.

[edit] Rocket Lightning

This is  a form of cloud discharge, generally horizontal and  at cloud base, with a luminous channel appearing to advance through the air with visually resolvable speed, often intermittently. The movement resembles the movement of a rocket, hence its name. it  is also one of the rarest of cloud discharges.

[edit] Lightning throughout the Solar System

Lightning requires the electrical breakdown of gas, so it  cannot exist in a visual form in the vacuum of space. However, lightning has been observed within the atmospheres of other planets, such as Venus and  Jupiter. Lightning on Jupiter is  estimated to be 100 times as powerful as, but fifteen times less  frequent than, that which occurs on Earth. Lightning on Venus is  still a controversial subject after decades of study. During the Soviet Venera and  U.S. Pioneer missions of the 1970s and  80s, signals suggesting lightning may be present in the upper atmosphere were detected.^[18] However, recently the Cassini-Huygens mission fly-by of Venus detected no signs of lightning at all.

[edit] Lightning safety

Thunderstorms are  the primary source of lightning. Because people have  been struck many miles away from a storm, seeking immediate and  effective shelter when thunderstorms approach is  an important part of lightning safety. Contrary to popular notion, there is  no 'safe' location outdoors. People have  been struck in sheds and  makeshift shelters. A better location would be inside a vehicle (a crude type of Faraday cage). it  is advisable to keep oneself away from any attached metallic components once inside (keys in ignition, etc.). As soon as one can hear thunder, one is  at risk of being struck.

Several different types of devices , including lightning rods and  electrical charge dissipators, are  used to prevent lightning damage and  safely redirect lightning strikes.

Nearly 2000 people per year in the world are  injured by lightning strikes, and  between 25 to 33% of those struck die. Lightning injuries result from three factors: electrical damage, intense heat, and  the mechanical energy which these generate. While sudden death is  common because of the huge voltage of a lightning strike, survivors often fare better than victims of other electrical injuries caused by a more  prolonged application of lesser voltage.^[19]

Lightning can incapacitate humans in four different ways:

• Direct strike
• 'Splash' from nearby objects struck
• Ground strike near victim causing a difference of potential in the ground itself (due to resistance to current in the Earth), amounting to several thousand volts per foot, depending upon the composition of the earth that makes up the ground at that location (sand being a fair insulator and  wet, salty and  spongy earth being more  conductive).
• EMP or  electromagnetic pulse from close strikes - especially during positive lightning discharges

In a direct hit the electrical charge strikes the victim first. Counterintuitively, if the victim's skin resistance is  high enough, much of the current will flash around the skin or  clothing to the ground, resulting in a surprisingly benign outcome. Splash hits occur when lightning prefers a victim (with lower resistance) over a nearby object that has more  resistance, and  strikes the victim on its way to ground. Ground strikes, in which the bolt lands near the victim and  is conducted through the victim and  his or  her connection to the ground (such as through the feet, due to the voltage gradient in the earth, as discussed above), can cause great damage.

The most critical injuries are  to the circulatory system, the lungs, and  the central nervous system. Many victims suffer immediate cardiac arrest and  will not survive without prompt emergency care, which is  safe to administer because the victim will not retain any electrical charge after the lightning has struck (of course, the helper could be struck by a separate bolt of lightning in the vicinity). Others incur myocardial infarction and  various cardiac arrhythmias, either of which can be rapidly fatal as well. The intense heat generated by a lightning strike can burn tissue, and  cause lung damage, and  the chest can be damaged by the mechanical force of rapidly expanding heated air. Either the electrical or  the mechanical force can result in loss of consciousness, which is  very common immediately after a strike. Amnesia and  confusion of varying duration often result as well. A complete physical examination by paramedics or  physicians may reveal ruptured eardrums, and  ocular cataracts may develop, sometimes more  than a year after an  otherwise uneventful recovery.

The lightning often leaves skin burns in characteristic Lichtenberg figures, sometimes called lightning flowers; they may persist for hours or  days, and  are a useful indicator for medical examiners when trying to determine the cause of death. They are  thought to be caused by the rupture of small capillaries under the skin, either from the current or  from the shock wave. it  is also speculated that the EMP created by a nearby lightning strike can cause cardiac arrest.

There is  sometimes spectacular and  unconventional lightning damage. Hot lightning (high-current lightning) which lasts for more  than a second can deposit immense energy, melting or  carbonizing large objects. One such example is  the destruction of the basement insulator of the 250-metre-high central mast of longwave transmitter Orlunda, which led to its collapse.

[ and  trivia">edit] Facts and  trivia

A bolt of lightning can reach temperatures approaching 28,000 degrees Celsius (50,000 degrees Fahrenheit) in a split second. This is  about five times hotter than the surface of the sun. The heat of lightning that strikes loose soil or  sandy regions of the ground may fuse the soil or  sand into glass channels called fulgurites. These are  sometimes found under the sandy surfaces of beaches and  golf courses, or  in desert regions. Fulgurites are  evidence that lightning spreads out into branching channels when it  strikes the ground.

Trees are  frequent conductors of lightning to the ground ^[20]. Since sap is  a poor conductor, its electrical resistance causes it  to be heated explosively into steam, which blows off the bark outside the lightning's path. In following seasons trees overgrow the damaged area and  may cover it  completely, leaving only a vertical scar. If the damage is  severe, the tree may not be able to recover, and  decay sets in, eventually killing the tree. Occasionally, a tree may explode completely, as in this Giant Sequoia struck in Geneva.^[21] it  is commonly thought that a tree standing alone is  more frequently struck, though in some forested areas, lightning scars can be seen on almost every tree.

Of all common trees the most frequently struck is  the oak. it  has a deep central root that goes beneath the tree, as well as hollow water-filled cells that run up and  down the wood of the oak's trunk. These two qualities make oak trees better grounded and  more conductive than trees with shallow roots and  closed cells.

• The odds of an  average person living in the USA being struck by lightning once in his lifetime has been estimated to be 1:280,000.^[22]

• The odds of having a friend or  family member struck by lightning in the USA in a lifetime has been estimated to 1:3000.^[22]

• Singapore has one of the highest rates of lightning activity in the world. [1]

• The city of Teresina in northern Brazil has the third-highest rate of occurrences of lightning strikes in the world. The surrounding region is  referred to as the Chapada do Corisco ("Flash Lightning Flatlands").

• The United States is  home to "Lightning Alley", a group of states in the American Southeast that collectively see more  lightning strikes per year than any other place in the US. The most notable state in Lightning Alley is  Florida.

• The saying "lightning never strikes twice in the same place" is  false. The Empire State Building is  struck by lightning on average 100 times each year, and  was once struck 15 times in 15 minutes. ^{[citation needed]}

• Jim Caviezel, the actor who played Jesus in the film The Passion of the Christ, is  reported to have  been struck twice by lightning during shooting. The assistant director Jan Michelini was struck twice.^[23]

• Golfers Retief Goosen and  Lee Trevino have  both been struck by lightning while playing.^[24]

• Although commonly associated with close thunderstorms, lightning strikes can occur on a day that seems devoid of clouds. This occurrence is  known as "A Bolt From the Blue" and  is due to the fact that lightning can strike up to 10 miles from a cloud.

• Lightning interferes with am  (amplitude modulation) radio signals much more  than FM (frequency modulation) signals, providing an  easy way to gauge local lightning strike intensity.

• Roy Sullivan has the record for being the human who has been struck by lightning the most times. Working as a park ranger, Roy was struck seven times over the course of his 35 year career. He lost a nail on his big toe, and  suffered multiple injuries to the rest of his body.^[25]

• Colombian soccer player Herman Gaviria a.k.a Carepa, was struck by lightning during a training session in Cali, Colombia and  died at the age of 37. Strangely, before starting the session, he said "Lightning is  not going to kill me."

• On average, lightning strikes the earth about 100 times every second.

In movies and  comics of the contemporary U.S. and  many other countries, lightning is  often employed as an  ominous, dramatic sign. it  may herald a waking of a great evil or  emergence of a crisis. This has often also been spoofed, with the uttering of certain words or  phrases causing flashes of lightning to appear outside of windows (and often scaring or  disturbing some characters). Some characters, such as Storm and  Thor have  the ability to control weather, which includes summoning bolts of lightning. While this is  usually typical of cartoons, it  has also been employed by regular TV shows and  movies. Various novels and  role playing games with fantasy tint involves wizardry of lightning bolt, weapon embodying the power of lightning, etc. The comic book character Billy Batson changed into the superhero Captain Marvel by saying the word "Shazam!", which called down a bolt of magic lightning to make the change. Flash II (Barry Allen) and  III (Wally West) were both granted their superspeed in accidents involving lightning. Also in the 1931 movie Frankenstein, lightning is  used as the catalyst for endowing life into the creature. In the Star Wars universe, the Sith have  the ability to use the Force to cast lightning from their fingertips.

The bolt of lightning in heraldry is  called a thunderbolt and  is shown as a zigzag with non-pointed ends. it  is also distinguished from the "fork of lightning". The lightning bolt shape was a symbol of male humans among the Native Americans such as the Apache (a rhombus shape being a symbol for females) in the American Old West.

The name of New Zealand's most celebrated thoroughbred horse, Phar Lap, derives from the shared Zhuang and  Thai word for lightning.

Some European languages have  a separate word for lightning which strikes the ground (as opposed to lightning in general). Often it's a cognate of the English word "rays."

Estimating distance of a lightning strike: The flash of a lightning strike and  resulting thunder occur at roughly the same time. But light travels at 300,000 kilometers in a second, almost a million times the speed of sound. Sound travels at the slower speed of 330 m/s in the same time, so the flash of lightning is  seen before thunder is  heard. By counting the seconds between the flash and  the thunder and  dividing by 3, you can estimate your distance from the strike and  initially the actual storm cell (in kilometers). Similarly, by dividing by 5, you can estimate the distance in miles.

[edit] See also

• List of light sources
• List of people famous for surviving a near-fatal event
• runaway breakdown

[edit] References

[edit] External links

Wikimedia Commons has media related to: Lightning

• Severe Weather UK
• Impressive lightning photography more  than 200 Lightning-Pictures
• Video of a heavy thunderstorm Source: German Weather Chronicles
• How cosmic rays trigger lightning strikes
• Article from How Stuff Works
• A Yahoo group about lightning safety and  power quality. Registration Required
• dmoz: Thunderstorms and  Lightning
• Lightning Safety Page - National Weather Service Pueblo Colorado Citat: "...This is  known as a "side flash". Many people who are  "struck" by lightning are  not hit directly by the main lightning channel, but are  affected by the side flash..."
• Lightning Facts
• Laser Beam Triggers Lightning Strike During Japanese Experiment
• Colorado Lightning Resource Center
• Webarchive: April 25, 1997 Sandia-led research may zap old beliefs about lightning protection at critical facilities; Triggered lightning tests leading to safer storage bunkers
• 2003-11-06, ScienceDaily: Thunderstorm Research Shocks Conventional Theories; Florida Tech Physicist Throws Open Debate On Lightning's Cause
• Huge oak tree destroyed by lightning, by Igor Chudov
• Austrian Lightning Detection and  Information System
• European Cooperation for Lightning Detection
• How to Photograph Lightning A page with both brief and  verbose instructions on taking lightning photos.
• Lightning strike to aircraft
• Lightning and  The Empire State Building NYC
• Lightning Gallery Flickr set of lightning from around the world
• Positive Lightning Strike Photo of a nearby positive lightning strike that nearly kills the Australian photographer
• Petrified lightning (fulgurites) from Central Florida
• United States Precision Lightning Network - Live lightning data map
• NASA Finds Lightning Clears Safe Zone in Earth's Radiation Belt
• AMS Glossary: Dart Leader
• NOAA: What is  Lightning?
• Theories of Lightning formation
• Lucky snapshot: lightning strikes chemical mill in Germany
• Lightning detection system shows lightning activity in the Tampa Bay, Florida area

[edit] Jets, sprites & elves

• Homepage of the Eurosprite campaign, itself part of the CAL (Coupled Atmospheric Layers research group
• March 2, 1999, University of Houston: UH Physicists Pursue Lightning-Like Mysteries Quote: "...Red sprites and  blue jets are  brief but powerful lightning-like flashes that appear at altitudes of 40-100 km (25-60 miles) above thunderstorms..."
  • Ground and  Balloon-Borne Observations of Sprites and  Jets
• Barrington-Leigh, C. P., "Elves : Ionospheric Heating By the Electromagnetic Pulses from Lightning (A primer)". Space Science Lab, Berkeley.
• "Darwin Sprites '97". Space Physics Group, University of Otago.
• Gibbs, W. Wayt, "Sprites and  Elves : Lightning's strange cousins flicker faster than light itself". San Francisco. ScientificAmerican.com.
• Barrington-Leigh, Christopher, "VLF Research at Palmer Station".
• Heavenly light show caught on film (Nature) - Requires subscription to news@nature.com.
• Sprites, jets and  TLE pictures and  articles