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For alternate uses, see Lightning (disambiguation).

(PD) Photo: M ircea Madau

Lightning over Lightning over Oradea in Romania

Lightning is a powerful natural electrostatic discharge produced during a thunderstorm. This abrupt electric discharge is accompanied by the emission of visible light and other forms of electromagnetic radiation. The electric current passing through the discharge channels rapidly heats and expands the air into plasma producing acoustic shock waves (thunder) in the atmosphere.

Early research

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

Benjamin Franklin also invented the lightning rod, endeavoured 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:

Whilst 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.

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.

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 discharge, or path of ionized air, called a "stepped leader", starts from a negatively charged region in the thundercloud and proceeds 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 clears 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.

How it is formed

The first process in the generation of lightning is the ejection of charged particles from the sun in what is called the solar wind. The Earth acquires an electric charge in its outer atmospheric layers, especially the ionosphere, from these particles. 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.

Charge separation theories

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:

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

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.

Gamma ray 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, these so-called Terrestrial Gamma-Ray Flashes (TGFs) were observed by accident, while he was documenting instances observed by the Compton Gamma Ray Observatory.

A 1996 study 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 that the TGF was of atmospheric origin and associated with lightning strikes.

Scientists from Duke University have 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 the RHESSI spacecraft. 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." Cummer, and co-researchers Wenyi Hu and Yuhu Zhai, described their analyses in a paper published in the journal Geophysical Research Letters. Their research builds on earlier work by David Smith and Liliana Lopez of the University of California.

In the new study, Cummer and his co-researchers made what he termed "very careful and continuous recordings" of lightning emissions in a targeted area over a four month period of 2004. They identified lightning episodes they could link in time and place to TGFs recorded by RHESSI in the tropical Caribbean region. "The Caribbean is 2,000-4,000 kilometers from our sensors - in the scale of things actually quite close. So we were able to say with very strong certainty whether lightning happened in the Caribbean at a specific time," said Cummer.

"If this were the operating mechanism, we should see enormous lightning strokes associated with every one of those TGFs," Cummer said. "But we found that this was unequivocally not the case." Instead, the lightning strokes his group analyzed were 50-500 times smaller than what should be required to create TGFs by runaway breakdown. Their report suggested that runaway breakdown at a much lower altitude, created within "strong fields in or just above the thundercloud," could have triggered the TGFs instead. "It still almost certainly has to be runaway breakdown that's creating these," Cummer said. "The only real possibility is that it's much closer to the cloud top, and linked to something else happening inside the cloud."

The analysis also disclosed that, on average, TGFs occurred 1.24 milliseconds before their associated lightning strokes. "That was something we absolutely were not expecting," Cummer said. "But the coincidence between the lightning and the TGFs we found is too good to be random. So, even if the TGFs precede the lightning, they are in some way connected."

Questions linger, though, about whether the TGF really does occur prior to the lightning stroke, or whether it occurs slightly after as an effect. For instance, the energy levels seen by the RHESSI spacecraft tend to drop in time over the duration of the TGF, suggesting a weakening effect rather than the strengthening buildup that would logically occur just before a lightning strike.

Furthermore, the accuracy of the RHESSI clock, and the reliant conclusion that TGFs precede lightning, has been called into question by a study of TGFs observed by the Compton Gamma Ray Observatory, the original spacecraft which "accidentally" discovered these TGFs. A recent study by Stanford University has made a similar analysis of TGFs from the original CGRO spacecraft, and found based on this dataset that TGFs in fact came after the lightning stroke by 1-3 ms. This is highly suggestive of a slight error in the recording clocks of either CGRO or RHESSI.

A recent comparison of a similar event between RHESSI and other spacecrafts indicated an offset of roughly 3 ms, implying that TGFs thought to be 1.24 ms early were actually occurring ~1.5 ms late, in line with the Stanford study of CGRO events. The CGROs recording clock compared accurately with all other spacecrafts during its mission.

TGFs occurring in the time following a lightning stroke is more suggestive of TGFs being generated at higher altitudes, 10s of km above the thundercloud, where the gamma rays can easily escape the very thin atmosphere without being absorbed.

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.

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 enhance 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 the 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[1][2], 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.

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.

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.[3]

Positive lightning is also now believed to have been responsible for the 1963 in-flight explosion and subsequent crash of 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), lasts for hundreds of milliseconds, with a discharge energy of up to 3x1011joule.

Types of lightning

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

Intracloud lightning, sheet lightning, anvil crawlers

Intracloud lightning is the most common type of lightning which occurs completely inside one cumulonimbus cloud, and is commonly called an anvil 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. Sheet lightning can be seen when lightning is close to the horizon. The individual strikes can't be seen, but simply light up the distant cloud.

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".

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.

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.

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.).

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.

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.[4]

Ball lightning

For more information, see: 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" [5]. 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 fireball diameter is usually standardized as 20–30 cm, 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 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).

Fireballs have been seen in tornadoes, and they have also split apart into two or more separate balls and recombined. Fireballs have 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.

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 are usually spawned by discharges of positive lightning between the cloud and the ground. Sprites can occur up to 50 km 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 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 [2] 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 [3]. 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 [4]. 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 [5]. 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. Rather, they may be due to differences in potential that result in current from the ozone layer.

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.

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 [6].

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.

Rocket lightning

A very rare and unexplained form which is slow enough for its movement to be visible, as with a rocket (hence the name).

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 [7]. However, recently the Cassini-Huygens mission fly-by of Venus detected no signs of lightning at all.

Lightning safety

File:Lightning animation.gif animation of a lightning strike

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.).

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.

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.

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 (photo of a tree being struck by lightning). 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 [8]. 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[6].

The odds of having a friend or family member struck by lightning in the USA in a lifetime has been estimated to 1:3000[6].

Singapore has the highest rate of lightning activity in the world. [9]

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.

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 [10].

Golfers Retief Goosen and Lee Trevino have both been struck by lightning while playing [11].

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 his big toe, and suffered multiple injuries to the rest of his body.[7]

Colombian soccer player Herman Gaviria a.k.a Carepa, was struck by a lightning during a training session in Cali Colombia and died at the age of 37. Strangely before starting the session he said "A 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). 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.

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 / Australia'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.

See also